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Understanding Nanocellulose−Water Interactions: Turning a
Detriment into an Asset
Laleh Solhi,*Valentina Guccini,*Katja Heise, Iina Solala, Elina Niinivaara, Wenyang Xu, Karl Mihhels,
Marcel Kröger, Zhuojun Meng, Jakob Wohlert, Han Tao, Emily D. Cranston, and Eero Kontturi*
Cite This: https://doi.org/10.1021/acs.chemrev.2c00611
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ABSTRACT: Modern technology has enabled the isolation of nanocellulose from plant-
based fibers, and the current trend focuses on utilizing nanocellulose in a broad range of
sustainable materials applications. Water is generally seen as a detrimental component when
in contact with nanocellulose-based materials, just like it is harmful for traditional cellulosic
materials such as paper or cardboard. However, water is an integral component in plants,
and many applications of nanocellulose already accept the presence of water or make use of
it. This review gives a comprehensive account of nanocellulose−water interactions and their
repercussions in all key areas of contemporary research: fundamental physical chemistry, chemical modification of nanocellulose,
materials applications, and analytical methods to map the water interactions and the eect of water on a nanocellulose matrix.
CONTENTS
1. Introduction B
2. Fundamentals of Cellulose−and Nanocellulose−
Water Interactions B
2.1. Overview of the Properties of Water B
2.2. Cellulose in Nature C
2.2.1. Origin and Basic Crystalline Structure of
Cellulose C
2.2.2. Cellulose Morphology and the Hierarch-
ical Structure of the Plant Cell Wall D
2.3. From Cellulose to Nanocellulose F
2.3.1. Types of Nanocellulose F
2.3.2. Role of Water in Nanocellulose Produc-
tion G
2.4. Nanocellulose−Water Systems: Properties
and Dynamics I
2.4.1. Water and Nanocellulose Interactions at
the Molecular and Supramolecular Level I
2.4.2. Behavior and Dynamics of Water within
Nanocellulose Matrices K
2.4.3. Nanocellulose Dispersions O
3. Role of Water in Nanocellulose Modification and
Applications: A Double-Edged Sword R
3.1. Pathways to Tune Nanocellulose−Water
Interactions R
3.1.1. Decreasing Nanocellulose Surface Hy-
drophilicity R
3.1.2. Increasing Nanocellulose Surface Hydro-
philicity T
3.2. Role of Water in Controlling Surface Mod-
ification Reactions U
3.2.1. Controlling Dispersion in Modification
Reactions by Tuning the Water Content U
3.2.2. Controlling Modification Outcomes by
Tuning Water Content U
3.3. Nanocellulose−Water Interactions in Materi-
als Applications V
3.3.1. Hydrogels X
3.3.2. Films, Membranes, Textile, and Coatings AC
3.3.3. Powders, Aerogels, and Foams AH
3.3.4. Reinforcing Nanofillers in Composites AK
4. Analytical Tools to Probe Nanocellulose−Water
System AM
4.1. Computational Methods to Uncover Water−
Nanocellulose Interactions AM
4.1.1. Cellulose Is Insoluble in Water AN
4.1.2. Cellulose Twist in Water AN
4.1.3. Eect of Hydration on Cellulose Dynam-
ics AN
4.1.4. Water Structure and Dynamics at Cellu-
lose Surfaces and within Fibril Aggre-
gates AN
4.1.5. Wetting and Water Sorption AO
4.1.6. Simulation of the Interactions of Func-
tionalized Cellulose with the Environ-
ment AQ
Special Issue: Sustainable Materials
Received: September 1, 2022
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© XXXX The Authors. Published by
American Chemical Society A
https://doi.org/10.1021/acs.chemrev.2c00611
Chem. Rev. XXXX, XXX, XXX−XXX
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4.2. Experimental Methods to Uncover Nano-
cellulose−Water Interactions AQ
4.2.1. Microscopic Methods AQ
4.2.2. Surface Energy and Mass Transport
Methods AT
4.2.3. Gravimetric Methods AW
4.2.4. Spectroscopic Methods AX
4.2.5. Thermal Analysis BD
4.2.6. Rheological and Mechanical Testing BD
4.2.7. Scattering Techniques BF
5. Future Trends in Nanocellulose−Water Interac-
tions BG
Table of Definitions and Abbreviations BH
Author Information BH
Corresponding Authors BH
Authors BH
Author Contributions BH
Notes BH
Biographies BH
Acknowledgments BI
References BI
1. INTRODUCTION
The term nanocellulose refers to anisotropic nanoparticles that
can be isolated from, in most cases, a processed plant cell wall.
While the early accounts of nanocellulose originate from the
mid-20th century, the first decade of the 21st century saw
process-related advances that enabled a more facile and ecient
isolation of nanocellulose for materials construction.
1
This
development in nanocellulose preparation and its applications
has unleashed an unprecedented scientific interest in cellulose-
based materials over the past 15 years.
2−14
Although much of the
attention has focused on utilizing nanocellulose as a building
block in new functional materials, the fundamental progress in
cellulose science stemming from nanocellulose research has also
been remarkable.
15−19
This review presents a comprehensive, critical coverage of a
topic which is dominant in both fundamental aspects and
materials applications of nanocellulose: cellulose−water inter-
actions. The presence of water or humidity in cellulose-based
materials is often seen as a detriment. Everyone knows what
happens to paper when you immerse it in water: it disintegrates
and loses its mechanical strength. Nanocellulose is made of
much smaller entities than pulp fibers, and because of its high
surface area, it takes up more water and the eect is even more
drastic. As high strength coupled with low density is one of the
main assets of nanocellulose, the strength loss in water is a major
issue. In general, water is seen as a nuisance, and the ensuing
problems are being tackled with “brute force” such as chemical
hydrophobization and the like. Quantification, localization, and
influence of water within a cellulose matrix has also been subject
to a number of analytical challenges throughout the history,
20
and it continues to be that way.
21
The purpose of this review is to point out how the presence of
water can be beneficial as well as detrimental in nanocellulose-
based systems, processes, and materials: isolation, chemical
modification, biomedical templates, responsive hydrogels,
sensors, smart emulsions, and so forth. Although intuitively
utilized since the ancient times, for example, Egyptians
exploiting wood swelling in water to seal leaking joints in their
boats, the systematic usage of water interactions has started to
emerge only within the past few years.
The specific response to water has its roots in the amphiphilic
nature of cellulose and its native crystallite structure. The
literature on fundamental aspects of cellulose−water inter-
actions spans roughly one century, albeit with a dramatic
upsurge during the past decade, fully covered in this review.
Understanding the dierent “types” of bound water and how to
measure them, is another crucial step to exploiting nano-
cellulose−water interactions, and as such, we extensively discuss
analytical tools including modeling and experimental ap-
proaches to elucidate the relationships between water and
nanocellulose.
A number of studies and reviews focus on the interactions of
water and other natural polymers such as chitin
22,23
and
collagen
24,25
in the literature. To our knowledge, a compre-
hensive review on such materials to the extent of the scope of this
review has not been published. In addition to presenting
fundamentals and characterization, this review focuses on new
nanocellulose-based materials which may suer (but just as well
benefit) from the presence of water. We see this as a vital
approach in the current research environment where “green”
solutions to chemicals and materials are intensively sought after.
Accepting (and taking advantage of) the presence of water and
the predictability of processing nanocellulose in water is also
important when we consider replacing nonaqueous solvents
with water in striving toward a more sustainable society.
Combined with the comprehensive nature of our approach,
spanning fundamentals and analytics, the all-inclusive take on
the role of water interactions in nanocellulose production,
modification, and applications is what distinguishes this review
from other recent reviews that touch on the subject of cellulose−
water interactions.
3,9,19,26−29
2. FUNDAMENTALS OF CELLULOSE−AND
NANOCELLULOSE−WATER INTERACTIONS
2.1. Overview of the Properties of Water
Water is usually considered as either a solvation agent or a
suspension medium in nanocellulose related research, and the
complexity of water is often overlooked. To understand
nanocellulose−water interactions, it is pivotal to briefly review
some of the relevant characteristics and main features of water.
To limit the scope of this discussion, we focus on liquid water at
moderate temperatures and pressures. For more comprehensive
reviews of water and its properties in more extreme conditions,
we guide the reader to relevant publications in the field.
30−33
The complex behavior of water results from a combination of
hydrogen bonding, dissociation behavior, and the complex
structural dynamics, influenced by temperature, pressure, and its
interaction with the interfaces and other molecules. As we
approach this topic, it should be noted that much controversy
still exists regarding the detailed mechanisms and dynamics of
water behavior at this level.
34
Also, the developments in the
simulation of water properties and dynamics
35−37
as well as
experimental techniques
38,39
have been recently reviewed
elsewhere.
IUPAC defines a hydrogen bond as “an attractive interaction
between a hydrogen atom from a molecule or a molecular
fragment X−H in which X is more electronegative than H, and
an atom or a group of atoms in the same or a dierent molecule,
in which there is evidence of bond formation”.
40
This definition
leaves the mechanism of this attractive interaction purposefully
open, as no single physical force can be found to be responsible
for the phenomena we observe as hydrogen bonding.
40
Indeed,
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the hydrogen bond is either described as a sum of several forces
such as electrostatic interactions, polarization, induction
interactions between multipoles, charge-transfer-induced co-
valency, or an independent interaction with unidentified
origin.
40
The consensus in the scientific community is that
there are three-dimensional dynamics and a random network of
hydrogen-bonded molecules in liquid water in which the
hydrogen bonds are continuously broken and reformed on the
time scale of femtoseconds to picoseconds.
41
Thus, there is a
broad distribution of possible energies and an indefinitely high
number of molecules involved in the hydrogen bonding network
of bulk liquid water.
42
This means that assigning a specific,
singular, hydrogen bonding energy to a liquid water system
would be misleading. Moreover, as water can be both an
acceptor and a donor of multiple hydrogen bonds, a distribution
of acceptor−donor states exists.
42
The 2 acceptor −2 donor
model, resulting in tetrahedral molecular ordering, seems to be
the dominant structure on average.
43
Due to the random
structuring of hydrogen bonds, a water cluster can be identified
as a subgroup of water molecules, which form comparatively
stable substructures in the time frame of hydrogen bond
formation and dissociation. These water clusters can influence
the structuring of hydrogen bonds in the surrounding medium
outside the cluster.
44
Although their very existence is not that
controversial, the exact structure, lifetime, and eects that
clusters have on the surrounding water medium are still under
debate. In addition to water clustering, other phenomena
characterized by the hydrogen water dynamics include proton
hopping (the exchange of protons between neighboring water
molecules)
45
and changes in the hydrogen bond structure of
water around OH−and H3O+ions.
46
Fluctuations in hydrogen
bond networks are experimentally accessible by computer
simulations,
47
albeit some spectroscopic methods also allow for
probing of the phenomena occurring on these time scales.
37
For
a more detailed discussion on how the various ways the
hydrogen bond energy of water has been approximated and
investigated we suggest a book chapter by Chaplin
48
and a
review by Cisneros et al.
35
for a more technical-oriented
approach.
In addition to water structuring in molecular clusters, the
hydrogen bond network of water is altered due to interactions
with interfaces or other molecules resulting in the reordering of
water molecules. Interesting examples of this phenomenon are
the structuring of water in the presence of solutes, at the surface
of water-dispersed colloids/particles, and even at hydrophobic
surfaces. We address the structuring of water around nano-
cellulosic materials in section 3.1 when the amphiphilic nature of
nanocellulose is discussed.
2.2. Cellulose in Nature
2.2.1. Origin and Basic Crystalline Structure of
Cellulose. Cellulose is a semicrystalline polysaccharide
composed of β-1,4-linked D-anhydroglucopyranose units
(C6H10O5), and it is biosynthesized from glucose through a
uridine diphosphate glucose intermediate by all higher-order
plants,
49
green algae,
50
as well as some specific marine animals
(tunicates)
51
and certain bacteria (namely those belonging to
the genera Acetobacter, Rhizobium, Agrobacterium, and
Sarcina).
52
Cellulose is the most ubiquitously present natural
polymer in both land and marine ecosystems and functions as a
highly eective natural carbon sink in terrestrial ecosystems
turning over almost 3.6 gigatons of carbon annually.
53
The
majority of cellulose is found in higher-order plants, and it
typically accounts for 40−50% of the mass of wood material
depending on the species of the plant source.
54
Cellulose biosynthesis is carried out by the cellulose synthase
complex or terminal complex (TC),
55
where simultaneously
upon their synthesis the cellulose polymer chains are assembled
into higher-order structures known as microfibrils, which are the
smallest supramolecular units of cellulose in nature. Microfibrils
are semicrystalline, slender threads that form the structural
scaold of the plant cell wall. In the native cellulose crystal,
sheets formed by hydrogen bonding are stacked on top of each
other through interplanar van der Waals forces.
56
Within the
sheets, the intramolecular hydrogen bonds in the native cellulose
I crystal are between HO(3)−HO(5) and HO(2)−HO(6),
whereas the major intermolecular bond forms between HO(3)
and HO(6) (Figure 1a for cellulose I and Figure 1b for cellulose
II). Overall, the hydrogen bond energy of cellulose ranges from
17 to 30 kJ mol−1, and the intermolecular hydrogen bond energy
is approximated to be around 20 kJ mol−1. In cellulose I, for
example, the density of hydrogen bonds is approximately 3.7 ×
1018 m−2along the 1(−1)0 crystallographic plane.
57
The widths and the shapes of the crystal, which determine the
width of the microfibril, dier according to the cellulose source.
As a rule of thumb, the higher the plant has climbed on the
evolutionary ladder, the thinner the crystal. Trees have the
thinnest crystals (∼3 nm), while algae have the widest (>20
nm). This observation can be rationalized by taking into account
that the crystallinity and order in the higher plant cell walls are
optimized to find a perfect balance between strength and
flexibility and to ensure the structural integrity of the organism
depending on their growth and environmental condition (e.g.,
wind, water availability).
58
The number of cellulose chains that make up the cellulose
crystallite in a microfibril is dependent on the source of the
cellulose, and it is still a matter of debate among the cellulose
community. For instance, in the case of wood cellulose, it was
traditionally accepted that each TC synthesizes microfibrils
consisting of 36 cellulose polymer chains (i.e., a 6 ×6 chain
cross-section, Figure 1c). Later, however, Jarvis and co-workers
suggested that 24 chains make up the crystal (Figure 1d),
61
and
more recently, models for 18 chain crystals have gained ground
(Figure 1e).
63
These 24 and 18 chain models appear currently
more accepted within crystallographers than the traditional 36
chain model. Regardless of the exact number of chains making
up the microfibril, it is important to understand that cellulose in
nature exists exclusively in the form of microfibrils and it is never
found in nature as single polymer chains or in a fully amorphous
form.
Native crystalline cellulose, also referred to as cellulose I, is
assembled with the cellulose polymer chains running parallel to
one another, and it exists as two dierent crystal structures (i.e.,
polymorphs): triclinic Iαor monoclinic Iβ.
64
Primarily, the
crystal structure of cellulose is governed by the conformation of
the TC, which in turn aects the morphology of the resulting
microfibril.
50
Linear TCs, for example, mostly produce Iαrich
cellulose and relatively wide microfibrils with high degree of
crystallinity, while rosette-shaped TCs produce thin, Iβ-rich
cellulose microfibrils.
65,66
Tsekos et al. showed a rough, but
indisputable, correlation between the shape of the TC and the
resulting cellulose microfibril.
67
It is important to note, however,
that both polymorphs coexist in all cellulose and their ratio
varies depending on the cellulose source.
68
Cellulose Iβis
dominant in higher-order plants and tunicate-synthesized
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cellulose, while cellulose Iαis the main component of celluloses
synthesized by algae and bacteria.
Two alternative hypotheses have been proposed to explain the
simultaneous presence of the Iαand Iβforms of cellulose. The
first is that cellulose Iαis synthesized by a dierent type of TC
from cellulose Iβ, whereas the second hypothesis is that the
dierent cellulose polyomorphs result from events that occur
after the synthesis of the cellulose polymer chains. For example,
it has been shown that bending can interconvert the crystalline
forms of cellulose I and that their ratio is very sensitive to the
angle through which the microfibril is bent.
69
Besides the native crystalline form, dierent cellulose
polymorphs exist, in which the crystalline structure of cellulose
Iαand Iβhave been altered through physicochemical treatments.
Cellulose II can be prepared by reorganizing the hydrogen
bonding network such that the cellulose chains run antiparallel
to each other in the crystal structure using either a mercerization
process where cellulose I is swollen in the presence of NaOH or
through the regeneration (i.e., solubilization and subsequent
recrystallization) of cellulose (Figure 1b).
70
Cellulose I readily
converts into cellulose II in an irreversible process, as cellulose II
is thermodynamically more stable than cellulose I.
71
A third
cellulose polymorph called cellulose III can also be reversibly
prepared by exposing cellulose I or II to liquid ammonia or
certain diamines.
72
However, cellulose I and II arguably garner
the most research interest because of their biological, industrial,
and scientific relevance, and cellulose I is overwhelmingly the
most dominant polymorph in all nanocellulose constructs.
2.2.2. Cellulose Morphology and the Hierarchical
Structure of the Plant Cell Wall. While cellulose microfibrils
are predominantly crystalline, there is direct evidence showing
that the microfibril structure in higher plants has regions of
disordered cellulose distributed along their length (Figure
2).
49,73
The length of the crystalline portions between these
disordered regions is primarily governed by the source of the
cellulosic material, and this is often depicted using the so-called
fringed-fibrillar model.
49,56,74−76
For example, the length of the
crystalline regions in cotton-derived cellulose is on average 125
nm (i.e., a degree of polymerization (DP) of 250), whereas that
of tunicate cellulose can be as high as 3 μm (DP = 6000).
77
The
inherent length of the crystalline portions of the cellulose
structure is linked to the leveling-o degree of polymerization
(LODP), corresponding to the DP at which the cellulose
structure become inaccessible for further degradation when
exposed to strong mineral acids or oxidizing agents at semidilute
concentrations.
78,79
Isolated native cellulose often gives crystallinity index values
between 60% and 90% when analyzed by X-ray diraction or
solid-state NMR spectroscopy. However, such values do not
directly indicate that, for example, 70% of the cellulose in the
microfibrils would be crystalline and the remaining 30% would
be disordered or “amorphous”. In comparison to the crystalline
regions, the disordered regions are reportedly very short:
accounts of 1−2 nm and 3−6 nm have been proposed,
73,80
i.e.,
they are more like defects in the crystallite rather than bulky
amorphous regions as they are often schematically depicted in
traditional literature. Much of the response of the disordered
cellulose in analytics likely comes from the microfibril surface
simply because the chains there have higher degrees of freedom.
As a result, the systematically higher crystallinity reported for
algae
50
or tunicates
51
comes largely from the fact that their
microfibrils, and therefore their crystallites, are wider than those
in higher plants (i.e., there are fewer surface cellulose chains).
1,81
Cellulose sample preparation prior to crystallinity measure-
ments can also aect the degree of crystallinity values obtained,
at least within a few percent range. Certainly, the frequency of
the disordered regions also plays a role here, but we believe that
the crystallite width is a more significant factor.
Furthermore, while nearly all studies assume that the
disordered cellulose regions are a part of natural microfibrils,
Figure 1. (a,b) Hydrogen bonding network in cellulose I and II. (a,b)
Reproduced from ref 59 under the terms of the CC-BY Creative
Commons Attribution 4.0 International license (CC-BY 4.0). Copy-
right 2021 Springer Nature. (c−e) Dierent chain models for cellulose
(c) the much debated 6 ×6 chain model. (c) Adapted with permission
from ref 60. Copyright 2010 American Chemical Society. (d) 6 ×4
chain model. (d) Reproduced under the terms of PNAS exclusive
License to Publish.
61
(e) 18-chain model (34443 form). (e)
Reproduced from ref 62 under the terms of the CC-BY. Copyright
2018 Springer Nature.
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some reports have suggested that these regions of disorder are in
fact a result of the processes involved in cellulose isolation. For
example, Atalla et al. put forward that native celluloses are
irreversibly transformed and develop the semicrystalline
character upon isolation at elevated temperatures.
82
These
issues are still under debate. Nonetheless, the semicrystalline
nature of cellulose is a fact of processed cellulosic materials, such
as virtually all nanocellulose types, and it adds to their
complexity as areas of high order are less susceptible to chemical
and biological attack.
83
The role of disordered regions in water-induced swelling of
cellulosic materials is not straightforward. There is a common
consensus that crystalline cellulose is impenetrable by water.
84
However, the disordered regions may be somewhat “accessible”,
but they are certainly not “swollen” by water because of their
short, defect-like texture. It has never been shown that the length
of a microfibril would increase when immersed in water.
However, small angle neutron scattering data have shown that a
somewhat higher concentration of D2O molecules can be
observed in the vicinity of the disordered segments.
85
Another often misunderstood fundamental issue with
cellulose−water interactions is hydrogen bonding. Despite the
undisputable importance of hydrogen bonding between water
molecules and nanocellulose at the interface in governing the
characteristics and properties of cellulose dispersions, we would
like to echo the conclusions of Wohlert et al. in their critical
review on the general role of hydrogen bonding in nanocellulose
structure and properties as a material. They conclude that
hydrogen bonding is one interaction among several, and its
relative contribution to the nanocellulose properties is highly
dependent on the specific conditions and cannot easily be
determined by intuition or even by analysis.
57,59
In any case, the combination of the insolubility of cellulose in
water with its ability to interact with water are fundamentally
important to sustaining its structural integrity. Nature has
utilized this feature of cellulose to engineer the incredible
structure that is the plant cell wall. The plant cell wall is made up
of three distinct regions: (i) the primary cell wall, (ii) the
secondary cell wall (made up of three layers), and (iii) the
middle lamella. Within the primary and secondary plant cell
walls, cellulose microfibrils are further assembled into microfibril
bundles and form a composite network structure with the two
other main plant cell wall components, namely hemicellulose
and lignin (Figure 2).
86−88
Within the plant cell wall, the microfibrils (and their bundles)
are orientated depending on which layer of the cell wall they
exist in (Figure 2). In the primary cell wall, the microfibrils form
very thin oriented layers with dierent orientations to one
another, forming the impression of a random network.
89,90
On
the other hand, the highly aligned microfibrils have a unique
microfibril angle in each of the three layers of the secondary cell
wall. This level of natural hierarchical engineering provides
plants with the necessary flexibility required for growth and
swelling in the presence of water while ensuring the plant has
sucient axial stiness.
91,92
When discussing the swelling of
cellulosic materials in water, one must remember that it is never
the individual cellulose microfibril that swells, it is always a
scaold where bulk water clusters between the microfibrils. As a
result of axial microfibril orientation in the secondary wall, the
plant fibers always swell copiously in a radial direction, but their
length of increase when immersed in water is negligible.
93
Plant fibers are always swollen by water as the major
component in their native growth environment, which imparts
the plants with necessary flexibility. It was earlier proposed that
water is held in a microporous gel of hemicelluloses and lignin
distributed as fine platelets within a cellulose skeleton.
94
Yet the
water content is strictly controlled by the presence of more
hydrophobic lignin in the cell wall. Conventional pulping
process including beating and bleaching (delignification) is
often undertaken when chemically processing plant fibers to, for
example, pulp for paper production. Rheological properties of
fiber−water suspensions where water acts as a suspension matrix
are of critical importance in many of papermaking process from
beating, screening, fractionation, dispersion flow in headbox,
sheet forming, and dewatering.
95
In general, delignified fibers
make up a strong network because the fibers are able to form
Figure 2. Schematic representation of the plant cell wall and cellulose fiber structure. CNFs and CNCs are extracted from cellulose fibers using
mechanical process and chemical methods (oxidation or acid hydrolysis), respectively.
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inter- and intrahydrogen bonds with each other due to cellulose
surface exposure after lignin removal.
93
Two general water
transport mechanisms including diusion and capillary flow
were suggested in cellulosic materials.
96
If the fiber network is
exposed to water, however, the fibers and the network lose their
integrity and mechanical strength because lignin is no longer
there to obstruct the water adsorption to cellulose surface due to
pore flow accompanied by a surface “hopping” mechanism.
96,97
A delignified fiber is also prone to irreversible loss of porosity
due to drying. Pores between the cellulose microfibrils are filled
with water in a swollen state and the pores disappear due to
capillary forces as the water is removed. When the fiber is re-
exposed to water, the pores reappear but not to the same extent
as before drying. This decrease in the swelling capability of a
fiber is referred to as hornification, and it has genuine practical
implications not only in papermaking and paper recycling but
also in nanocellulose production.
98−103
Hornification is often
(usually without explicit evidence) attributed to “irreversible
hydrogen bond formation between neighboring microfibrils
upon drying”. To our perception, a more likely reason could be
hydrophobic interactions where the hydrophobic sites of the
cellulose crystals in microfibrils partially aggregate. Such
hydrophobic bonding is less likely to be cleaved by water
upon re-exposure than hydrogen bonding. We must acknowl-
edge, however, that several accounts refer to co-crystallization or
association of hydrophilic sites as a culprit for hornification, with
obvious involvement of (also) hydrogen bonding between the
microfibrils.
104,105
The issue remains unsettled within the
community. Hornification or similar phenomena with micro-
fibril aggregation have even been reported to occur in air-dried
cotton fibers upon exposure to HCl vapor,
106
a preliminary
phase in one type of nanocellulose isolation procedure.
107
Moreover, the loss in swelling capability is not restricted to fully
delignified samples and it has also been observed to an extent for
lignin-containing samples such as wood and mechanical
pulp.
108,109
The situation is further complicated by the fact
that hornification is known to be at least partially reversible with
introduction of mechanical force in the system, a process
referred to as beating in the papermaking sciences.
110
In
addition, the response of the dimensional behavior of paper to
relative humidity changes is a reflection of individual cellulose
fiber changes in macroscale manner.
111
The torsional response,
i.e., twist, in drying of a collapsed fiber is a function of the
microfibril angle, the fiber length, and the factional linear
shrinkage across the microfibrils, which is closely related to the
nanoscale twist of nanocellulose (will be explicitly discussed in
section 2.3 and section 4.1). All in all, drying of wood and other
native specimen is a more complex aair with a series of
structural rearrangements taking place, involving physical
deformations such as bending, buckling, or twisting of the
fibrous cellulose bundles.
112−115
2.3. From Cellulose to Nanocellulose
2.3.1. Types of Nanocellulose. Nanocellulose refers to
cellulosic materials which have at least one dimension in the
nanoscale. A vast majority of nanocellulose consists of
anisotropic nanoparticles with varying aspect ratios although
spherical nanocelluloses have also been reported.
116,117
Nano-
cellulose, namely cellulose nanofibrils (CNFs), cellulose nano-
crystals (CNCs), and bacterial cellulose (BC), have garnered a
vast amount of research attention due to their incredible
versatility. Their high aspect ratios, high surface area to volume
ratios, abundance of surface hydroxy groups, and high strength
enable them to be used in a wide variety of potential
applications.
118,119
While all nanocellulose grades exhibit the
aforementioned properties, CNFs and BC are distinctly dierent
materials to CNCs, as attested visually in Figure 3.
120,121
CNFs are essentially isolated cellulose microfibrils (Figure 2
and Figure 3a). In consequence, they are flexible and
semicrystalline threads, with diameters in the nanoscale (average
width of 2−50 nm) but lengths in the micrometer range (1−15
μm).
51,102
A series of chemical (e.g., a 2,2,6,6-tetramethylpiper-
idin-1-yl)oxyl (TEMPO)-mediated oxidation
125
) and/or enzy-
matic pretreatments (e.g., cellulases),
126
followed by substantial
mechanical defibrillation,
127
are necessary to liberate CNFs
from their biological matrix, i.e., the plant fiber, given the strong
interactions between cellulose microfibrils and the tightly knit
hierarchical structure of the cell wall.
128
During the mechanical
defibrillation, high shear forces are applied to isolate single
CNFs which have dimensions dependent on both the isolation
method and the source of the cellulose. The surface charge of
Figure 3. Typical appearance of nanocellulose observed by TEM (a,b)
and SEM (c). (a) CNF
122
and (b) CNC.
123
(a) Adapted with
permission from ref 122. Copyright 1998 John Wiley and Sons. (b)
Adapted with permission from ref 123. Copyright 1991 The Royal
Society of Chemistry. (c) SEM image of BC membrane.
124
(c) Adapted
with permission from ref 124. Copyright 2008 Elsevier.
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F
CNFs depends on the amount of residual hemicellulose, namely
xylan with methylglucuronic acid groups, and the choice of
pretreatment. Specifically, the CNFs prepared by TEMPO-
mediated oxidation pretreatment (TOCNF) carry very high
charge densities, with approximately every second anhydroglu-
cose unit on the surface bearing a carboxylic group when the
maximum degree of oxidation is applied. Full details on CNF
isolation can be found in the relevant reviews.
102,129
TEMPO-
mediated oxidation is also the only pretreatment method that
manages to truly individualize microfibrils into CNFs. With
other isolation methods, the CNFs always consist of (at least
partially) microfibrillar bundles.
129
CNCs (sometimes called nanowhiskers) on the other hand,
are highly crystalline, rod-shaped nanoparticles, with widths
between 3 and 50 nm and lengths ranging from 50 nm to 10 μm
(Figure 3b) depending on the cellulose source.
130
The most
common source for CNCs is cotton because of the wide
availability and relative purity of ordinary laboratory filter paper;
the typical widths of cotton CNCs range from 5 to 20 nm and
lengths from 50 to 300 nm (with an average at ∼120 nm).
Similar to CNFs, CNCs are also prepared using a top-down
approach. By contrast, however, CNCs are typically isolated
using an acid-catalyzed hydrolysis, which selectively degrades
the more reactive disordered regions of the cellulose microfibril
and leaves intact the crystalline regions.
79,131−133
The length (or
DP) of CNCs is largely governed by the LODP of their source
material (see section 2.2.2 for a description of LODP and Figure
2). The conventional method to isolate CNCs is to perform a
hydrolysis reaction in the presence of concentrated sulfuric acid
at elevated temperatures, which in addition to hydrolyzing the
disordered regions introduces charged sulfate half ester groups
on the CNC surface, thus imparting colloidal stability.
134−137
Recently, many alternative isolation methods for CNCs have
surfaced, based on, e.g., oxidation and esterification, but these
approaches often require the additional presence of a strong
mineral acid to perform the hydrolysis of disordered regions.
138
A critical factor is that CNCs require some type of a surface
charge group to come up with a stable colloidal dispersion in
water. While sulfated CNCs are still overwhelmingly the most
produced and studied, CNCs stabilized by phosphate
139,140
and
carboxylate groups
107,133,141,142
have gained ground in recent
years. For comprehensive accounts of CNC preparation, the
interested reader is referred to recent reviews on the
topic.
138,143,144
One of the distinguishing characteristics of
CNC dispersions is that they spontaneously form (lyotropic)
chiral nematic liquid crystals,
145
a feature that has spawned a
sizable branch of CNC research over the past 30 years.
146−148
The current trend in the literature is to regard BC as the third
member of the nanocellulose family because it is produced by
various bacterial species from low molecular weight sugars using
a bottom-up approach (Figure 3c). BC is directly synthesized as
a hydrated nanofiber network, i.e., a hydrogel,
149,150
and unlike
CNF and CNC preparation, it requires no isolation steps apart
from washing away the bacteria and growth culture medium
with mild alkali. Although the bottom-up preparation route for
BC is distinctly dierent from the other nanocellulose types, BC
is essentially a form of CNF as it consists of semicrystalline,
flexible threads. In contrast to plant-based CNFs, BC nanofibers
are like flat ribbons, that is, their cross-sectional dimensions are
rectangular: ca. 7 nm high and 20−140 nm wide.
119
After rinsing
o the bacteria, BC is also the purest form of nanocellulose
without remnants of hemicellulose or lignin and virtually
without any charge. The chemical purity is among the reasons
why BC has been popular, particularly in biomedical
applications.
151
Both CNCs and CNFs show a longitudinal twist with a right-
handed chirality, which ultimately originates from the crystalline
structure of cellulose.
152,153
Molecular dynamics (MD)
simulations have established the twist in molecular cellulose
154
as well as in cellulose I crystal.
155,156
Combining computational
and experimental data, Conley et al. quantified a twist of 800 nm
period for wood-based CNC.
154
As the CNCs from typical
sources like wood, cotton, or ramie have lengths spanning 50−
300 nm, the long periodicity in the twist may be the reason as to
why the twist is rarely visually evident in microscopy images of
common CNCs. Despite this, the twisting of the cellulose crystal
ultimately causes the formation of chiral nematic liquid crystals
in CNC dispersions.
A noteworthy physical distinction in aqueous dispersions of
CNFs and CNCs is that CNFs form gels at very low
concentrations (<1 wt %), whereas CNCs are fluid dispersions
of fairly low viscosity.
9
CNCs do gel, but usually the gelling point
is above the critical concentration for liquid crystal formation
(ca. 10 wt %).
157
These dierences in water binding capacity are
a partial reason for the dierent approaches in applications-
related research concerning CNFs and CNCs. CNFs are often
applied as scaold structures in hydrogels, or as entangled
networks, whereas CNCs are utilized when, for example,
discrete particles are needed or sophisticated chemical
modifications are deployed for self-assembly or responsive
materials.
2,27
This distinction in applications is by no means a
strict one, but it has set an underlying trend for research over the
past 15 years.
2.3.2. Role of Water in Nanocellulose Production.
There is no doubt that water plays an important role in the
production of nanocellulose, particularly in the hydrolysis
reaction utilized to isolate CNCs (discussed in detail in section
2.3.2.2).
158
However, even when not directly involved in
chemical reactions of nanocellulose production, the presence
of water is pivotal. Figure 4 summarizes the role of water in
nanocellulose preparation.
2.3.2.1. Cellulose Hydrolysis. Hydrolysis is a chemical
reaction of a substance with water, leading to the decomposition
of both the substance and water.
159
The isolation of CNCs
typically occurs through a controlled hydrolysis reaction in
which the β-1,4-glycosidic bonds in the disordered regions of the
microfibril are cleaved by the addition of a water molecule
(Figure 4a). Therefore, all CNC production routes rely on the
presence of water as a reagent. Ultimately, as a result of
hydrolysis of cellulose, glucose is released in a process called
saccharification. However, under ambient conditions, this is a
very slow reaction, and it is not applicable for practical
purposes.
160
The hydrolysis of cellulose can be expedited via
dierent catalysts such as acids and bases, enzymes, or by using
subcritical and supercritical water as a reaction medium. While
the hydrolysis in subcritical and near critical conditions of water
has been rarely used for the isolation of CNFs,
161,162
it has been
more actively demonstrated in the production of CNCs. For
example, the use of subcritical water (120 °C and 20.3 MPa for
60 min) allows higher diusion, activity, and ionization of water
but leads to relatively low yields of CNCs (around 20%).
163,164
Also, in a recent study, supercritical carbon dioxide and enzymes
were studied to hydrolyze the disordered regions of the cellulose
microfibrils to produce CNCs.
165
As mentioned earlier, the most
common method to isolate CNCs is through an acid catalyzed
hydrolysis using concentrated aqueous mineral acids such as
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sulfuric acid, phosphoric acid, and hydrochloric acid to yield
charged or uncharged CNCs (such as the case for HCl
hydrolyzed CNC). Recently, Paakkonen et al. reported the
production of carboxylated CNCs via HCl gas hydrolysis,
followed by a TEMPO-mediated oxidation, leading to lower
acid consumption and more eortless process steps toward
easier CNC purifications.
79,107
The use of HCl gas to produce
CNCs from cellulose fibers relies on the water naturally present
on the cellulose surface: water is able to dissociate the HCl and
carry out the acid hydrolysis.
79,107
In addition, solid acid
catalysts (such as carbon catalysts with weakly acidic groups,
polymer-based acids, magnetic solid acids, and lignin-based
catalysts) have been used.
166−171
Bronsted acid active sites on
the solid catalysts oer advantages such as selective cellulose
hydrolysis, long catalyst lifetime, reusability, reduction of acid
pollutants, and reduction of the cost of wastewater treatment.
However, the contact between the active sites and cellulose
remains a challenge because both reactant and catalyst are
present in solid phase. The role of water as the reaction medium
is crucial to improve the accessibility of catalyst to cellulose.
Additionally, water can act as a catalyst for a autohydrolysis
process, as hydronium ions (H3O+) formed on the surface of
catalyst further promote cellulose hydrolysis.
172−174
Despite the
multiple reports on gas or solid based methods to hydrolyze
cellulose into nanocellulose, these processes remain to be less
common in comparison to liquid acid hydrolysis.
In acid catalyzed hydrolysis, the reaction mechanism involves
the activation of the glycosidic oxygen by protonation before
water addition,
175
and the rate of hydrolysis is dependent on
both the acid concentration (i.e., fraction of water) and the
temperature. At very high acid concentrations and temperatures,
cellulose undergoes complete degradation into singular
sugars.
176
Therefore, for the purposes of implementing acid
catalyzed hydrolysis in CNC production, reaction conditions
must be strictly controlled.
177
Both the temperature and acid
concentration must be suciently high to cleave the glycosidic
bonds in the disordered regions of the microfibril in a timely
manner but low enough to keep the crystalline regions intact
(Figure 3b). In the conventional sulfuric acid hydrolysis to
produce CNCs, the concentration is generally set to 64−65 wt
%, which is fairly close to the value where total hydrolysis/
dissolution of crystalline cellulose occurs (72 wt %). Here, the
water content is very low because of the formation of oxonium
ions, and this is integral for enabling the esterification of sulfate
groups which must take place in order to ensure the necessary
colloidal stability of CNCs. Furthermore, it is important to note
that during the acid hydrolysis of cellulose there exists a
competition between the dehydration of cellulose (i.e., cleaving
of the glycosidic bonds) and the dissolution of lower DP sugars
resulting from the hydrolysis.
79,178−180
While the use of enzymes has been reported for the isolation
of CNCs, they are typically implemented in the pretreatment of
cellulosic substrates prior to high shear mechanical treat-
ments.
181
Hydrolysis of the cellulosic substrates through
enzymatic hydrolysis significantly decreases the amount of
energy required to mechanically isolate individual cellulose
microfibrils from cellulose fibers. In nature, the degradation of
cellulose is accelerated by more than 17 orders of magnitude by
cellulases, which include a variety of enzymes called glycoside
hydrolases (or glycosidases) that catalyze the hydrolysis of the β-
1,4-glycosidic bonds of cellulose.
182
Enzymatic hydrolysis
involves complex interactions between enzyme, cellulose, and
the reaction environment, and although the complete
mechanism of action of the above-mentioned enzymes is still
unknown
183
it has been shown that enzyme folds and crevices
are formed in water into which the substrates fit.
184
Dierent
cellulases will catalyze the hydrolysis at dierent locations along
the cellulose polymer chain, and all of these enzymes act
synergistically in order to fully degrade cellulose to glucose for
the production of biofuels, for example.
185
In the production of
Figure 4. Schematic summary of the role of water in nanocellulose
production. Water serves as (a) reagent
198
and (b) swelling agent,
199
(c) an essential medium for tertiary structures of enzymes
200
used for
nanocellulose production, and (d) medium
195
in the hydrolysis
reaction, (a) Adapted with permission from ref 198. Copyright 2011
John Wiley and Sons. (b) Adapted from ref 199 under the terms of CC-
BY. Copyright 2021 Elsevier. (c) Adapted from ref 200 under the terms
of CC-BY 4.0. Copyright 2021 Springer Nature. (d) Adapted from ref
195 under the terms of CC-BY. Copyright 2020 American Chemical
Society.
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H
CNFs, however, the enzymatic activity of cellulases is carefully
controlled to minimize undesired cellulose degradation.
Furthermore, the isolation of CNFs is often aided by the
introduction of enzymes such as xylanases, laccases, and lytic
polysaccharide monooxygenases, which have an anity for the
glycosidic bonds in other polysaccharides (hemicelluloses in
particular).
186−190
2.3.2.2. Water as a Medium in Nanocellulose Production.
Cellulose and cellulosic fibers are insoluble and relatively inert in
water under ambient conditions. Nonetheless, the hygroscopic
nature of cellulose enables the swelling of cellulose fibers as a
result of water sorption, which is key in the production of CNFs.
Water-induced swelling of cellulosic fibers “opens” their
structure (Figure 4b), increasing their accessibility, and in turn
facilitates the penetration of chemical reagents and activity of
enzymes (Figure 4c) during the pretreatment stages of CNF
production. Additionally, these pretreatment methods often rely
heavily on the water present to act as the reaction medium (e.g.,
in the case of TEMPO-mediated (Figure 4d) or enzymatic
oxidation).
191
Pretreatment methods such as swelling or partial
dissolution in ionic liquids or deep eutectic solvents (DES) are
also heavily dependent on water, further highlighting its
importance in the many possible routes available for nano-
cellulose production.
192−195
It is important to note that even in
production methods where water does not play an active role in
the breakdown of the cellulose structure to the nanoscale (e.g.,
nanocellulose production through oxidation using an electron
beam), water is omnipresent in the purification, workup (e.g.,
alkaline treatment, sonication, and high-pressure homogeniza-
tion), and often storage stages of nanocellulose produc-
tion.
195−197
2.4. Nanocellulose−Water Systems: Properties and
Dynamics
Before introducing the interactions between water and nano-
cellulose specifically, it is important to highlight the distinction
between the swelling of cellulosic (plant-based) fibers and
nanocellulose networks in water. In nature, the geometrical
constraints of the cell wall set by their hierarchical structure,
especially with rigorous microfibril alignment in the secondary
wall, greatly restrict the swelling capacity of fibers. As already
stated, swelling in water is highly anisotropic for fibers, as the
volume increase occurs solely in the lateral dimension. The
isotropic nature of most nanocellulose networks, together with
the significant increase in specific surface area (SSA), leads to a
much higher water sorption capacity per mass of cellulose. In
conjunction with this increase in SSA comes an increase in the
accessibility of surface hydroxy groups which can easily interact
with water. Furthermore, any charged groups introduced to the
surface of the nanocellulose during isolation will also alter its
ability to interact with water. Phenomenologically, the nano-
cellulose−water interaction involves multiple overlapping
phenomena such as hydration, condensation, wetting, and
diusion, which are all mediated by various interaction forces
including hydrogen bonding, electrostatic interactions, and van
der Waals forces. These processes unfold along dierent length
scales, which ultimately gives nanocellulose its extraordinary
hygroscopic character.
2.4.1. Water and Nanocellulose Interactions at the
Molecular and Supramolecular Level. 2.4.1.1. Accessibility
of Cellulose Chains to Water. The accessibility of cellulose
chains to water is governed by the availability of hydroxy groups
on the surface of a cellulose crystal. While cellulose−cellulose
hydrogen bonds often take precedence over cellulose−water
hydrogen bonds (hence the insolubility of cellulose in water),
there is also an abundance of hydroxy groups on the crystallite
surface that have the propensity to hydrogen bond with
water.
201
A common method by which to quantify the
availability of hydroxy groups in cellulosic materials is to
substitute the hydrogen in available hydroxy groups for
deuterium through a water/deuterium oxide (H−D) solvent
exchange.
202
The availability of hydroxy groups for solvent
exchange is dependent on a number of factors: (i) their position
in the cellulose chain (i.e., 2-, 3-, or 6-position), (ii) whether they
are within the ordered or disordered region of the cellulose
microfibril, and (iii) whether they are located on the surface or
embedded within the crystallite (microfibril). The hydrogen
atoms in the HO(2) and HO(6) hydroxy groups can act as
hydrogen-bond donors to water, but the HO(3) behaves as a
hydrogen-bond acceptor from water and donor to their
intrachain neighbors O(5) (see, Figure 1). The accessibility of
these hydroxy groups on the surface of cellulose crystals
correlates with the H−D exchange behavior. For a specific
hydroxy group to be available for the H−D exchange, it must be
able to donate a hydrogen atom via hydrogen bond to a water
molecule.
203
For this reason, the HO(3) does not participate in
the H−D exchange.
203
Indeed, the HO(2) and HO(6) are more
prone to moisture absorption, while the HO(3) has a lower
accessibility, given the fact that the intramolecular hydrogen
bond with O(5) has a predominant role in stabilizing the
cellulose structure.
204
In addition to the molecular position of
each hydroxy group, the crystalline structure of cellulose plays a
crucial role in the cellulose−water interactions (and therefore
−OH group accessibility). On the crystallite itself, water
accessibility is also based on the geometrical requirements of
the available hydroxy groups of the cellulose that come into
contact with water molecules.
155
The degree of crystallinity of the cellulose substrate plays a
role in extreme cases: the accessibility of hydroxy groups in
amorphous cellulose (as measured through deuteration using
dynamic vapor sorption (DVS)) is estimated to be 63%, whereas
the equivalent accessibility for microcrystalline cellulose is 51%,
indicating that a higher crystallinity leads to lower −OH group
accessibility.
205
Similarly, multiple works in the literature
correlate the dierence in water uptake capacity of CNFs and
CNCs to the dierence in their degree of crystallinity and
(supposedly) consequent hydroxy group accessibility.
206,207
As
mentioned earlier, CNFs are isolated cellulose microfibrils
which exhibit the semicrystalline structure, whereas CNCs
represent the crystalline portion of the cellulose microfibril. As
such, it has been speculated that the more frequent presence of
the disordered region leads to a higher water uptake capacity in
CNFs than CNCs.
208,209
Yet there is no evidence of increased
swelling due to an increased number of disordered regions in
microfibrils, although (as already mentioned) data published by
Nishiyama et al. showed the hydrogen atoms in the hydroxy
groups of the disordered domains were in fact susceptible to
deuteration.
73
It is also evident that water interacts solely with
the surface of CNCs and is unable to penetrate (at least to any
significant degree) into the crystal structure,
208,210
and
deuteration is indicative of the number of available surface
hydroxy groups
206,211
unless substantial temperature and
pressure is applied.
212
Instead of the disputed role of the crystallinity index, the
hydroxy group availability in native cellulosic structures is rather
governed by the size of the cellulose crystallite, as shown by
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Driemeier and Bragatto in their seminal work on microcrystal-
line cellulose with varying crystallite widths.
213
Also, the amount
of residual hemicellulose plays a marked role in general water
uptake and must not be confused with hydroxy group
accessibility in cellulose per se.
213
Particularly with CNFs,
hemicellulose is often intentionally left in the structure because
it facilitates the separation of microfibrils from one another.
214
Another factor in accessibility with water is the imperfect
packing of aggregated crystallites that may allow a concentration
of water molecules in voids created by the phenomenon.
213
Therefore, when examining the dierences in hydroxy group
availability and subsequent water uptake capacity, it is important
to also consider dierences in morphology, chemical composi-
tion, and flexibility. We will discuss the dierence in water
uptake capacity of assemblies of dierent nanocelluloses in
section 2.4.1.2.
The accessibility of cellulose to water may also be governed by
its polymorphism. Some studies have indicated that nano-
celluloses of cellulose II have higher water sorption capacity than
those with a cellulose I crystal structure due to the possible
changes in the crystallite dimensions and the decrease in overall
crystallinity as a result of the mercerization process used to
convert cellulose I to cellulose II.
215,216
Therefore, unlike
cellulose I that has small disordered regions that cannot
generally be regarded as fully “amorphous”, cellulose II has
been speculated to exhibit genuine crystalline−amorphous
transition, akin to many synthetic polymers.
217
Water uptake is also substantially influenced by the number of
charged groups in the cellulose matrix because they directly
contribute to the osmotic pressure.
218
Intrinsically linked to the
osmotic pressure, the induced electrostatic forces direct the
water molecules and determine much of the texture of the
nanocellulose matrix, as governed by the Derjaguin−Landau−
Verwey−Overbeek (DLVO) theory
219
The counterion on the
charged group also has a pronounced impact. For example, the
sulfate half ester group’s counterion on CNCs directly aects the
critical coagulation concentration following the Hofmeister
series (N(CH3)4+ < NH4+ < Cs+< Rb+< K+< Na+< Li+), which
also correlates with the interactions they have with water
molecules.
220−222
2.4.1.2. Wettability of Cellulose. The mundane definition of
hydrophilic/hydrophobic character of a surface is to have the
static water contact angle below or above 90°, respectively.
Striving for higher thermodynamic accuracy, alternative takes on
the issue have used Gibbs free energy of hydration (ΔGsl): van
Oss, for example, proposed that hydrophobic compounds attract
each other in water when their ΔGsl >−113 mJ m−2and repel
each other when ΔGsl <−113 mJ m−2.
223
Moreover, factors
such as surface roughness and morphology, porosity, and fouling
all play an important role in the spreading of a liquid at the air/
solid interface.
224
In addition to, and intrinsically linked to, the hydroxy group
availability, native cellulose crystals are amphiphilic (as
demonstrated in the cross-sectional schemes of the microfibril
in Figure 1).
225−228
This amphiphilicity is present at molecular
and supramolecular level of cellulose,
229−231
and it is governed
by the structural and conformational order in addition to the
roughness, purity, and porosity of the assembled nanocellulose
structures. Cellulose can also undergo conformational changes
to accommodate the surrounding medium.
231,232
The dis-
tinction between hydrophilic and hydrophobic faces in the
cellulose I crystallite is straightforward (Figure 1), but their
behaviors are not. For example, molecular dynamic (MD)
simulations have shown that the hydrophilic 110 face, which is
the most represented in the external morphology of the native
fibers, possesses a water contact angle of 43°, while the other
hydrophobic 100 face shows a contact angle of 95°.
233
As a side note, the noncellulosic materials adsorbed on the
surface of cellulose can change the surface characteristics. Dried
CNFs have been reported to accumulate a layer of noncellulosic
material on the surface, which renders them less reactive than
one might expect of a material rich in hydroxy groups.
232
As a
conclusion, it is fair to define nanocellulose as a family of
cellulosic materials with amphiphilic nature, whereby the free
energy of hydration depends on morphological factors on all
length scales.
Materials with similar surface energies are inherently
compatible, suggesting that understanding the surface energies
of both water and nanocelluloses can give us a rough picture of
their potential interactions. Surface energy can be divided into a
dispersive (i.e., hydrophobic) and a polar (i.e., hydrophilic)
component, the former describing the ability of a surface to
participate in long-range London type nonpolar interactions and
the latter in short-range “polar” interactions. In the case of
Table 1. Values for the Dispersive and Polar Surface Energies (γ) of Various Cellulosic Materials
a
cellulosic material γD (mN m−1)γp (mN m−1)γT (mN m−1) method of quantification
b
ref
hardwood α-cellulose 31.9 iGC-SEA 237
hardwood α-cellulose extracted with acetone 47.4 iGC-SEA 237
Avicel MCC 31.8 23.9 55.7 CA 238
Avicel MCC 51.8 0 51.8 TLC 239
Sigmacell 20 52.9 4.2 57.2 TLC 239
Whatman paper 32.1 20.2 52.3 CA 238
Technocel fibers 20 CA 240
amorphous cellulose beads 70.5 iGC-SEA 241
TEMPO-oxidized CNF 42−46 iGC-SEA 242
enzymatic CNF 51.5 iGC-SEA 242
cellulose II, critical CO2dried 49.6 6.1 55.8 iGC-SEA 243
cellulose II, freeze-dried from t-BuOH 52.3 6.9 59.1 iGC-SEA 243
bacterial cellulose 47.2−58.3 iGC-SEA 244
amorphous cellulose ca. 35 ca. 17 ca. 52 CA 245
a
γDrefers to the dispersive component, γPto the polar component, and the γTto the total surface energy.
b
Abbreviations for method of surface
energy quantification: inverse gas chromatography−surface energy analyzer (iGC-SEA); thin layer chromatography (TLC); contact angle
measurements (CA).
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liquids interacting with surfaces of diering surface energies, one
can discuss their interfacial compatibility in terms of wettability
(or the ability of water to spread over a surface). A surface can be
considered wettable when its θwith a liquid is between 0°and
90°and not wettable at θabove 90°.
234
We will discuss the water
sorption capacity (or wettability) of nanocellulose assemblies in
sections 2.4.2.4,4.1.1, and 4.2.2. Simply put, the wettability and
the water retention of nanocellulose assemblies is highly
dependent on their overall crystallinity, surface chemistry (i.e.,
charged, uncharged, or chemically modified), and purity of the
nanocellulose along with the roughness, morphology, and
porosity of its assemblies.
235
In section 2.4.1.1, we described the ambiguous role of
crystallinity in the accessibility of hydroxy groups in cellulosic
materials. One can imagine that on the nanoscale the wettability
of crystalline cellulose with water is also dependent on its crystal
structure and the subsequent degree of amphiphilicity.
However, a comparison of the surface energies of nanocelluloses
extracted from several plants did not reveal a significant
dierence (regardless of variability in crystallinity), most likely
due to the analogous surface chemistries as a result of the
similarity between the biological processes of cellulose
production in nature.
236
To provide the reader with an idea of
the order of magnitude of the surface energy of cellulose, a few
reported values are shown in Table 1.
In general, the total surface energy of cellulose is between 50
and 60 mN m−1although the contribution of the dispersive and
polar components can be significantly dierent, depending on
the type of the cellulose in question.
It is evident that altering the surface chemistry of cellulose will
have a significant impact on its interactions with water. The
surface energies of nanocelluloses are highly tunable through a
multitude of surface modification routes,
3,118,119,246−252
which
can render the material more hydrophilic or less hydrophilic.
These modification pathways will be discussed in detail in
section 2.1.
It should also be noted that nanocellulose structures can
readily swell upon exposure to water. However, the dynamic
changes caused by the wetting and consecutive drying of these
structures may significantly alter the perceived surface energies
of the nanocellulose surfaces.
232,245,253,254
For example, CNFs
and CNCs dried through freeze-drying tend to have a lower
surface energy than those dried by air-drying, spray-drying or
supercritical-drying, which is linked to their dierent state of
aggregation.
255
2.4.1.3. Water Insolubility and the Eect of Water on
Cellulose Dissolution. Cellulose is insoluble in water and other
conventional solvents, inorganic and organic alike. However,
several solvent systems for cellulose include water as a central
component. N-Methylmorpholine N-oxide (NMMO)/water,
NaOH/water, and urea/NaOH/water are among the most
commonly used water-containing cellulose solvents. Unusually
rapid dissolution of cellulose has been reported in urea/LiOH/
water and urea/NaOH/water,
256
albeit at temperatures below 0
°C (Figure 5). The role of water in cellulose dissolution diers a
great deal depending on the solvent. In NMMO, for example, a
rather precise water content of 13.7 wt % is necessary to establish
the monohydrate form of NMMO, necessary for dissolving
cellulose. In many recipes for dissolving cellulose, water plays a
role in swelling and improving the accessibility of the chains to
the solvent. Chen et al. have studied the kinetics of the
dissolution and swelling of dierent cellulose fibers in the ionic
liquid 1-ethyl-3-methylimidazolium acetate ([EMIM][OAc])
and reported that the solvent power was modified from very
good (neat ionic liquid) to moderate (with 5 wt % water) and
weak (15 wt % water). They showed that while the rate of fiber
dissolution in neat ionic liquid depends on fiber accessibility and
solvent viscosity, the water-induced decrease in solvent power
dominates the general fiber behavior.
257
Furthermore, factors
such as cellulose DP, degree of crystallinity, morphology, surface
chemistry, degree of substitution, and the surface tension of the
solvent all play a pivotal role in the solubility of cellulose.
Because dissolution is not a prominent or often-utilized
phenomenon with nanocellulose (it obviously destroys the
nanoscale morphology), we shall not discuss it further. Several
reviews discussing the mechanism of cellulose dissolution in
various solvent systems are available in the literature.
258−263
2.4.2. Behavior and Dynamics of Water within Nano-
cellulose Matrices. 2.4.2.1. Network Formation and
Viscoelastic Behavior. Nanocellulose dispersions form arrested
Figure 5. Schematic representation of dispersion, swelling, and dissolution of cellulose in water.
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phases (i.e., a state of kinetic arrest), in the case of CNFs even at
very low solids contents. Dierent factors such as nanocellulose
aspect ratio and flexibility, surface charge density, counterion of
surface charge groups, along with ionic strength of the
dispersion, contribute to critical concentration at which this
phenomenon occurs.
130,264−266
The formation of an arrested
phase is driven by the decrease of nanocellulose mobility either
by increasing the suspension concentration or by reducing the
eects of electrostatic or steric repulsion between nanocellulose
fibrils/crystals.
265
In either case, once the nanocellulose
suspension reaches a critical concentration, it will go through
a transition from a dispersed liquid-like state to an arrested solid-
like phase. Depending on the dominant interparticle forces, two
kinds of ideal arrested phases exist. In a system dominated by
electrostatic Coulomb repulsion (more specifically from
electrostatic double-layer repulsion due to osmotic pressure),
a decrease in the interparticle distance is hindered by so-called
caging eects, which leads to the formation of a colloidal
glass.
265,267,268
When van der Waals attractive forces are the
dominant forces, the increase in concentration leads to the
formation of a gel characterized by a percolated network often
with a given fractal dimension.
265,267,268
In nanocellulose
suspensions, both repulsive and attractive forces play a role.
For dilute suspensions (i.e., volume fractions below 0.05 wt %),
concentrating the nanocellulose suspension will lead to the
formation of a mostly reversible colloidal glass with a threshold
concentration that is inversely proportional to the aspect ratio of
the nanocellulose. On the other hand, increasing the ionic
strength of a dilute nanocellulose suspension will result in a
screening of repulsive forces between nanocellulose fibrils/
crystals and the suspension will transition into an irreversible
gel.
265
At higher solids content, the dynamics involving the
formation of an arrested phase may be more complicated. For
example, the colloidal interactions of carboxylated CNF
suspensions with concentrations ranging between 0.5 and 4.9
wt % are dominated by electrostatic Coulomb repulsion in the
lower range of concentrations and by van der Waals attraction
forces in higher range.
121
The transition from a suspension to an arrested phase is quite
dierent between CNF and CNC suspensions. Due to their rod-
shaped structure and inflexibility, CNCs form a gel at relatively
high concentrations. The critical gelation concentration of CNC
suspensions is approximately 10 wt %. However, this is of course
dependent on their aspect ratio, purity, ionic strength, and
Figure 6. Schematic representation of wetting and hydration of nanocellulose. (a) Water and nanocellulose interactions from a supramolecular
hierarchical point of view.
26
(a) Adapted from ref 26 under the terms of CC_BY. Copyright 2020 John Wiley and Sons. (b) Wetting of cellulose
nanofibers, highlighting the dierent states of water (bulk, free, and bound water) within nanocellulose networks. (c) Wetting of cellulose nanocrystals
(in green) surrounded by adsorbed water (light blue)
271
(c) Adapted with permission from ref 271. Copyright 2015 American Chemical Society.
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surface charge density. At critical concentrations below their
gelation concentration, CNCs have a tendency to phase separate
into a chiral nematic (anisotropic) phase in which the alignment
of CNCs results in birefringence and an isotropic phase where
the CNCs remain in suspension and repulsive electrostatic
interactions dominate.
130
In the case of CNFs, the formation of
an anisotropic phase is hindered by the early onset of an arrested
phase promoted by their tendency to form entanglements as a
consequence of their higher aspect ratios and their flexibility.
Nevertheless, CNFs can form anisotropic nematic phases under
some conditions.
121,269
2.4.2.2. “Types of Water” within Nanocellulose Networks.
Water within a nanocellulose network exists either as free water,
which fills any voids due to capillary forces, and bound water,
which interacts with the cellulose at specific sorption sites. At
this stage, it is important to make a clear distinction in
terminology: any water that is taken up by a cellulosic matrix is
absorbed. However, absorbed water does not necessary interact
with the cellulose through molecular interactions. When an
interaction between the cellulose and water does occur, water is
said to adsorb on the cellulose. Another means by which to
distinguish the nature of water within a cellulose matrix, is that
bound (or adsorbed) water is water present at moisture contents
far below capillary condensation (i.e., the saturation point),
while free (or absorbed) water is the water present in the matrix
far above capillary condensation (i.e., close to saturation)
(Figure 6a).
270
With many routine experimental methods, such
as dynamic vapor sorption (DVS) or water retention value
(WRV), it is dicult, if not impossible, to distinguish between
absorbed and adsorbed water, and hence, the predominant term
for quantified water uptake is often “water sorption”.
Interestingly, the properties of water within a nanocellulose
network are highly dependent on whether the water is free
(absorbed) or bound (adsorbed) (Figure 6b) based on the
melting and freezing behavior of water within a nanocellulose
network (as measured through dierential scanning calorimetry
(DSC) and explained in detail in section 4.2.5), water can be
categorized as (i) free water, (ii) nonfreezing bound water, or
(iii) freezing bound water.
272,273
The thermal properties of free
water are the same as pure water, while bound water shows lower
transition temperatures due to its strong interaction with
cellulose surfaces (primarily hydroxy groups) mediated by
hydrogen bonding and the consequent restructuring of local
water environments and nanoconfinement.
270,274
Bulk water
surrounding nanocellulose is distinguishable from free water, as
it does not cause observable swelling in the cellulose matrix as in
the case of free water.
275
It is possible to detect whether water bound to cellulose is
freezing bound water (or surface bound water) or nonfreezing
bound water (or confined bound water) depending on its
mobility as measured through 2H NMR and 1H NMR.
270
This
classification is particularly interesting (explained in more detail
in section 4.2.4), as it allows one to directly connect the
properties of water with its distribution inside cellulosic matrices
such as fibers and nanocellulose networks. The characteristics of
these networks have a great eect on the physicochemical
properties of nanocellulose derived materials and on the
properties of sorbed water.
208
Another way to distinguish
dierent types of water is thermoporosimetry, also discussed in
detail in section 4.2.
CNFs form percolated fractal networks due to the formation
of arrested phases upon an increase in concentration during
which the 3D network of nanofibers shrink, decreasing the
packing space between CNFs and forming agglomerates.
121
Alternatively, phenomena such as coagulation, cross-linking, or
ion exchange can be used to promote the formation of CNF
networks. At the fiber saturation point, all of the intra-
agglomerate (CNF−CNF interface) and interagglomerate
(agglomerate−agglomerate interface) pores are fully hy-
drated.
208
At the interagglomerate level, the surface bound
water is located at the surface of the nanofiber belonging to two
dierent agglomerates, while at the intra-agglomerate level, the
confined bound water is present at the nanofibers interface
belonging to the same agglomerates.
270
Upon increasing
hydration, the surface bound water becomes gradually more
and more mobile due to the high accessibility between cellulose
agglomerates. In contrast, the confined bound water is only
marginally influenced by the hydration level due to the lower
accessibility within the agglomerates
270
Due to these dynamics,
the surface bound water also is sometimes called “movable or
mobile bound water”, while the confined bound water as “fixed
or immobile bound water”.
2.4.2.3. Water Sorption Dynamics. The dynamics of water
sorption by nanocelluloses are highly dependent on whether the
nanocellulose is exposed to water vapor or liquid water as the
sorption of liquid water is governed by hydrostatic pressure and
capillary forces, which are not present in the sorption of water
vapor. Generally speaking, the degree of water vapor sorption by
nanocellulose is significantly lower than its liquid counter-
part.
276−278
Nanocelluloses can uptake a significant amount of water due
to their extremely high surface area to volume ratio and their
abundance of accessible hydroxy groups. Furthermore, the
swelling of nanocellulose networks exposes even more surface,
resulting in a higher number of hydroxy groups available for
water sorption. Because they are the primary sites of interaction
for water on the nanocellulose surface, rate of sorption, and
desorption of water in nanocelluloses has been associated with
the accessibility of hydroxy groups.
215
The accessibility is
governed by factors such as surface charge content (and charge
counterion), degree of aggregation, geometric constraints,
porosity, crystallinity, and the thermal history of the nano-
cellulose (see also section 2.4.1.1).
A number of quartz crystal microbalance with dissipation
(QCM-D) and surface plasmon resonance spectroscopy (SPR)
studies have been carried out on CNC
271,279
and CNF
280−282
model films to understand the swelling of nanocellulose
networks. The kinetics of CNC film swelling as a function of
solvent ionic strength and CNC surface charge was evaluated in
an SPR study by Reid et al.
283,284
Interestingly, in this work, the
total water uptake capacity of the CNC films was independent of
both CNC surface charge and the ionic strength of the solvent,
seemingly due to the restrictions on swelling capacity resulting
from the presence of van der Waals forces. However, it was clear
that the rate of swelling was greatly impacted by surface charge
and ionic strength. Niinivaara et al. gained quantitatively similar
results in a combined QCM and ellipsometry study on CNC
thin films: apparently a 1 nm layer of water surrounds the CNCs
at high (>90%) relative humidity values (Figure 6c).
271
Similarly, other works have also demonstrated that the water
sorption capacity of (nano)celluloses generally increases with
increasing surface charge content, in addition to changes in
charge counterion, ionic strength, and pH.
134,285
Hakalahti et al. studied the water vapor absorption
mechanism and dynamics into model carboxylated CNF
films.
282
They showed that below 10% relative humidity (RH)
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water vapor is adsorbed mainly onto the surface of CNFs
through specific interaction (e.g., hydrogen bonding), whereas
at humidity values between 10% and 75%, multilayers of water
molecules were built up inside the CNF network following a
Flory−Huggins model. When the RH exceeded 75%, the water
vapor condensed in the CNF network via cluster formation,
promoting the swelling of the thin film. During sorption, the
water volume fraction increased from 0.21 at 75%RH to 0.59 at
97%RH.
282
The water vapor uptake capacity of CNFs is generally higher
than CNCs due to the flexibility of the network, higher hydroxy
group accessibility, and residual lignocellulosic components
such as hemicellulose. At 95%RH, CNFs have at least 10% more
moisture than CNCs. Because of their rigidity, CNCs do not
form entangled networks such as CNFs. Comparing celluloses I
and II, cellulose II has higher uptake due to the dierence in
crystallinity, texture, and overall morphology of the samples.
215
2.4.2.4. Mass Transport. The mass transport of liquid water
in porous cellulosic materials occurs as a result of capillary flow
(or wicking), which is often modeled using eq 1 as established
independently by both Richard Lucas
286
and Edward W.
Washburn,
287
which provides information on how liquids
move through porous media and can be used to characterize
the surface energies of powdered solids.
254,288,289
=x
r
t
cos
2
l2
(1)
where xis the distance traveled, γlis surface tension of the liquid
(i.e., water), ris the radius of the (circular) flow channel, θis the
contact angle (assumed static) between the solid surface and the
liquid (i.e., cellulose and water), ηis the dynamic viscosity of the
liquid (i.e., water), and tis time.
On the contrary, the mass transport of water vapor through
porous materials proceeds via diusion; for dense materials, a
solution-diusion mechanism is usually assumed.
290
The
diusion driven mass transport of water vapor through porous
materials is sensitive to changes in temperature and in relative
humidity (and thereby partial pressure).
291
As in the case of
wetting and sorption, material properties such as thickness,
292
crystallinity,
293
hydrophilicity,
294
along with porosity, and pore
size and structure,
295
all influence the rate of mass transport and
are all applicable to materials such as nanocelluloses.
As most assembled nanocellulosic structures are porous, the
relevant phenomena for water vapor transport through nano-
cellulose materials are Fickian diusion, Knudsen diusion, and
surface diusion. Fickian diusion refers to classic diusion
governed by local dierences in chemical potential, where the
interactions between the diusing molecule and the solid
material are insignificant (i.e., when the porosity is high and pore
size is large).
296
Knudsen diusion, on the other hand, occurs
when the pore size of the solid material is comparable to or
smaller than the mean free path of the diusing molecule,
leading to significant interactions between the vapor and the
solid.
297
Surface diusion, on the other hand, occurs when
molecules are mostly adsorbed on a surface, only to jump to the
next adsorption site.
290
All of these mechanisms may be found in
water vapor mass transport across cellulosic materials and
nanocelluloses, depending on their porosity, pore size, and
surface energy.
253
While some eorts have been made to model
the observed water vapor transport of various cellulosic materials
by these mechanisms, most of the literature on the uptake of
water vapor by cellulose does not oer a full theoretical
explanation to the experimentally observed behavior.
290,291,298
2.4.2.5. Dewatering of Nanocellulose. Recently, Sinquefield
et al. published an exhaustive review on the dewatering and
drying of nanocellulose.
299
Maintaining the nanoscale properties
of nanocelluloses upon dewatering/drying represents a distinct
challenge, given the structural changes and irreversible
aggregation which occur during the process. The most common
dewatering procedures used for nanocelluloses include
centrifugation,
300
pressing,
301,302
filtration,
303−306
shear stress
induced dewatering,
307−311
and solvent exchange followed by
solvent evaporation.
312,313
More recently, Guccini et al. have
used forward osmosis to reproducibly dewater CNFs
suspensions into hydrogels with a solid content up to 12 wt %.
Using this approach, they were able to retain the viscoelastic
properties of the CNFs upon redispersion.
314
After dewatering, a
further drying stage is often required to bring nanocellulose to
the dried state. The most common technologies used for drying
are air and oven drying,
126,315−317
freeze-drying,
318−325
super-
critical CO2drying,
21,315,326,327
and spray drying.
315,327−329
Each of these drying techniques results in nanocellulose
structures with dierent properties, such as thermal stability,
degree of crystallinity, and char residue (upon heating or
carbonization). For example, CNF films/aerogels prepared
through supercritical drying have a lower stability and degree of
crystallinity than those prepared by air drying or spray drying.
327
As such, it is important to note that the drying history of
nanocelluloses and nanocellulose-based materials must be
carefully taken into consideration, given the influence of the
drying method on their physicochemical properties.
Hornification is one of the main phenomena leading to the
changes in nanocellulose materials upon drying, presumably by
co-crystallization or irreversible binding of hydrophobic sites in
the microfibril (see also section 2.2.2). Compared to fibers,
hornification is exacerbated to a great deal in a nanocellulose
networks because of the immense surface area of CNFs or
CNCs. Nanocelluloses can be eciently dried at temperatures
below 100 °C under vacuum or by freeze-drying, but upon
exposure to ambient conditions, atmospheric moisture will
promptly be resorbed into the material.
330,331
In fact, in ambient
conditions, air-dried nanocelluloses contain between 2 and 5 wt
% residual moisture.
327
Hornification usually becomes domi-
nant already at higher water contents, however, leading to a
permanent decrease in hydroxy group accessibility. Additionally,
nanocelluloses with surface moieties such as carboxyl and
aldehyde groups may undergo chemical cross-linking under
these conditions, further enhancing the eects of horn-
ification.
332
As a result, the dehydrated nanocelluloses are
unable to be returned to their initial state upon rehydration.
The accessibility of the hydroxy groups in both redispersed
CNCs and CNFs is reported to be reduced by ca. 84% and 82%
upon drying, respectively. The structural collapse of nano-
cellulose during drying predominantly has negative eects on its
redispersibility,
21
chemical modification,
333
and swelling ability.
In the process of nanocellulose preparation, hornification of the
source material plays a role as well. For example, pulp fibers
subjected to hornification prior to TEMPO-mediated oxidation
(in the isolation of CNFs) require higher energy and consume
more chemicals in comparison to the never-dried pulp.
334
In
fibers, dierent strategies such as beating, addition of bulking
agents such as sucrose and glycerol, or derivatization with
spacers such as poly(ethylene glycol) (PEG) have been applied
to prevent hornification.
335
It is important to note that although
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the eects of hornification upon drying are usually significant,
some sources of nanocellulose, such as Cladophora, show
inherently lower extent of hornification.
50,336
Due to the
significant eect of hornification, it is common for nano-
celluloses to be stored in their “never-dried” form in order to
prevent any additional complications when utilizing these
materials.
21
Should nanocelluloses require drying, the eect of
hornification (and as such the potential to redisperse the dried
material in water) can be limited through the addition of salt or
the exchange of counterion on surface charge groups (e.g.,
replace H+counterion with Na+), which hinders the formation
of new cellulose−cellulose bonding during drying.
337
2.4.3. Nanocellulose Dispersions. Nanocellulose can be
colloidally stable in water but only with a sucient amount of
surface charge, as the inherent attraction between the nano-
particles aects dispersion stability. If nanocellulosic particles do
not have sucient surface charge, a material with dispersion
properties comparable to cellulosic powders and macrofibers is
created due to aggregation. This is similar to the conventional
pulp suspensions and have been discussed in the session of
aqueous fiber suspensions in a conference proceeding in
1977.
110
The intermolecular hydrogen bonding between the
surface hydroxy groups as well as van der Waals bonding
between the hydrophobic sites in nanocellulose play a role in
aggregation in many media, such as organic solvents, polymer
matrices, and even water. Physical thinning, ultrasonic
dispersion, and high-pressure homogenization can weaken
these hydrogen bonds and are methods used to facilitate the
(re)dispersion of nanocellulose. Introduction of surface charge
or long chain molecules to increase either the electrostatic
repulsion or steric hindrance between nanocellulose particles
have also been used to improve dispersion.
247
In all cases,
dispersion of nanocellulose in water requires high energy input
(e.g., sonication, microfluidization, high pressure homogeniza-
tion) to either liberate individual nano-objects during
production or redisperse prepared nanocellulose from con-
centrated/dried forms (Figure 5).
2.4.3.1. Fundamentals of Colloidal Stability of Nano-
cellulose in Water. 2.4.3.1.1. Electrostatic Repulsion. The
surface charge density and electrostatic repulsion are important
factors influencing the dispersibility of nanocellulose in water,
which are aected by the environmental conditions, such as pH,
temperature, and salt concentration in aqueous media.
247,338
Stable nanocellulose dispersions in water can generally be
obtained when the absolute value of zeta potential is higher than
30 mV.
247,339
Sulfate half ester,
135−137
sulfonate,
340,341
carbox-
yl,
342−346
phosphate half ester,
139,140,347−349
phosphonate,
350
quaternary amine,
351−354
and amino
355
functionalities have
been used to introduce ionic groups onto the surface and
thereby improve the dispersion.
247
The neutral sodium-form of
CNCs dried by evaporation, lyophilization, or spray-drying is
more easily redispersed in comparison to CNCs in acid form.
356
Monovalent salts in the medium can improve the dispersion
through hydrogen bond blocking between hydroxy groups and
reducing the aggregation of CNFs and can also potentially help
to regenerate hydrogen bonds between water and nanocellulose
during the redispersion steps.
337
It is important to bear in mind
that the presence of divalent cationic charge in the medium, will
result in cross-linking and more aggregation for the nano-
celluloses with anionic charge on the surface (discussed more in
section 3.3.1).
Water can play a secondary role in desulfation of sulfate half
ester groups on the surface of sulfuric acid hydrolyzed CNCs
that leads to destabilization and aggregation.
357,358
Desulfation
happens fairly rapidly by acid- or alkaline-catalyzed de-
esterification in water. Auto-catalyzed desulfation in acid form
CNC suspensions occurs slowly at ambient conditions and fast
at higher temperatures and results in loss of surface charge, in
addition to aggregation, desulfation has been shown to aect
thermal stability, liquid crystal properties, and rheological
behavior. Dried solid acid form sulfated CNCs also undergo
rapid desulfation in contact with the humidity in air, in which
water acts as a medium for de-esterification reaction.
357
2.4.3.1.2. Steric Stability. Neutral polymers can be
immobilized on the surface of nanocellulose and expand in the
medium to gain configurational entropy. As the grafted polymer
layers overlap, steric or entropic repulsion between nano-
cellulose particles is generated.
359
The length of grafted
polymers plays an important role in the stability of the
dispersion of nanocellulose in water. The common strategies
for polymer induced steric repulsion are grafting PEG chains
onto the cellulose surface, silylation of the surface, polymer-
ization from the cellulose surface (e.g., surface-initiated electron
transfer atom transfer radical polymerization or SI-ATRP), and
adsorption of a polymeric surfactant for nonaqueous dis-
persions.
247,359−361
Araki et al. covalently conjugated aminated
PEG to carboxylated CNCs and the dispersion of the resulting
PEG-grafted CNCs in NaCl solution remained stable. After
freeze-drying, these PEG-grafted CNCs could be redispersed in
water and chloroform easily.
362
Interestingly, these PEG-grafted
CNCs were also used as dispersants for fluorescent probes
within cells.
363
Similarly, poly(ethylene oxide) (PEO) grafted
CNCs were dispersed stably in water for several months without
precipitation.
247,364
Some plant-based nanocelluloses disperse
well in water even after oven drying due to residual pectins on
the surface.
365
The molecular weight and content of pectin
control its inhibiting eect on the aggregation of CNFs during
drying process. Attaching surfactants such as cetyltrimethylam-
monium bromide (CTAB) onto uncharged (and colloidally
unstable) CNC via electrostatic adsorption has been shown to
improve their dispersibility in water.
366
2.4.3.2. Manipulating Nanocellulose Dispersions in Water.
2.4.3.2.1. Controlling Aqueous Nanocellulose Dispersion
Stability by Surface Modification and Blending with
Responsive Polymers. When responsive polymers are grafted
onto the surface of nanocelluloses, the dispersion stability in
water can be controlled by changing environmental conditions.
Thermoresposive polymers such as poly(2-hydroxyethyl meth-
acrylate) (PHEMA) or poly(N-isopropylacrylamide) (PNI-
PAM) have been grafted onto or from nanocellulose matrices to
control the water response of these systems via temperature
changes,
367
while both thermoresponsive and polyelectrolyte
brushes respond to changes in ionic strength.
368
Yi et al.
synthesized the first temperature-sensitive poly[2-
(dimethylamino)ethyl methacrylate] (PDMAEMA) grafted
CNCs by ATRP. In lower temperatures than the critical
temperature, the grafted PDMAEMA chains were in extended
conformation, leading to a good dispersibility in water. At higher
temperatures, water dispersibility decreased.
369
Similar thermor-
esponsive CNCs grafted with poly(N-isopropylacrylamide)
(PNIPAM) were reported by Zoppe et al. Higher ionic strength,
graft ratio, and degree of polymerization decreased the
dispersibility in water.
370
Azzam et al. synthesized Jeamine
polyetheramine grafted CNCs, which dispersed well in water
(and other media) due to the surface carboxyl groups and
polymer brushes; furthermore, the dispersion stability could be
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tuned by ionic strength, pH, and temperature.
247,371
Other
examples of responsive polymer-grafted CNCs prepared by
radical polymerization in water (i.e., not “controlled polymer-
ization” reactions) include pH-responsive polyvinylpyridine
372
and dual temperature and pH responsive PDMAEMA;
373
a full
review of polymer grafted CNCs, their functionalization routes,
and behavior can be found elsewhere.
374
Larsson et al. tuned the
interaction between CNF and water by adsorbing charged, heat-
responsive block copolymers onto the fibers. They observed the
transition of dispersed modified CNFs in water to a macro-
scopically aggregated state by heating, during which the
adsorbed block copolymer transitioned through a critical
solution temperature.
375
Apart from surface modification, blending with dierent
polymers can also be used to create novel forms of nanocellulose
dispersions. Bai et al. have reported the formation of water-in-
water liquid crystal emulsions of CNCs with permeable colloidal
assemblies. They showed CNCs spontaneously self-assemble
into a helical arrangement with the coexistence of nonionic,
hydrophilic polyethylene glycol (PEG), and dextran. These two
polymer solutions are thermodynamically immiscible. Stable
water-in-water emulsions are easily prepared by mixing CNC/
PEG and CNC/dextran solutions, where micrometric CNC/
PEG form the dispersed droplets and CNC/dextran form the
continuous phase. Over time, this emulsion breaks into an
upper, droplet-lean isotropic phase and a bottom, droplet-rich
cholesteric phase. Osmotic pressure gradient between PEG and
dextran phases results in target transfer of CNCs across the
water/water interface to reassemble into a liquid crystal-in-
liquid crystal emulsion with global cholesteric organization. The
authors observed that the colloidal particles in the two
immiscible phases experience short-range interactions and
form long-range assemblies across the interface.
376
2.4.3.2.2. Controlling Aqueous Nanocellulose Dispersion
Properties by Drying. Water removal techniques have been
applied as an “adjustment tool” in nanocellulose preparation
with specific dispersion ability and stability in mind. As discussed
earlier, redispersion in water and other solvents or polymeric
blends is strongly aected by the drying method. In general,
nanocellulose obtained by evaporation from aqueous suspen-
sions is extremely dicult to redisperse, while freeze-dried
sulfated CNCs and TOCNFs can be redispersed in water with a
correctly chosen counterion.
21,377−379
CNFs are less commonly
dried due to their inherently entangled nature and lower charge,
which leads to more diculty in redispersing them. However,
some successful drying processes and additive dispersants have
been introduced for this purpose.
21
Hu et al. reported that
intrinsic adsorption of hemicellulose imparted a good
redispersibility on mechanically defibrillated nanocellulose via
good water accessibility of soluble hemicellulose to water
comparing to that of cellulose.
380
Foster et al. reported that the
concentration of the CNC dispersion being dried influences
redispersibility in water, lower concentrations lead to more
redispersible dried materials. When redispersing CNCs in water
from dried powders, low concentration suspensions are easiest
to achieve, but if a high concentration is needed, it is best to
prepare a dispersion below 2 wt % with sonication and then
gradually add more CNCs with repeated sonication steps.
21
It
has also been shown that a combination of surface modification
and drying techniques can be used to obtain materials with
tailored dispersibility. For example, directly adding capping
agents (such as specific counterions, polymers, and surfactants)
prevents the agglomeration of CNFs during their dehydration
(hornification) leading to CNFs with noteworthy dispersibility
and colloidal stability.
381,382
2.4.3.2.3. Nanocelluloses as Dispersing Agents. The ability
of nanocellulose to partition at solid/liquid, liquid/liquid, and
gas/liquid interfaces has opened up new avenues to control
dispersions containing nanocellulose.
383
Nanocellulose charac-
teristics such as size, charge, and polymorph aect their surface
properties, and their ability to stabilize interfaces and act as
dispersants.
247
While nanocelluloses are not strictly “surface
active”,
384
their amphiphilicity is governed by the crystalline
polymorph
385
and any surface modification.
386
Besides, any
colloidal particle has a tendency to enrich at interfaces by
default.
387
Pickering emulsions were one of the first applications
of these findings with potential in the pharmaceutics/drug
delivery, personal care, food, cosmetics, and porous materials,
etc.
384,388,389
Amphiphilic particles like nanocellulose can be
wetted by both water and oil and are excellent stabilizers that are
essentially irreversibly adsorbed at the oil/water interface and
prevent droplet coalescence compared to typical molecular
surfactants. Emulsions, and particularly high internal phase
emulsions, allow for formulated products with significantly less
water overall but the ability to process nanocellulose under
favorable and predictable aqueous conditions.
390
CNFs have a
higher aspect ratio than CNCs and often have a higher
adsorption capacity and wettability by oil, which could result
in more stable emulsions.
391
However, their entangled nature
often leads them to act more as a rheological modifier in the
continuous water phase than as a Pickering stabilizer.
384
Latexes
are an extension of emulsion systems because they are made by
emulsion or suspension polymerization, they can also be greatly
improved by nanocellulose incorporation, where the role of
nanocellulose varies from being a monomer−water interface
stabilizer, to a water phase additive, or even a polymer particle
“seed” with either active or passive participation in the chemical
reactions. Importantly, nanocelluloses can be used as dispersants
to improve the interfacial compatibility and prevent the
agglomeration/aggregation of other noncolloidally stable
particles.
247
Amphiphilic nanocellulose is associated with 2D
nanomaterials via hydrophobic interactions eciently, whereas
the hydrophilic surfaces help to disperse nanocellulose-bound
2D nanosheet in aqueous media. Surface charges stabilize the
nanocellulose-bound 2D nanomaterial dispersions in water
through Coulomb repulsion, where nanocellulose−water
interaction is vital. Nanocellulose has been intensively used as
dispersing agent for 2D nanomaterials such as graphene
392−394
and MXenes,
395
through intercalation. In aqueous environment
exfoliation occurs via double electrostatic layers build up on 2D
nanomaterials that can overcome the van der Waals
interactions.
396
Similarly, other nanoparticles, such as metal
oxides,
349,397,398
quantum dots,
399
metal organic frameworks
(MOFs),
400
polymer nanofibers,
398
and salt nanoparticles
401,402
have also been stabilized by nanocellulose to disperse them in
water. Lastly, the unique templating ability of nanocellulose can
be used to mediate the nucleation and growth of metal
nanoparticles, inhibiting nanoparticle aggregation, improving
their dispersion uniformity and stability, and in many cases,
enhancing their catalytic or electrochemical function, as
reviewed previously.
403,404
Interestingly, Gonzales et al. rationalized that if a surfactant
with high HLB (hydrophilic−lipophilic balance) is able to
stabilize oil in water emulsions, then the same type of material
could hinder the formation of water in oil emulsions. They used
nanocellulose as an inhibitor (demulsifier) of water in crude oil
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Table 2. A List of Covalent and Noncovalent Methods for Decreasing Hydrophilicity of Nanocellulose
altering the hydrophilicity of nanocellulose surfaces
surface modification reagents used required
conditions role/consequence of water potential target application ref
Covalent Hydrophobization
esterification carbonyl chlorides, acid anhydrides anhydrous undergoes side reactions sensors, mechanical reinforcement, biomedical
materials, (super)hydrophobic interfacial materials 487−493
darboxylic acids, active esters (potentially
activating agents for in situ generation) anhydrous/
aqueous reaction byproduct that decreases yields 490
darbamate formation isocyanate anhydrous causes side reactions polyurethane composites 494
etherification alkyl chlorides anhydrous may cause phase separation, side reactions
depending on substrates and reaction conditions introduction of functional groups, e.g.,
carboxymethylation 495−501
epoxides aqueous/
anhydrous causes side reactions introduction of functional groups, e.g., quaternization 495−500
chlorination (potential
subsequent etherification) thionyl chloride anhydrous causes side reactions further modification
oxidation (potential
subsequent amination) periodate aqueous solvent further modification 503
amidation EDC/NHS or comparable coupling agents,
primary or secondary amines aqueous solvent selective modification, biomolecule immobilization 504−507
amination aldehyde/keto-groups on cellulose, amines,
reducing agent aqueous/
anhydrous solvent protein immobilization 503,508
silylation chlorosilanes anhydrous causes side reactions stable dispersion in organic media 509−511
alkoxysilanes anhydrous/
aqueous solvent porous hydrophobic adsorbent materials 512,513
polymer grafting (ATRP) ATRP-agent, vinyl monomers anhydrous/
aqueous solvent tailor-made reinforced hydrogels 514,515
polymer grafting (ROP) epoxides/cyclic lactones anhydrous chain transfer agent giving rise to bulk ROP covalent composite materials 416
Noncovalent Hydrophobization
polymer adsorption hydrogen bond acceptors, countercharge
carrying polymers, hemicelluloses anhydrous/
aqueous dispersion medium, antisolvent thermoplastic polymer composite materials 516,517,518,
424,519−528
counterion exchange surfactants, quaternary ammonium ions, aqueous dispersion medium, antisolvent cellulose dispersion in nonpolar media 424,522,
529−534
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emulsion formation, which is a challenge during crude oil
extraction and processing.
405
3. ROLE OF WATER IN NANOCELLULOSE
MODIFICATION AND APPLICATIONS: A
DOUBLE-EDGED SWORD
3.1. Pathways to Tune Nanocellulose−Water Interactions
As discussed earlier a few times, cellulose is inherently
amphiphilic,
229−231
and altering its surface energy is possible
through a wide range of surface modification methods. Due to
the natural anity between cellulose and water, it is clear that
water plays an important role in the modification of nano-
cellulosic surfaces. Although most covalent routes are based on
organic reactions where water is usually detrimental, several
synthetic approaches for nanocellulose modification can
accommodate having water as a solvent.
74,251,386,406−422
One
of the main motivations to surface modify nanocellulose is to
improve its compatibility with other materials. For example, the
dissociation of charged sulfate half ester groups in water ensures
the colloidal stability of CNCs, but if one wants to handle them
in nonpolar solvents (which can then be used as further
modification medium), they tend to aggregate. Similarly, while
as-prepared nanocelluloses are inherently incompatible with
many petroleum-based polymers, hydrophobized nanocellu-
loses may be more easily dispersed throughout a composite
matrix. Additionally, the use and functionality of assembled
nanocellulose structures may require a specific surface energy
(e.g., hydrophobicity in nanocellulose aerogels designed for
separating oil from water). In this chapter, we focus on surface
modification methods which tune the amphiphilic nature of
nanocelluloses, and we further go on to highlight the role of
water in these modification procedures.
We re-emphasize that the distinction between hydrophilic/
hydrophobic is oversimplified in the literature, particularly when
defined by static contact angle measurements. Still, it is a
definition that has stuck and is widely recognized and, as such,
we feel that adhering to these terms has significance in the
scientific community.
When considering surface modification of nanocellulose, it is
not only the nature of the substituents being grafted but also the
degree of substitution that influences the hydrophilicity/
hydrophobicity. Furthermore, contact angle measurements are
highly sensitive to surface roughness, which can be altered after
surface modification such that interpretation of results is not
always straightforward. In this context, Cunha et al. showed that
for the esterification of nanocellulose with trifluoroacetic
anhydride even a modest degree of substitution of 0.04 had a
strong eect on the hydrophilicity of the surface, resulting in a
static water contact angle of 126°.
423
This demonstrates how
small changes to the cellulosic surface may have large impacts on
the wetting characteristics due to both chemistry and top-
ography.
3.1.1. Decreasing Nanocellulose Surface Hydrophilic-
ity. In this section, we focus on surface modification approaches
that aim to decrease the hydrophilicity of nanocelluloses and the
role of water in these reactions. This often-termed “hydro-
phobization” of nanocellulose leads to the disruption of the
solvation of surface structures by water by capping the available
hydroxy groups through the attachment of hydrophobic
moieties,
424−427
grafted polymers,
428−436
or even nanopar-
ticles.
437−443
Generally, the reasons to hydrophobize the surface
of nanocellulose include increasing the hygromechanical
stability (keeping good mechanical properties in the wet
state), improving the compatibility with hydrophobic polymers
or solvents or reducing the eects of hornification upon
drying.
12,444−446
The palette of the available modification
pathways is very diverse, and a full review of these reactions is
out of the scope of this paper. The reader is encouraged to follow
the details of modification reactions in the previous publications
on this topic.1,3,118,119,128,246,247,249,251,252,384,
406,408,415,447−486 However, a short overview of the most
important routes to nanocellulose surface hydrophobization is
covered here and summarized in Table 2.
3.1.1.1. Covalent Nanocellulose Hydrophobization. The
formation of esters
487−490
and carbamates on the surface of
nanocellulose usually requires anhydrous environments, which
is facilitated by drying techniques or the use of nonaqueous
reaction media, such as gas phase esterification.
491−493
The
esterification of cellulose hydroxy groups with carboxylic acids,
anhydrides, and acid derivates is arguably the most versatile tool
for surface modification due to the variety of reaction pathways,
including simple Fischer esterification, acid chloride or
anhydride alcoholysis, and transesterification, which can be
conducted in a liquid or gas phase, depending on the moiety to
be attached.
490
Polymer grafting through ring-opening polymer-
ization or surface initiation for (controlled or not) radical
polymerization on the surface via (trans)esterification have been
also widely reported in the literature.
416,515
Similarly, the
grafting of isocyanates onto cellulose surfaces requires an
anhydrous environment to avoid side reactions of the reactive
electrophiles. The high reaction speed and yield of the addition
reaction, the availability of isocyanates, and easy access to
polyurethane grafting techniques add to the popularity of this
approach.
494
Etherification via thionyl chloride is also performed
in nonaqueous environments, substituting the cellulose hydroxy
groups with chloride.
502
This activation facilitates subsequent
substitutions, as the halogen is a better leaving group compared
to hydroxy moieties. Eyley and Thielemans employed a
chlorination approach on CNFs, followed by substitution of
the chloride group with azide to obtain clickable cellulose
moieties.
501
Some reactions for hydrophobization, such as silanization, can
be carried out in both aqueous and anhydrous environ-
ments.
513,535−541
Reactive chlorosilanes undergo hydrolysis in
aqueous environments, therefore, they are used to silanize
nanocellulose in anhydrous environments. These reagents are
chosen for their high reactivity and therefore short reaction
times when in contact with cellulose. However, the need for
tedious solvent exchange procedures and the release of
hydrogen chloride are drawbacks in the synthesis of silylated
components by chlorosilanes.
509−511
Alternatively, correspond-
ing alkoxysilanes can be used to obtain the same desired
compounds. These reagents are not as sensitive to water but are
less reactive, and therefore, reaction times are increased.
512,513
While the deprotonated cellulose hydroxy groups act as
nucleophiles in etherification reactions (explained in the next
paragraph), auxiliaries can be employed to transform the
cellulose component into an electrophile. Historically, this
involved the use of thionyl chloride as explained previously.
501
Alternatively, in etherification via employing tosyl chloride
reactions, the hydroxy groups can be converted into tosylates,
which in turn are better leaving groups than chlorides. While the
tosylation of cellulose is usually carried out in a nonaqueous
environment such as DMA/LiCl,
542
the reaction can also be
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conducted in water.
543
Guo et al. used tosylation on nano-
cellulose to prepare clickable cellulose azides.
544
Some other cellulose modification reactions are safe to be
carried out in water. Etherification of surface hydroxy groups can
easily be achieved by reacting cellulose with glycidylethers or
chloro-compounds, most prominently chloroacetic acid, in a
basic aqueous solution.
495−500
Basic media are needed to
activate the surface hydroxy groups via deprotonation. The
formed alkoxide species are suciently nucleophilic to add to
epoxide moieties or to substitute halogens. The oxidation of
nanocellulosic surfaces in aqueous media to aldehydes and acids
(which initially increases the hydrophilicity of the surface and on
its own will be discussed in section 3.1.2) followed by further
functionalization is an important pathway to preparation of
hydrophobized nanocellulose surfaces. As mentioned in the
previous section, oxidation is also considered to be a very
common step in the isolation of dierent types of nanocellulose,
often ending up in carboxylate moieties on the sur-
face.
342,545−547
Nanocellulose surfaces containing carbonyl
groups (such as in aldehydes and ketones) can be used as
substrates for Schi’s base reactions, including the attachment of
amines,
503,508
while the carboxylic groups can further be
modified through activated peptide coupling.
504−506
A variety
of coupling agents have been developed over the past decades to
facilitate peptide synthesis and can be used for this purpose in
aqueous environments.
507
An overview of covalent routes to
nanocellulose modification are depicted in Figure 7.
3.1.1.2. Noncovalent Hydrophobization Routes. In addition
to chemical pathways to decrease the hydrophilicity of the
surface, physical interactions are other ways to hydrophobize
nanocellulose surface in aqueous and nonaqueous systems.
Physical surface hydrophobization (essentially adsorption or
counterion exchange) can be achieved in a facile and cost-
eective process without the need for complex or multiple
chemical reactions. These physical routes usually preserve the
nanocellulose crystallinity and morphology better, but they are
weaker and more likely to be reversible,
386
compared to
chemical modification via covalent bonds. Often, nanocellulose
surface modifications via physical interactions are based on
electrostatic interactions or polymer adsorption.
548−554
The simplest practical way to approach adsorption as a
modification system is to use water-soluble polymers.
555
For
example, carboxylated CNFs and amphiphilic diblock copoly-
mer poly(methyl methacrylate-b-acrylic acid) form a stable
complex. Due to the hydrophobic poly(methyl methacrylate)
block, the surface modified CNFs exhibit good dispersibility in
dierent organic solvents including DMF, DMSO, ethanol, and
methanol.
516
In addition to hydrophilic interactions, the charges on the
surface of nanocelluloses can be employed to physically modify
the surface. For example, the negative surface charge on CNCs
prepared by sulfuric acid hydrolysis (i.e., sulfate half esters) and
CNFs prepared via TEMPO-mediated oxidation (i.e., carboxyl
groups) can interact with polyelectrolytes.
103,517,518
Hydrophilic
CNFs can be hydrophobized through the adsorption of
octadecyl amine resulting in their dispersibility in solvents
such as toluene, tetrahydrofuran, and isopropyl alcohol.
However, this surface modification is unstable and will desorb
over time.
424
Esker et al. utilized the adsorption of hydrophobic
polyelectrolytes (i.e., cationically derivatized hydrophobized
dextran polyelectrolytes) to decrease the surface water on
sulfated CNCs and expedite their dewatering kinetics. Using
electrostatic interactions, block copolymers based on quater-
nized poly(2-(dimethylamino)ethyl methacrylate) and poly-
caprolactone can also be attached to anionic nanocellulose
surfaces, resulting in a decrease in the surface energy of the
cellulose surface. The benefit of modification pathways such as
this is that they can be carried out in an aqueous environ-
ment.
424,519−522
Yahia et al. used poly 1-[4-(bromomethyl)-
phenyl]-1,2,4-triazole to modify the surface of CNCs through
Figure 7. Most common initial reactions of cellulose, as the foundation for more elaborate modifications and their compatibility (blue)/sensitivity
(yellow) to water.
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electrostatic adsorption, which significantly altered their electro-
phoretic mobility and enabled their dispersion in acetone and
acetonitrile.
556
They even took the modification a step further
by substituting the bromine with bis(trifluoromethane
sulfonyl)imide, which resulted in nanocellulose complexes
able to disperse in tetrahydrofuran and ethyl acetate.
386
A simple counterion exchange of the charged groups on
nanocellulose surfaces can also be used to tune surface energy in
order to decrease self-aggregation and improve compatibility
with nonpolar solvents and hydrophobic polymers. In particular,
substituting the counterion of CNCs with an imidazolium and
phosphonium cation improves CNC compatibility with
polymers such as epoxy and polystyrene and decreases their
ability to interact with water; phosphonium ion exchanged
CNCs absorbed 30% less water than the sulfate CNCs in their
sodium form.
534
In the case of CNFs, a mixed system of sodium
and tetraethylammonium counterions has been used to control
CNF film hydrophilicity. The oxygen and water vapor
permeabilities of said films increased with an increase in the
molar ratio of bulky tetraethylammonium counterions.
529
Physical adsorption of surfactants onto nanocellulose is
another strategy to reduce the capacity for intermolecular
hydrogen bonding. Common surfactants such as cetyltrimethy-
lammonium bromide (CTAB), dimethyldidodecylammonium
bromide (DDAB), and dimethyldihexadecylammonium have
been demonstrated to stabilize nanocellulose dispersions in
THF.
522
Additionally, amphiphilic polymers, which contain
both polar and nonpolar moieties, can interact with nano-
cellulose surface such that their polar end interacts with the
nanocellulose and the nonpolar remains in solution.
523
Addi-
tionally, nonionic surfactants, such as sorbitan monostearate,
have been used to modify CNC surfaces to improve their
dispersibility in organic solvents and prevent self-aggregation.
530
The acid phosphate ester of ethoxylated nonylphenol surfactant
(Beycostat A B09) was also reported to improve the
dispersibility of CNCs in chloroform.
531
Recently, Kontturi et
al. presented a nanocellulose surface hydrophobization
technique through the adsorption of hydrophobic polymers
(e.g., polystyrene and poly(trifluoroethylene)) from aprotic
solvents, resulting in nanopapers with water vapor uptake ability
yet a strong repellency for liquid water, but this method is
restricted to macroscopic substrates such as nanopaper and does
not work for individual nanoparticles like CNFs and CNCs.
524
Carbohydrates and soluble cellulosic materials have also been
used to tune the interactions of nanocellulose surfaces with
water. Larsson et al., for example, altered the behavior of
composite films of both CNFs and CNCs with water through
the adsorption of water-soluble hydroxypropylmethylcellulose
(HPMC),
525
while numerous works have shown altered CNC−
water interactions as a result of insoluble oligosaccharides
precipitated onto the surface of nanocellulose.
526−528
Adsorp-
tion of water-soluble polysaccharides is indeed an often applied
approach to alter the surface of (nano)cellulose, but in most
cases it does not result in more hydrophobic surfaces. In fact, in
many cases the surface becomes more hydrophilic through the
attachment of polysaccharides, specifically methylcellulose,
hydroxyethylcellulose, guars, carboxymethylcellulose, and hemi-
celluloses have been demonstrated.
3.1.2. Increasing Nanocellulose Surface Hydrophilic-
ity. In this section, we focus on surface modification pathways to
increase the hydrophilicity of nanocelluloses, and we highlight
the role of water in these procedures. Despite the hygroscopic
nature of cellulose, its amphiphilicity can hinder the use of
nanocelluloses in applications where high levels of hydro-
philicity are required, such as in the case of superabsorbents, for
example. As with surface hydrophobozation methods, both
covalent and noncovalent pathways can be implemented. Table
3is a summary of the most important nanocellulose surface
hydrophilization modification routes and the role of water in
these procedures.
3.1.2.1. Covalent Nanocellulose Hydrophilization. The
hydrophilicity of nanocelluloses can also be improved by
grafting hydrophilic polymers (e.g., acrylate based super-
absorbent polymers),
370,416,557
although grafting hydrophilic
polymers is less explored than grafting hydrophobic polymers.
Kaldeus et al. reported an all-water-based procedure for
“controlled” grafting of hydrophilic polymers from CNFs;
polymers and copolymers of PEG, methyl ether methacrylate,
and poly(methyl methacrylate) were synthesized by ATRP from
the CNF surface in water. This was made possible through an
amphiphilic macroinitiator that was electrostatically immobi-
lized on the CNF surface, facilitating both hydrophobic and
Table 3. A List of Covalent and Noncovalent Methods for Increasing Hydrophilicity of Nanocellulose
altering the hydrophilicity of nanocellulose surfaces
surface
modification reagents used required
conditions role/consequence of water potential target application ref
Covalent Hydrophilization
esterification mineral acids aqueous solvent dispersion/gelation control 136,
137,
347
oxidation TEMPO, hypochlorite aqueous solvent dispersion/gelation control 411
polymer
grafting acrylates (polyelectrolytes) aqueous solvent for monomer, dispersant for
cellulose, grafted polymer covalent composite
materials 370,
416,
557
PNIPAM (thermoresponsive) aqueous solvent for monomer, dispersant for
cellulose, grafted polymer covalent thermoresponsive
composite materials 367
Noncovalent Hydrophilization
counterion
exchange monovalent counterions aqueous dispersion medium “salting-in” eects for
improved colloidal
stability
220
chaotropic co-
ions chaotropes according to the hofmeister series
with equal charge to the cellulose aqueous dispersion medium defined swelling of cellulose
hydrogels 558
dilution any solvent, not
fully anhydrous reduction of hornification by
dilution 332
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hydrophilic monomer polymerization in water.
514
Here, the
precise mapping of water interactions would be an important
fundamental undertaking as the grafted nanocelluloses possess a
corona that can hold rather vast amounts of bound water
compared to pristine (or charged) CNFs and CNCs that have
bound water primarily on their surfaces.
3.1.2.2. Noncovalent Nanocellulose Hydrophilization
Routes. The most popular approach to increase the hydrophilic
character of nanocellulose is to impart additional charge onto
the surface.
559
Nearly always, nanocellulose bears some degree
of surface charge directly after production with a certain degree
of tailoribility based on reaction conditions. With TEMPO-
oxidation
79,132,342,560
and periodate oxidation,
503
the surface
charge can be varied by up to an order of magnitude, whereas
sulfuric acid hydrolysis “harshness” can only aect the degree of
sulfation within a relatively small window.
561
In addition to mere
hydrophilic character, subtle changes in charge caused by
varying degrees of TEMPO-oxidation have been applied to
control wetting and antifouling properties.
562
However, it is
important to bear in mind that surface charge makes the
behavior of nanocellulose in water susceptible to ionic strength.
The easiest way to suppress electrostatic interactions is to
increase the ionic strength of a nanocellulose suspension and
this will lead to nanoparticles aggregation and loss of colloidal
stability according to the DLVO theory.
220
Furthermore, the
counterion on the nanocellulose surface charge group aects the
water interactions and leads to dierent coagulation behavior in
water and other solvents.
220
Zhang et al. illustrated the eect of
cosolvent choice (i.e., sodium salts of various anions) on the
swelling behavior and thus the mechanical properties of
polymeric hydrogels.
558
While these principles also apply to
cellulose, the overall ionic strength may lead to a screening of the
surface charges and in turn to dierent levels of interaction
between water and (aggregated) nanocelluloses.
563
3.2. Role of Water in Controlling Surface Modification
Reactions
Unfortunately, the presence of water can be detrimental in some
nanocellulose surface modification mechanisms, due to its
incompatibility with the required chemical reagent or reaction.
In these cases, it is crucially important to remove any residual
water prior to initiating the modification of nanocellulose
surfaces. An anhydrous chemical environment can be achieved
through techniques such as solvent exchange, the subsequent
use of molecular sieves, or drying by evaporation and diusion.
However, each of these techniques has its own inherent
drawbacks. Drying solvents is tedious and requires working in
conditions where contact with air (and ambient moisture) is
strictly prevented by working with a vacuum line for example.
Molecular sieves, on the other hand, can introduce impurities
into the reaction medium, which can also inhibit the surface
modification reactions of nanocellulose. There are also other
methods for water removal prior to chemical modification, such
as adding alkaline materials or exploiting ionic liquids while
water is being removed.
312
While solvent exchange is a fast and
relatively easy method to remove water from nanocelluloses,
residual water often remains in the system afterward. For surface
modification techniques requiring anhydrous conditions, one
would typically use fully dried nanocellulosic substrates, but it is
important to recognize the eect of hornification (irreversible
aggregation) on the eciency of these chemical modifica-
tions.
332,564,565
Indeed, many accounts utilize solvent exchange
or extensive drying step to introduce nanocellulose into
anhydrous media, perform reactions on the nanocellulose, and
report a certain degree of substitution. However, they do not
necessarily disclose the morphology of nanocellulose after the
reactions. It may be that in some cases large swathes of, for
example, CNC particles remain completely unmodified in larger
aggregates that have accommodated all the substitutes on their
surfaces. All in all, it appears that aqueous medium is the safest
place to work on comprehensive modification of nanocellulose
in a way that all nanoparticles are evenly modified.
3.2.1. Controlling Dispersion in Modification Reac-
tions by Tuning the Water Content. Solvent-exchange often
leaves behind a fraction of residual water in nanocellulose
systems,
510
which might be detrimental to some organic
reactions, but minute amounts of water may under specific
conditions play a positive role, for example, in CNC dispersion
in organic media.
247
Viet et al. found that a small amount of
water is critical to colloidally stabilize sulfated CNCs in polar
aprotic solvents such as DMSO and DMF. Freeze-dried CNCs
with residual water contents of 6−8% were able to form stable
and well dispersed suspensions. Yet further removal of moisture
using molecular sieves resulted in the aggregation of the CNCs.
Interestingly, when 0.1% w/v of water was reintroduced into
mixture, a stable suspension reformed. The authors concluded
that a small amount of water (<0.2% w/v) is indeed necessary
for a colloidally stable dispersion.
566
Belgacem et al. investigated
the eect of residual water on the particle size of the CNC
aggregates by dispersing CNCs in water and subsequently
carrying out a solvent exchange with ethanol and acetone. When
measured through dynamic light scattering, the apparent size of
the solvent exchanged CNCs was ca. 300 nm, whereas dried
CNCs redispersed directly into acetone had an apparent size of
ca. 635 nm. They concluded that the interactions of residual
water with CNCs were not totally lost during solvent exchange,
leading to a more stable suspension in acetone.
567
Chang et al.
systematically investigated the influence of water on the
redispersion of CNCs in DMF. CNCs with a residual moisture
content of ca. 4% have the same hydrodynamic radius as the
theoretical radius of monodispersed CNCs, indicating that the
CNCs were completely dispersed in the DMF.
568
3.2.2. Controlling Modification Outcomes by Tuning
Water Content. When macroscopic nanocellulose substrates,
such as films or nanopapers, are subjected to modification, the
role of water is ambiguous. The high swelling capacity of CNF
networks in water means that under aqueous conditions, the
whole network, including its interior, will be modified. The
water-immersed modification can also impair the bonding in the
network if the fiber−fiber bonds are cleaved and modified. By
contrast, when utilizing nonswelling hydrophobic solvents, only
the surface of the network will be modified, and the network
bonding remains intact. In this realm, Kontturi et al. used aprotic
solvents instead of water as medium to adsorb hydrophobic
polymers on the surface of nanopapers in the study already
quoted in relation to the adsorption-based methods in section
3.1.1.2. Due to the limitations to the ability of these solvents to
swell the nanofibers, these nonporous nanopapers were
hydrophobized only on their surface. This resulted in CNF
nanopapers with tunable hydrophobicity on the surface, while
their water vapor absorption capacity was demonstrated to be
similar to the pristine, untreated nanopapers. The strong
mechanical properties, based partially on the hydrogen bonding
between the CNFs have been retained, while the vapor
transmission of the surface-hydrophobized nanopapers may be
useful in applications such as textiles or building insulations.
524
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Sometimes water is not a good solvent for one of the reagents
in a cellulose modification reaction, and in these cases, although
water does not interfere with the reaction per se, the
concentration of the reagent in water is too low, and this
prevents the reaction to proceed to completion. Odabas et al.
cationized bleached kraft pulp in systems with dierent water-
miscible organic solvents. Replacing 90% of the water with 2-
propanol and particularly with tetrahydrofuran yielded higher
degrees of substitution and increased reaction eciency. The
degree of substitution depends on the concentration of the
cationization reagent in the reaction medium. Replacing most of
water with tetrahydrofuran resulted in a higher concentration
while maintaining supramolecular properties such as crystal-
linity and polymer chain integrity.
569
Although this particular
study has been done on pulp fibers, the concept is likely
applicable to all cellulosic materials, including dierent nano-
cellulose types.
When working with nanocellulose suspensions in water (or
other solvents), there is a strong eect of concentration on
viscosity and potential flow alignment.
570−573
Unless a
controlled environment is maintained around the sample,
water evaporation can occur during the course of the
modification reactions and significantly change the concen-
trations, viscosity, and alignment due to flow. Water evaporation
can (and should) be reduced by implementing a closed or reflux
system during surface modification reactions.
3.3. Nanocellulose−Water Interactions in Materials
Applications
The following subchapters highlight the connection between
water interactions and nanocellulose material applications,
providing an overview of where water can be a benefit or
detriment. Moreover, we discuss dierent synthetic and
technical approaches toward tailoring the water response in
nanocellulose-based materials for certain applications. Depend-
ing on the production and processing methods, dierent
morphologies and hybrid systems containing nanocellulose
can be obtained, e.g., colloids and emulsions, hydrogels, films
and membranes, aerogels and foams, and nanofillers, which oer
a range of properties useful across various fields (Figure 8).
The overview on nanocellulose applications is not exhaustive.
Rather, we aim at exploring how water interactions are
influencing the functionality of the materials described by
focusing on relevant examples on a certain class of applications
where nanocellulose has been used as an integral component.
For the readers interested in exhaustive treatises, substantial
review articles have been published, and we refer to those
reviews with each relevant topic throughout this chapter. Table 4
summarizes the water related applications of nanocellulose in
dierent forms (dispersion, hydrogel, film, aerogel, and powder)
and important properties of each form for the mentioned
application.
Figure 8. Dierent morphologies and systems of nanocellulose materials: (a) film,
574
(b) emulsion,
388
(c) hydrogel,
575
(d) aerogel,
319
(e) spray-dried
powder,
299
(f) coating,
576
(g) membrane,
577
(h) nanocomposite.
578
(a) Adapted from ref 574 under the terms of CC-BY. Copyright 2014 John Wiley
and Sons. (b) Adapted with permission from ref 388. Copyright 2011 American Chemical Society. (c) Adapted with permission from ref 575.
Copyright 2015 American Chemical Society. (d) Adapted with permission from ref 319. Copyright 2005 The Royal Society of Chemistry. (e) Adapted
with permission from ref 299. Copyright 2020 American Chemical Society. (f) Adapted from ref 576 under the terms of CC-BY. Copyright 2020
Multidisciplinary Digital Publishing Institute. (g) Adapted with permission from ref 579. Copyright 2018 John Wiley and Sons. (h) Adapted with
permission from ref 578. Copyright 2014 Elsevier.
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V
Table 4. Applications and Properties of Various Nanocellulose-Derived Materials Regarding Water
a
application dispersion hydrogel film aerogel powder
drug delivery carriers (water sorption, swelling, bio-
compatibility)
emulsifiers in formulations (am-
phiphilicity)
tissue engi-
neering
scaolds (water sorption, swel-
ling, biocompatibility, mass
transport, mechanical
strength)
diagnostics sensors (anisotropy, self-assem-
bly, water sorption, mass
transport, swelling)
robotics actuators and responsive materi-
als (anisotropy, self-assembly,
water sorption, mass transport,
swelling)
energy separators, binders, and electrodes in batteries, batteries, energy
harvest devices, and super capacitors (mechanical strength,
water sorption, mass transport, surface charge, high surface
area, self-assembly)
flexible elec-
tronics
coatings on wearable devices (mechan-
ical toughness controllability with
water content, anity toward cellu-
lose-based fibers)
textile coatings on textiles (mechanical tough-
ness controllability with water con-
tent, anity toward cellulose-based
fibers)
packaging gas barriers (high surface area, mechanical strength, structural
integrity tunability with water, surface modification)
membranes liquid barriers (wet mechanical
strength, surface modification)
absorbents site specific absorbents in mem-
branes (wet mechanical
strength, surface modification)
foams in hydrophobic and hydrophilic liquid
superabsorbent (high surface area, porosity,
surface modification, water sorption, mass
transport)
composites nanofillers as reinforcement
agents (wet mechanical
strength, surface modification,
high surface area)
food rheology modifiers in processed food
(water dispersibility, gel formation,
shear-depending on viscosity, bio-
compatibility)
emulsifiers in processed food
(amphiphilicity)
cosmetics rheology modifiers in formulations
(water dispersibility, gel formation,
shear-depending viscosity, biocom-
patibility)
emulsifiers in formulations (am-
phiphilicity)
a
This table summarizes the main areas of application of nanocellulose that are directly connected to the interactions with water. There are numerous details that are not reflected in this table.
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W
3.3.1. Hydrogels. CNFs, including BC, inherently form
weak, physical hydrogels via hydrogelation at low concentrations
(0.5−1.5 wt %) owing to their flexibility and ability to
entangle.
9,580,581
In contrast to CNFs and BCs, the rod-like
CNCs have a limited ability to entangle. Nevertheless, as
described in section 1, CNC aqueous suspensions transition
from liquid crystalline to birefringent viscoelastic phases at
elevated concentrations.
571,582
Altogether hydrogels are a widely
reviewed
9,580,583,584
class of nanocellulose-based materials. A
recent review by Ajdary et al., in particular, gives a
comprehensive view on nanocellulose-based hydrogels, inspired
by nature toward advanced applications.
26
Figure 9 illustrates the
overview of water interactions in hydrogels, their tuning, and
their eect on dierent applications. As explained earlier, the
ability of nanocellulosic hydrogels to bind and retain water is a
direct result of the nanocellulose surface chemistry, aspect ratio,
and flexibility, as well as of the 3D network microscopic and
macroscopic structure. Water−network interactions and swel-
ling are therefore related to processes on the molecular
(hydration) and supramolecular (wetting, capillarity, and
diusion) level.
26
However, excessive amounts of water can
result in structural disintegration over time, especially in the
presence of mechanical stress.
266,585−590
The water ratio impacts
the behavior of nanocellulose gels under mechanical stress and
makes the prediction of this behavior dicult.
591
Many of the
nanocellulose hydrogels are formed physically by ionic
interactions or physical cross-linking, which become weaker
and eventually might fail with increased water content.
592,593
Covalent cross-linking is the main strategy to minimize the
detrimental eects of excessive water on hydrogel structural
integrity (Figure 10a). The cross-linked hydrogels swell in
excessive water and might change their shape, but normally
these changes are less apparent than non-cross-linked hydrogels
and the material usually shows better mechanical properties over
time and under stress.
594−599
Hydrogels of CNFs prepared by
TEMPO-mediated oxidation have a hierarchical porosity in wet
state in the meso- and macroporous range depending on their
solid content. Subsequently, their mechanical properties and the
characteristics of water within the nanofiber network depend on
their hierarchical porosity.
314
3.3.1.1. Tuning Hydrogel Behavior through Water Inter-
actions. Surface modification/functionalization
248,600,601
and/
or cross-linking
595,602,603
have been widely used to tune the
gelation behavior of nanocellulose and the network properties of
their hydrogels. CNCs, for instance, form physically cross-linked
nanofibrillar hydrogel networks in the presence of metal salts,
604
with a sol−gel transition appearing at CNC concentrations far
below (e.g., with 50 mM Ca2+ at 2 wt % CNC) the range of
pristine CNCs.
571
In the case of CNFs, cation cross-linking has
been used to tune the mechanical properties of the hydrogels,
i.e., via charge screening of the repulsive charges on the
negatively charged nanofibril surfaces.
592
Hydrogels made of
hydrophobized cellulose have also been reported in an attempt
to tune the hydrogel properties. Nigmatullin et al. synthesized
CNCs that bind to each other through associative hydrophobic
interactions. In this process, the sulfated CNCs were modified
with hydrophobic moieties such as octyl groups. These octyl-
CNCs form significantly stronger gels at lower concentrations
than parent CNCs. Atomic force microscopy (AFM) studies
revealed favorable interactions between remnant starch and
octyl-CNCs, which is thought to be the source of the dramatic
increase in gel strength.
605
They also harnessed these hydro-
phobic interactions to assemble water-soluble macromolecules
and nanoparticles into a transient hybrid network, forming
thermosensitive hydrogels with tunable rheological proper-
ties.
606
3.3.1.2. Matrices and Carriers for Biological and
Pharmaceutical Entities. As explained in section 1, water
holds its pronounced role of being favorably involved in
cellulose functions in nature along the dimensional hierarchy
from macroplant down to nanofibers. Water is of paramount
importance for the bioactivity in plant cell wall and plant growth.
The physiological and biomechanical properties of lignocellu-
lose are strongly influenced by the interaction of water with the
biopolymer components within cell wall ultrastructure.
610−613
Similar nanocellulose−water interactions, for example, freezing
bound water mediated between the biopolymer matrix, and the
surrounding water, contribute to the biocompatible function-
ality in several applications of nanocellulose outside their natural
occurrence.
614,615
As a result, nanocellulose has been applied in biomedical
fields, such as drug delivery and tissue engineering (Figure
10a,b).
406,581,616−620
In particular, nanocellulose-based hydro-
gels have attracted enormous interests to be utilized as a
biocompatible substrate via dierent engineering technologies
due to structural similarity with collagen.
621,622
A large category
of these systems are the hybrid hydrogels of nanocellulose and
hydrophilic natural polymers such as hemicelluloses,
623−632
pectins,
633−636
lignin,
624,637−640
chitosan,
641−653
algi-
nate,
575,608,654−666
and gelatin.
661,667−673
Whereas CNCs have
been mostly used in hybrid hydrogels (Figure 10b),
27
CNFs find
wide-reaching applications as single-component systems,
especially in the biomedical realm covering cell cultures, drug
release, tissue engineering, and wound healing.
9,406
Their high
water content is an essential prerequisite for biocompatibility,
and their nanostructure, porosity, and tunable mechanical
properties can oer a biomimetic environment for cell
immobilization and cell support.
674,675
Moreover, network
flexibility, porosity, and water content enable diusivity, i.e.,
the uptake, transport, and release of low- and high-molecular-
Figure 9. Summary of nanocellulose−water interactions in hydrogels.
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X
Figure 10. Nanocellulose−water interactions in hydrogels for biomedical applications (a,b), sensors (c), and actuators (d,e). (a) Schematic
representation of an injectable CNC−poly(oligoethylene glycol methacrylate) (POEGMA) nanocomposite hydrogel with aligned CNCs directing the
dierentiation of skeletal muscle myoblasts into oriented myotubes in situ after culture for 8 days.
607
(a) Adapted with permission from ref 607.
Copyright 2017 American Chemical Society. (b) SEM images of aerogels produced by drying magnetic CNC and alginate hydrogel beads (i) 0%
magnetic CNC (A0), (ii) 4.7% magnetic CNC (A1), (iii) 11.1% magnetic CNC (A3), (iv) 20% magnetic CNC (A6), (v) 33% magnetic CNC (A10),
and (vi) time profile of ibuprofen release from alginate hydrogel beads. The presence of magnetic CNCs improves the physical and mechanical
properties of the alginate hydrogel beads, increasing the swelling degree in water, and decreasing the rate of drug release.
608
(b) Adapted with
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Y
weight compounds, as exploited in controlled drug delivery
(Figure 10.
676−680
Besides hybrid hydrogels and single-component bulk CNF
hydrogels, membranes, films, and microbeads based on swollen
Figure 10. continued
permission from ref 608. Copyright 2018 Elsevier. (c) Flexible and responsive chiral nematic cellulose nanocrystal/poly(ethylene glycol) composite
films as humidity sensors.
609
(c) Adapted with permission from ref 609. Copyright 2018 John Wiley and Sons. (d) Cyclic bending and recovery of a 90
mm diameter and 23 μm thick graphene/CNF nanopaper (15.2% graphene) upon exposure to two human breaths at 0 and 127 s. (e) Proposed folding
mechanism of graphene/CNF nanopaper. Graphene flakes and CNFs in the nanopaper are held together by hydrophobic interactions among graphene
flakes and hydrophilic and hydrogen bonding among CNFs. Upon exposure to moisture, the distances between CNF−CNF expand as hydrophilic and
charge surface groups on CNFs become ionized by water. (d,e) Adapted with permission from ref 393. Copyright 2009 The Royal Society of
Chemistry.
Figure 11. Nanocellulose−water interactions in anisotropic nanostructuring: (a,b) Schematic representation of CNC alignment while exiting the
printer’s nozzle,
705
(c) directional swelling of the material with aligned CNCs in water, used to create controlled water response.
705
(a−c) Adapted
from ref 705 under the terms of CC-BY. Copyright 2021 Elsevier. (d,e) In situ polarization rheology to study shear induced CNC alignment.
704
(d,e)
Adapted from ref 704. Copyright 2018 American Chemical Society. (f) SEM images of aerogels cross-section (the XY-plane perpendicular to the ice
crystal-growth direction) with morphologies ranging from fibrillar (F) to columnar (C) to lamellar (L) and their combinations, dependent on A-CNC
(aldehyde-functionalized cellulose nanocrystals):H-POEGMA (hydrazide-functionalized poly(oligoethylene glycol methacrylate)) weight ratio and
A-CNC+H-POEGMA concentration. Scale bars are 20 μm. (right) Aerogels cast in cylindrical molds.
706
(f) Adapted with permission from ref 706.
Copyright 2016 American Chemical Society. (g) (left) Schematics showing the formation of a selectively aligned CNF film on a substrate. (right)
Photographs of the CNF deposition with a few carbon nanotubes (CNTs) as a visible tracer on top of a transparent PET film.
707
(g) Adapted with
permission from ref 707. Copyright 2019 Elsevier. (h) Inducing nematic ordering of cellulose nanofibers using osmotic dehydration, images of the
CNF suspensions with dierent concentrations between cross-polars.
121
(h) Adapted from ref 121 under the terms of CC_BY. Copyright 2018 The
Royal Society of Chemistry.
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Z
CNF networks, that is, somewhat geometrically constrained
hydrogels, have been investigated in this context.
681
For
example, the eects of nanocellulose charge density and fibril
size on the mechanical properties of the films in liquids were
investigated. Swelling behavior of the films was studied in
deionized water, in complete cell culture medium (DMEM),
and in CaCl2solutions. Cell culture media and CaCl2solutions
reduced the swelling of the films observed in deionized water,
most probably due to a bridging eect (physical cross-linking)
by the calcium ions. The reduction was proportional to the
charge of the nanocellulose. The possibility to tune the softness
of a surface by the level of oxidation can potentially be a way to
influence cell behavior through mechanical cues.
682
An
emerging application of CNF-based hydrogels is support for
microalgae with the aim of creating the next generation of solid-
state photosynthetic cell factories. The combination of trans-
parency, hydrophilicity and water retention, biocompatibility,
good mechanical properties, and appropriate porosity ensures
better performance than the alginates traditionally used as
substrate.
683,684
Furthermore, controlling water content and
thereby binding of water on nanocellulose enables the
manipulation of the diusion of components within the
hydrogel.
314
This permits the regulation of diusion of the
photosynthetic products inside and out of the cell factory matrix.
3.3.1.3. Anisotropic, Responsive, and Smart Materials. The
specific response of nanocellulose to water is the direct
functional foundation of a vast area of materials applications
including humidity sensors and actuators. Precise tuning of the
nanocellulose water-response, chemically or by physical means,
enables, e.g., ultrahigh network swelling or water-response
gradients in the nanoscale. As an externally eective trigger,
water can be harnessed by sorption and desorption with
cellulose chains bringing up multiscale movements for
sensing.
685,686
3.3.1.3.1. Water Responsive Materials. The nanocellulose−
water interactions have been used as a tool to tune mechanical
properties. Capadona et al. developed responsive CNC-
reinforced composites where the formation and disruption of
a percolating CNC network was selectively and reversibly
modulated via a response to water as a trigger. DMA analysis
showed that the observed changes in modulus, elongation at
break, and tensile strength are the result of switching o the
cellulose−cellulose interactions by water molecules and not a
simple result of a plasticizing eect. The authors concluded that
the stiness reduction achieved in the rubbery ethylene oxide−
epichlorohydrin 1:1 copolymer nanocomposite is related to the
decoupling of the stress-transferring rigid CNC network by
water molecules as they competitively make hydrogen bonds. As
a result, this switching is fully reversible, and the nanocomposite
reverses to its original stiness upon drying.
687
Annamalai et al.
studied the incorporation of cellulose nanocrystals into soft
hydrophobic styrene butadiene rubber matrices to create water-
responsive mechanically adaptive nanocomposites. In the dry
state, all nanocomposites show higher tensile storage moduli
than the neat styrene−butadiene rubber (SBR) or the SBR latex.
Upon submersion in deionized water, a dramatic reduction of
modulus was observed, which was connected to disengagement
of the percolating CNC network due to mostly competitive
hydrogen bonding of water molecules with the CNCs (solvation
eects in the hydrophilic groups play also a role).
688
Zhu et al.
reported reversible formation and disruption of a CNC
percolation network in an elastomeric thermoplastic polyur-
ethane (PU) matrix that ultimately led to a rapidly switchable
shape-memory eect (SME) activated by water. The researchers
concluded that a combination of chemomechanical adaptability
of the CNC percolation network and the entropic elasticity of
the PU facilitates shape fixity for temporary deformation in the
dry state and shape recovery in the wet state.
689
Furthermore, hybridization with other natural or synthetic
(macro)molecules (e.g., polymers, peptides) or nanoparticles
has been a popular strategy to introduce multifunctionality or
stimuli-responsive properties into nanocellulose-based hydro-
gels.
690
The application prospects of these hybrids or multi-
component hydrogels are broad including, e.g., bioactive tissue
scaolds,
596,655,670
ophthalmics,
691,692
self-healing materi-
als,
693,694
high water containing fertilizers,
695
and fingerprint
detectors.
696
The hydrogel network structure, in nanometer and
micrometer scale, defines wetting characteristics, capillarity, and
water diusion. Moreover, the spatial orientation and
distribution of (nanocellulosic) elements decides whether the
network swells isotropically or in an anisotropic fashion, the
latter paving the way toward swelling gradients and controlled
movement. Several techniques exist for precise 3D design of
hydrogel network structures, among them, 3D printing as
probably the most popular one.
3.3.1.3.2. Alignment, Anisotropy, 3D, and 4D Printing. The
transformation of digital design to on-demand manufacturing
has been one of the predominant trends within the past decade
and has established new technologies entering the market.
697
Especially in the biomedical realm, additive manufacturing
enables a customized design of, e.g., biomimetic tissues via
tailoring the macroscopic hydrogel structure.
622
CNC and CNF
hydrogels inherently bear shear-thinning properties, which is a
prerequisite for direct ink writing (DIW), the 3D printing
technology of choice for hydrogels.
622
Accordingly, the shear
during extrusion induces an alignment of the nanoparticles
introducing nanoscale anisotropy into the network (Figure
11a,b).
698,699
This anisotropy is not only a very relevant attribute
from a mechanical, thermal, and cell functional point of view,
e.g., guiding cell dierentiation in hydrogel-based biotissue
mimics,
9,700
but also in terms of directing the networks’ water
response. Gladman et al. for instance, exploited the alignment
during DIW of CNF embedded in a soft acrylamide matrix
during DIW for the design of bioinspired shapes with
anisotropic swelling characteristics, converting water-response
gradients into a controlled mechanical movement.
701
Gradient
swelling dynamics of the shapes, thus, introduce a fourth
dimension, and the transition of 3D to 4D printing has become
an emerging trend in the realm of stimuli-responsive or
multifunctional hydrogel systems with prospects, e.g., in soft
robotics or biomedical devices (Figure 11d,e).
702
In the case of
CNC hydrogel inks, the alignment of the nanocrystals during 3D
printing has been observed by a strong birefringence, confirming
the anisotropy in the printed scaolds (Figure 11a,b).
703
Hausmann et al. made an attempt to understand the interplay
between the concentration of CNC suspensions, the applied
shear stress, and the dynamics of particle alignment using in situ
polarization rheology.
704
They showed that the precise adjust-
ment of shear, extensional flow, and printing nozzle geometry
can eectively tune the CNC orientation from full alignment to
core−shell architectures (Figure 11a).
Freeze-casting is another way toward structural anisotropy
(more explanation in section 3.3.3.1) (Figure 11f).
706,708,709
Chau et al., for instance, prepared CNC-based hydrogels,
starting from fabricating aerogels by simultaneous freeze-casting
and cross-linking.
706
The morphology of these aerogels, ranging
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from fibrillar, columnar, or lamellar, was thereby tailored by
precisely controlling the composition of a CNC/poly-
(oligoethylene glycol methacrylate) precursor dispersion and
the freeze-casting temperature. De France et al. used a strong
magnetic field for the in situ alignment of CNC/poly-
(oligoethylene glycol methacrylate) nanocomposite hydrogels,
which were simultaneously cross-linked via rapidly forming
hydrazone bond formation, a promising approach toward
injectable, anisotropic hydrogels for in vivo tissue engineering
(Figure 10a).
607
Another way to introduce anisotropy into
nanocellulose suspensions was proposed by Guccini et al.
121
The authors obtained induced nematic order in CNF
suspensions over a wide CNF concentration range (0.5−4.9
wt %) by osmotic dehydration (Figure 11h). Figure 11
summarizes the attempts to prepare anisotropic nanostructures
from nanocellulose.
3.3.1.3.3. Actuators, Robotics, and Sensors. The controlled
movement of hydrogels, realized via aligning the cellulose
nanoparticles, as demonstrated by Gladman et al.,
701
has an
exceptional potential for hydrogel-type actuators, soft robotics,
or sensor materials (Figure 10c−e). Nanocellulose-based
biomimetic actuators that combine hydrophilicity gradients in
the material with exploiting the fast dynamics
710
of hydration
have experienced an increasing popularity.
711−713
Kuang et al.,
for instance, showed that CNFs align selectively upon solvent
evaporation (Figure 11g). They used this self-assembly behavior
to design extremely strong (∼1000 times lifting weight ratio)
actuators with reversible shape-morphing properties upon
hydration and dehydration.
707
Zhu et al. used CNCs as active
coating on poly(vinyl alcohol-co-ethylene) substrates driving
humidity-induced actuation.
714
These actuators could bend or
twist depending on the CNC alignment direction. In a dierent
example, Wang et al. showed that also asymmetric exposure of
CNF thin films to water vapor can lead to controlled movement
governed by the humidity dierence across the film.
715
Xu and
Hsieh transformed the aqueous exfoliated graphene by
amphiphilic nanocellulose into moisture-responsive foldable
actuators, as discussed in section 2.4.3. The CNF/graphene film
was easily obtained by vacuum filtration into nanopapers that
exhibited rapid moisture triggered motion attributed to the
highly accessible, charged CNF surfaces.
393
Exploiting the
responsiveness of nanocelluloses to water, CNCs and CNFs
have been further explored as 1D, 2D, and 3D scaolds for
sensor applications,
716−718
some of them for humidity sensing.
Kafy et al., for instance, designed humidity sensors based on
homogeneous CNC/graphene oxide (GO) composite films, in
which changes of the surrounding relative humidity stimulated a
change of the composite’s relative capacitance.
719
Similarly,
Solin et al. used a hybrid film of carbon nanotubes and CNF to
detect humidity by a means of altered electrical resistivity due to
accumulating water in the conductive film.
720
Another principle
toward nanocellulose-based sensors relies on the reversible
swelling upon moisture contact and dehydration of chiral
nematic CNC films coassembled with poly(ethylene glycol)
609
or treated with N-methylmorpholine-N-oxide solution. This is
accompanied by a color change, thus allowing easy detection of
humidity changes (Figure 10c).
609,721
Also exploiting the
sensitivity of CNCs to moisture, Sadasivuni et al. developed a
proximity sensor based on a graphene oxide-modified CNC
(CNC/GO) sprayed in layers on polymer substrates bearing
interdigitated electrodes.
722
This setup oered a controlled
proximity sensitivity without physical contact, which was
investigated by measuring the current−voltage−resistance of
the samples. The key for this application was the sorbed water on
the CNC surface serving as an electron donor at low relative
humidity, which changed the film resistance. At high relative
humidity, the ionization of water to H3O+ions contributed to
the overall conductivity of the material. Additionally, the
presence of CNCs in the composite improved the film structure
and spatial resolution as an important structural prerequisite for
a fast sensor response and signal recovery.
3.3.1.4. Mechanical Load Bearing Hydrogels. Although
nanocellulose hydrogels have water in their structure, the mere
measurement of their mechanical properties does not give too
much information on water interactions, and these studies will
not be covered here.
9,583,594,596,723−728
In many of these cases,
Figure 12. Summary of nanocellulose−water interactions in barrier films.
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Chem. Rev. XXXX, XXX, XXX−XXX
AB
nanocellulose was simply added to an existing hydrogel
formulation to tune their water holding capacity (refs 641,
654,655,658,691,692,729−735).
3.3.1.4.1. Turning Mechanical Properties of Hydrogels with
Water. Water certainly plays an ambiguous role in the
mechanical properties of nanocellulose materials. Structurally,
with a trace amount of water, the crystallization of cellulose was
increased with ordered molecular arrangement in disordered
regions.
736,737
Similar to other biopolymers, such as chitosan
and collagen, the molecular packing of natural cellulose takes a
more ordered structure in the presence of water.
736,738
Consequently, the mechanical strength of cellulose increases
in a characteristic amount of water. In a CNF hydrogel, water is
not only considered a biocompatible element, but it also acts as a
cross-linker for holding the gel integrity,
739,740
up to a certain
level after which the water is only loosely retained within the
CNF network and not contributing to the swell of the nanofiber
network.
314
Beyond cellulose−cellulose adhesion, water can be
essential for nanocellulose adhesion to other materials for
producing a certain cellulose nanocomposite or a reinforced
thermoplastic.
274
However, water has also weakening eects on
the nanocellulose hydrogels. It may cause the deterioration of
mechanical strength over time by weakening cellulose−cellulose
adhesion within the gel.
204,687,688
At lower water contents,
moisture can be used for a plasticization eect that enables the
bending of wood material without breaking it.
741,742
The same
principle is utilized in the production of mechanical pulps from
wood chips or logs.
743,744
3.3.2. Films, Membranes, Textile, and Coatings. From a
technical point of view, many films, membranes, textiles, and
coatings are categorized as either hydrogels, aerogels, or colloids
and could be discussed in sections 2.4.3,3.3.1, and 3.3.3. Hybrid
materials made by combining cellulose fibers and nanocellulose
belong to this group as well. In the following section, we focus
mainly on permeability and barrier properties of these materials
and their application in packaging, filters, textile industry, and
energy related and electronic devices with respect to water. As
mentioned before, the availability of a large number of hydroxy
groups in cellulose causes high water vapor adsorption and
therefore increasing water vapor permeability and poor barrier
properties.
298
The eect of moisture on the mass transport of
water within cellulosic materials is among the most crucial
questions when the industrial use of (nano)celluloses is
concerned. These eects are of importance in applications
such as barrier films,
745
absorbents,
746,747
wound care
products,
748
biomedical materials,
692
as summarized in Figure
12.
3.3.2.1. Cellulose Nanopapers. Films of cellulose nanofibers
are commonly known as cellulose nanopapers.
749,750
Because of
the high surface area of CNF and the density of the nanopaper
network, the mechanical strength of air-dried nanopaper is
outstanding, running up to 200 MPa in tensile strength.
751,752
Contrary to conventional paper, nanopaper can also be
transparent due to the nanoscale width of the CNFs.
753
The
proposed applications of nanopaper range from advanced
packaging solutions to electronic supports.
Perhaps the most well-known eect of water on cellulosic
materials is demonstrated when water accumulates on relatively
dry networks, such as paper or nanopaper, whereupon an
imminent strength loss is encountered.
754,755
In technical terms,
this represents a poor hygromechanical stability of the
nanopapers. The mechanical properties of nanopapers depend
on mastering structure formation processes and understanding
interfibrillar interactions as well as deformation mechanisms of
cellulose nanofibrils in bulk. Benitez et al. showed how dierent
dispersion states of cellulose nanofibrils and dierent relative
humidity values influence the mechanical properties of these
nanopapers. The materials undergo a humidity-induced
transition from a predominantly linear elastic behavior in dry
state to films displaying plastic deformation due to disengage-
ment of the hydrogen-bonded network and lower nanofibrillar
friction at high humidity. A concurrent loss of stiness and
tensile strength of 1 order of magnitude was observed, while
maximum elongation stayed near constant. Multiple yielding
phenomena and substantially increased elongation in strongly
disengaged networks, swollen in water, show that strain at break
in such nanofibril-based materials is coupled to relaxation of
structural entities, such as cooperative entanglements and
aggregates, which depend on the pathway of material
preparation. The results demonstrate the importance of
controlling the state of dispersion and aggregation of nanofibrils
in water by mediating their interactions and highlight the
complexity associated with understanding hierarchically struc-
tured nanofibrillar networks under deformation.
756
3.3.2.1.1. Tuning Material Structural Integrity Using Water.
Integrity of nanocellulose materials, which is perceived to play a
role in mechanical disintegration of nanocellulose materials
upon wetting is also strongly aected by water. Swelling is a
detriment in many composites where the dimensions and the
structure should be preserved, but it can be a benefit in
applications such as 4D printing or biodegradable materials,
where interaction with water and the change in shape is
expected.
When it comes to preparation of flexible CNC chiral nematic
films with optical and sensor applications, structural integrity
and stability in water is important. Strategies such as cross-
linking can limit swelling of the organized chiral nematic films
and improve their structural integrity and stability in water and
facilitate their use as freestanding template substrate for
conducting polymers or metal oxides to form flexible chiral
nematic photonic hybrids.
757
A crucial challenge in characterization of nanocellulose
materials is the lack of any standard practice on how to take
the swelling into account when reporting mechanical properties
at dierent relative humidity levels or when measuring the
mechanical properties of fully hydrated materials, which limits
comparisons between dierent studies. Walther et al. reviewed
the current approaches and proposed a potential best practice
for measuring and reporting mechanical properties of wet
nanocellulose-based materials, highlighting the importance of
swelling and the correlation between mechanical properties and
volume expansion.
758
3.3.2.1.2. Improving Humid/Wet Mechanical Properties.
Tremendous eorts have been taken to minimize the adverse
eect of water on mechanical properties. Most of the studies
with focus on improving mechanical properties of nanocellulose
materials measure the strength, modulus, or ductility to assess
the mechanical performance of the material in humid or wet
state in comparison to dry state to confirm that the applied
modification or blending techniques have improved hygrom-
echanical stability. In this regard, the focus of these studies is
mostly on the blending or modification they used to obtain
waterproof or water-resistant materials rather than interactions
of water and nanocellulose. Addressing these cases one by one is
out of the scope of this review (some of them have been
mentioned in section 3.1), and we refer the readers to the
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Chem. Rev. XXXX, XXX, XXX−XXX
AC
original publications for details.
444,759−761
Here we focus on
studies that provide deeper analysis of the interaction with water
in their method to improve the hydromechanical stability.
Osterberg et al. presented a rapid method to prepare robust,
solvent-resistant, CNF films that can be further surface-modified
for functionality by hot pressing the films. Drying of the films
using high pressure and heat resulted in a film with good
resistance to solvents. The films could be soaked in both polar
and nonpolar solvents (including water) for more than 18 h.
They swelled considerably in the solvents. However, their wet
strength remained high, and they were easy to handle in the wet
state.
760
The enhanced properties are due to a decrease in the
film porosity, which restricts the solvent diusion through the
film. In addition to the densification of the film, the hot-pressing
aects the hydroxy groups at the surface, further restricting
solvent penetration. This system results in structure-controlled
hydrogels upon wetting that limits swelling and therefore the
loss of mechanical properties by excessive water penetration.
760
Shimizu et al. prepared TOCNF films, dried them, and soaked
them in aqueous MgCl2, CaCl2, AlCl3, and FeCl3solutions to
change the counterion and form TOCNFs−COOM films. Dry
TOCNF−COOM films showed high Young’s moduli and
tensile strength. They found out that TOCNF films with
aluminum and iron(III) carboxylates showed good mechanical
properties in the wet state. These results are explained in terms
of the high water resistance of the films, which is caused by the
formation of interfibrillar electrostatic cross-linkages through
multivalent metal ions, limiting swelling and consequent loss of
structural integrity.
762
Wang et al. pursued the design of ordered
hard/soft nanocomposite structures with balanced supra-
molecular interactions for biomimetic applications. They
established a drying procedure that induces a high orientation
Figure 13. Nanocellulose−water interactions in films in packaging (a,b) and energy related applications (c): (a) Oxygen and water vapor permeability
of untreated and thermally treated (100, 125, 150, and 175 °C for 3 h) CNF films,
798
(a) Adapted with permission from ref 798. Copyright 2011 The
Royal Society of Chemistry. (b) Oxygen permeability as a function of relative humidity for CNF films after varying thermal treatment times (0.5, 1, and
2 h) at 100 °C.
760
(b) Adapted with permission from ref 760. Copyright 2013 American Chemical Society. (c) Electricity generation mechanism of
biological nanofibrous generator: (i) Typical polarized photo of nematic CNF dispersion and TEM images of TEMPO−CNFs,
799
(ii) optical and
SEM images of aerogel fabricated by directionally freezing the CNF dispersions,
799
(iii) schematic illustration of hydrated channels around and
between TEMPO−CNFs with the carboxyl content of ≈1.39 mmol g−1and the thickness of 2 mm in a nanogenerator.
799
(iv) Open circuit voltage
(Voc): (1) Generated by biological generators from dierent CNFs, (2) variation upon dierent relative humidity values of the air flow (flow velocity:
15 cm s−1), (3) variation upon dierent carboxyl contents of TEMPO−CNFs, (4) variation upon dierent air flow directions.
799
(c) Adapted with
permission from ref 799. Copyright 2019 John Wiley and Sons.
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AD
of CNCs in a matrix of carboxymethylcellulose (CMC) at high
level of reinforcements (50 vol %). They showed alignment in
thick bulk films and reported synergetic improvement with a
simultaneous increase of stiness, strength, and work of fracture
as a function of the degree of alignment. They showed that the
decline in the mechanical properties of such waterborne
biobased nanocomposites at high relative humidity can be
canceled out by using supramolecular modulation of the ionic
interactions by exchanging the monovalent Na+counterion,
present in CMC and CNC, with Cu2+ and Fe3+. This leads to a
synergetic improvement of the mechanical properties at 90%
relative humidity.
763
Ansari et al. reported on an interface tailoring route to prepare
PEG-grafted CNF to address hygromechanical instability.
Modified CNF nanopaper shows significantly improved
mechanical properties under moist and wet conditions. Fracture
surfaces of CNF films soaked in water showed distinct layers and
fibrils pulled out in bundles, while modified CNFs showed a
dense structure due to polymer grafting, protecting structural
integrity and maintaining mechanical properties in water.
444
Hakalahti et al. bonded TEMPO/NaClO2oxidized CNF with
poly(vinyl alcohol) (PVA) covalently to render water stable
films. Pure CNF films and CNF−PVA films in the dry state
showed similar humidity dependent behavior in the elastic
region, while in wet films PVA had a significant eect on the
stability and mechanical characteristics of the films. Influence of
the amount and the degree of hydrolysis of PVA on the
mechanical properties of the films were also investigated.
759
Duran et al. partially modified CNFs by chemical means to
create a shell of derivatized cellulose that surrounds the
crystalline core of native cellulose. Through the dierent
modifications, they aimed at creating a toolbox to tune the
properties of CNF materials for specific demands. In total, nine
dierent chemical modifications using dierent aqueous-based
procedures were used as chemical pretreatments before CNF
production through homogenization. Films produced from the
dierent CNFs were mechanically tested, and it was found that a
combination of periodate oxidation and borohydride reduction
resulted in a high strain-at-break. The presence of carboxylic
acids led to an increase in tensile strength and Young’s modulus,
but a decrease in strain-at-break was also observed. The
introduction of aldehydes, resulted in brittle films, but also a
decrease in the moisture sorption rate, while the modulus even
at high relative humidity was maintained.
445
Zhang et al. suggested an approach for designing a water
resistant, assembled nanopaper through controlled and
irreversible aqueous complexation of oppositely charged
cellulose nanomaterials. They produced cationic cellulose
nanocrystals and tempo oxidized anionic cellulose nanofibers
and adjusted the features of the nanopaper by altering the
cationic CNC/anionic TOCNF ratio. the water resistance and
water vapor permeability of obtained NC complexed nano-
papers were improved. The full charge neutralization of
oppositely charged NCs created a water-stable nanopaper with
a wet strength of 11 ±3 MPa after immersion in water for 24
h.
764
Some scientists utilized nature inspired designs to waterproof
nanocellulose films. For example, inspired by plant epidermis,
Heredia-Guerrero et al. sprayed a cutin like coating, aleuritic
acid, a polyhydroxylated fatty acid, on CNC films and
polymerized the monomers by hot-pressing. Measurement of
Young’s modulus and hardness and water uptake and water
vapor transmission rate indicated that this design enhances the
robustness and waterproof behavior of CNC films.
765,766
3.3.2.2. Films and Coatings as Barriers. 3.3.2.2.1. Liquid vs
Gas Barrier in Nanopapers. As explained in section 2.4.2.4,
transportation of gases and liquids in nanocellulose materials
proceeds dierently, and it is easier to block the flow of liquid
water with its high cohesion and surface tension than that of
individual water molecules in vapor phase. Water has generally
detrimental eects on the barrier properties of nanocellulose. In
the context of cellulose nanopapers, the intention is often to
produce a barrier film, and the focus of the research field has
largely been on how to improve the barrier properties in moist
conditions.
745,755,767−771
However, this has been proven to be a
dicult task due to the tendency of cellulose to interact with and
rearrange itself in the presence of moisture.
210,245,772−777
Nanopapers exhibit excellent oxygen barrier properties in dry
state, and they have been candidates for food packaging for a
long time.
778
However, the strong hygroscopic character of the
nanofibers limits their use in environments with high relative
humidity (Figure 13a,b). Intercalation of water molecules
between cellulose chains weakens the interfibrillar bonds
between adjacent nanofibers, which leads to a decrease of the
films’ barrier properties.
755,779,780
Therefore, a common
approach to use nanocellulose materials for packaging is to
modify or blend them to increase the hydrophobicity of the
surface of the material.
766,781−794
Even some studies reported
selective gas separation properties of these materials.
753,795−797
3.3.2.2.2. Strategies to Improve Nanocellulose Barrier
Properties. Nanocellulose materials have been modified in
dierent ways to make their barrier properties more resistant to
moisture, (mentioned earlier in section 3.1.1). Polyethylenimine
(PEI) surface functionalization, silylating, cross-linking with
polyamide epichlorohydrin resins, cross-linking with chitosan,
and compounding with montmorillonite are among numerous
strategies that have been used to improve gas barrier properties
at high relative humidity.
3
Rodionova et al. carried out
heterogeneous acetylation of microfibrillated cellulose to modify
its physical properties, specifically improving barrier properties
and at the same time preserving the morphology of cellulose
fibrils for application in packaging.
800
Kubo et al. mixed aqueous
dispersions of CNF with sodium counterions (CNF-COONa)
and CNFs with tetraethylammonium counterions (CNF-
COONEt4) with various weight ratios. CNF-COONa/NEt4
films were prepared by casting and drying with dierent Na/
NEt4molar ratios. The film density, Young’s modulus, tensile
strength, hydrophilic properties, and oxygen and water vapor
permeability could be tuned by controlling the Na/NEt4molar
ratios in the films by the eect of bulky versus small
counterion.
529
However, it is important to note that mere bulk
hydrophobization of the films is usually not enough to improve
gas barrier properties. Generally, ecient water vapor barriers
have not been achieved after bulk hydrophobization treatments
that render the nanopapers nonwettable by liquid water.
801
For
example, chemical esterification of CNF with the aim of
reducing bulk hydrophilicity and producing hydrophobic
cellulose nanopaper with reduced moisture sensitivity has
resulted in a decrease in barrier performances toward oxygen
and water vapor.
755,779,780
So far, the most ecient ways to block
vapor transmission have been reported to include a (con-
tinuous) hydrophobic layer on top of the nanopaper,
801−803
such as a applying a coating layer of hydrophobic cellulose via
hornification or a coating layer of other hydrophobic polymers
such as wax via deposition.
804,805
In the context of food
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AE
packaging the traditional strategy is to employ cellulosic
materials in multilayered or sandwiched systems, which are
eective due to isolation of the cellulosic materials from the
humid environment and their protection from the eects of
moisture.
760,806−812
CNFs can also be applied as coatings to other cellulosic
materials to improve barrier properties. Yook et al. prepared
various types of CNFs coated on linerboard and wood-free
paper to evaluate the barrier properties of these papers against
air, liquid water, water vapor, oxygen, and grease. The average
fibril size and hydrophobicity were strongly related to the barrier
properties. CNFs with smaller fibril sizes and hydrophobized
CNFs improved the water resistance. Air resistance and oxygen
barrier properties and grease resistance were related to the
average fibril diameter of CNFs.
813
3.3.2.3. Filters, Membranes, Textiles, and Absorbents. In
the context of liquid sorption and permeability, the interactions
between nanocellulose and water have been used to design
materials as filters and membranes. There are numerous
literature accounts on the application of nanocellulose materials
as membranes. Besides the often-touted biobased “sustain-
ability” aspect of nanocellulose, their more hydrophilic nature in
comparison to common synthetic hydrophobic materials used
for membranes, improves their resistance against fouling to
some extent, not to mention that regenerated cellulose has been
used for over a century to produce membranes and filters for
dierent applications, including water treatment.
2
Nanocellulose may increase the hydrophilicity of hybrid
membranes to improve water diusion, but this may reduce their
mechanical properties. For polyethylenimine (PEI)-CNC films,
for example, the Young’s modulus at relative humidity values of
30%, 42%, and 64% were found to be 16, 12, and 3.5 GPa,
respectively.
814
If the polymer that nanocellulose is mixed or
modified with is highly hydrophilic, such as starch-based
polymers, the nanocellulose actually decrease water sorption
and diusion. Using inherently hydrophilic polymers prevents
the reduction of the mechanical properties but reduces the water
flux through the nanocellulose membrane, simultaneously.
Therefore, there seems to be a trade-o between the flux and
mechanical strength of nanocellulose composite mem-
branes.
64,815,816
Network diusivity, combined with oering sorption sites for
pollutants, are key for the use of nanocellulosic absorbent filters
in environmental engineering applications
816,817
and oil and gas
production.
818
Secondary surface modification, cross-linking, or
hybridization with other natural or synthetic polymers enables
pollutant-specific membrane designs, with enhanced sorption
capacities and rates, as well as mechanical and structural
integrity, e.g., to withstand water flow for an extended
time.
4,450,819−821
The sorption of organic contaminants from
water has also been demonstrated with modified nanocellulose
matrices. The inherent hydrophilicity of nanocelluloses should
be reduced to improve the anity of the material for
hydrophobic compounds. Increasing the hydrophobicity for
this purpose has been achieved by inclusion of both organic and
inorganic functionalities. Xiong et al., for example, designed
flexible and multifunctional CNF membranes, coated with
titanate−bismuth oxide for the synergistic treatment of anion/
cation-containing oily water.
822
In another study, atomic layer
deposition of TiO2nanoparticles onto the surface of nano-
cellulose aerogels created a low-energy surface on the fibers to
yield a hydrophobic and oleophilic material that can absorb oil
and a variety of organic solvents from the surface of water at a
capacity of 80−90% vol/vol.
823
Similarly, depositing triethoxyl-
(octyl)silane or poly(dimethylsiloxane), or freeze-drying
methyltrimethoxysilane onto CNF aerogels resulted in even
higher sorption capacities.
512,824,825
Graphene oxide/nano-
cellulose composites and poly(dopamine)/BC membranes
also showed good sorption capacities.
816,826,827
A distinctive field of applications of nanocellulose that might
require water repellence in some cases is the textile industry.
Colloidal dispersions of CNFs are good alternatives to
dissolution/regeneration into cellulose II in filament spinning.
Cunha et al. investigated the eects of postmodification of wet-
spun CNF filaments via chemical vapor deposition of organo-
silanes with dierent numbers of methyl substituents. Various
surface structures such as continuous and homogeneous coating
layers, or three-dimensional and hairy-like layers, reduced the
surface energy, which significantly aected the interactions with
water. Mechanical testing revealed that the wet strength of the
modified filaments were almost 3 times higher than that of the
unmodified precursors, while the final product maintained the
moisture buering capacity and breathability.
828
3.3.2.4. Energy Storage Devices. Nanocellulose-based
mesoporous structures, flexible thin films, fibers, and networks
have been often used in photovoltaic devices, energy storage
systems, mechanical energy harvesters, binders, separators,
structural supports, and catalyst supports, demonstrating the
potential of this material in several energy-related fields.
3,829−842
The presence of surface-adsorbed water opens completely new
perspectives for nanocelluloses in electronics.
3.3.2.4.1. Aqueous Ion Batteries. Nanocellulose attracted
numerous interests in serving as a promising building block for
electrolyte wettable and thermally resistant separators as an
alternative to synthetic polymers in gel polymer and solid
polymer electrolytes in diverse rechargeable battery systems
including established Li-ion and Li−S to next generation Na-ion,
K-ion, and Zn-ion systems.
842,843
Nanocellulose is also capable
of inducing an increase in viscosity of electrolytes solutions even
at minuscule concentrations. High porosity oered by nano-
cellulose network as electronically insulating physical barrier is
desirable in separator design to ensure good electrolyte
retention and enhanced ionic conductivity as well as prevent
internal short-circuit. Nanocellulose also holds a prevalent role
of enabling good anity for liquid electrolytes ensuring
preferential interaction with the salt anion to improve salt
solubility and cation transference in the electrolyte. Mittal et al.
combined CNFs and CNCs constructed a mesoporous
hierarchical structure as gel polymer electrolytes, ensuring a
close contact with metallic Na outperforming conventional
fossil-based separator in sodium ion batteries.
844
However, the
detrimental eect of water on mechanical properties of
nanocellulose networks used in energy storage applications,
the eect of water on degradation of lithium salts in Li-ion
batteries (LIBs) is a concern. Dierent strategies have been
reported to address this issue and unlock the potential that
nanocellulose oers in terms of mechanical properties, environ-
mental benignity, and versatility. The manufacturing process of
CNFs is essential to improve the performance of CNFs as both
an electrode component and a separator. Flexible paper
electrodes were also simply obtained using as low as 4 wt %
CNFs (prepared by TEMPO-mediated oxidation) as binder.
845
Such low amount minimizes the moisture introduced to the cell
and increases the capacity of the electrodes by total weight. A
further study by Lu et al. investigates the optimal processing
parameters to enhance the cathode in terms of mechanical and
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AF
electrochemical performance, achieved using high surface
charge (ca. 1.5 mmol g−1) and low defibrillation degree.
845
Using a facile paper-making method, Kim et al. assembled CNFs
prepared by TEMPO-mediated oxidation into asymmetric
mesoporous separators that represented a more environmentally
benign approach compared to conventional fossil fuel-derived
materials.
846
Furthermore, the performance of CNF based
separators were crucially improved by substituting the native
Na+ions of the carboxylate groups and adding 2 wt % of vinylene
carbonate to remove Na+deposition and suppress gas
evolution.
847
In addition to solvent exchange to address the
issue of Li salt degradation in LIBs, Jabbour et al. and
Leijonmarck et al. suggested that prolonged thermal treatment
of cellulose papers can also solve this problem.
848,849
Generally,
cellulose, with its intrinsic chemical and physical properties and
microstructures, oers a perfect matrix for manufacturing
environmentally friendly LIBs. The good compatibility of
cellulose with water allows the utilization of aqueous electrolytes
instead of hazardous organic solvents, and the safety concern of
LIBs can therefore be greatly alleviated.
850
Water can also be
used to remove unwanted chemicals in the process of binder
preparation. Natural cellulose dissolved in ionic liquids can be
used for production of binders. After the preparation of cellulose
solution in ionic liquid and mixing graphite and LiFePO4(LFP)
for anode and cathode, respectively, and coating the slurry on
metal foils, the electrodes can be immediately immersed in water
to extract the ionic liquid. Water is miscible with ionic liquids but
does not dissolve cellulose. In this phase-inversion process, the
ionic liquid is completely recyclable.
851
3.3.2.4.2. Fuel Cells. Recently, Vilela et al. have reviewed the
use of nanocellulose-based materials in polymer electrolyte fuel
cells.
577
In such application, the high-water anity coupled with
the mechanical properties of nanocellulose are an excellent
combination. Compared to the commercial ionomer Nafion,
nanocellulose-based membranes can be fabricated thinner
without compromising their gas barrier properties, a factor
that can potentially decrease the cell resistance. Bayer et al.
852
incorporated CNF and CNC nanopapers into membrane
electrode assemblies replacing the commonly used and
expensive ionomer membrane Nafion or nanocellulose/Nafion
composites.
853,854
The researchers showed a superior perform-
ance of the nanocellulose membranes in the fuel cells at high
operating temperatures up to 80 °C, with a continuous increase
of conductivity with the temperature.
852
Moreover, the
conductivity of CNF and CNC membranes increased sharply
with the relative humidity as water acts as a charge transport
medium. A decisive role was also given to the surface charge
groups (−COO−for CNF and −OSO3−for CNC) functioning
as proton acceptors and donors. CNC-based membranes
showed good conductivity and gas barrier properties, attribut-
able to their higher surface charge density and crystallinity. As in
the case of LIBs, the physicochemical properties of the
nanocellulose are crucial in terms of aspect ratio and surface
charge of CNFs in their performance as polymeric electrolytes.
The decrease in conductivity between 80 and 70%RH, as
described by Bayer et al., can be suppressed by using nanofibers
with a high amount of surface water to produce well-defined and
homogeneous membranes, in particular with thin nanofibers
(ca. 2 nm) and high surface charges (ca. 1.5 mmol/g).
855
In the
same study, Guccini et al. achieved a proton conductivity
exceeded 1 mS cm−1at 30 °C between 65 and 95%RH, which is
2 orders of magnitude larger than with previously reported
nanocellulose materials.
852,856
Furthermore, despite being 30%
thinner, a lower hydrogen crossover than with conventional
Nafion membranes was observed, likely given by the
combination of the excellent mechanical properties of CNFs
and the homogeneous membrane structure.
3.3.2.4.3. Hydrovoltaic Eect. The interaction of nano-
cellulose with water can generate electricity, a phenomenon
which has been denoted as hydrovoltaic eect.
857
In brief, this
eect is based on dierent water activities, including diusion,
evaporation, and flow, which generate water gradients through
solids. An electrical pulse results from the concentration
gradient of H+ions.
858
The ability of nanomaterials, including
the eagerly advertised carbon nanomaterials, to harvest electric
energy from flowing water and moisture, is based on their
exceptional sensitivity toward adsorbed species, providing the
ideal 1D nanospace for water binding and a rapid transport.
859
In a recent contribution, Li et al., translated the hydrovoltaic
eect to naturally derived nanomaterials, including CNFs, for
harvesting energy from moist air flow (Figure 13c).
799
They
have shown that, analogous to ion channels of cytomembranes,
these bionanomaterials can capture moisture from air through
hydrated nanochannels due to their inherent hygroscopic
properties and surface charges. Accordingly, in a continuous
air flow, the dynamic balance between water adsorption and
evaporation produced a streaming potential through the aerogel
membranes resulting in an open-circuit voltage. Nanocellulosic
materials have also been used in solar evaporation systems. A
typical interfacial solar evaporator includes a photothermal layer
on top, which upon solar irradiation converts light to thermal
energy (heat). It also includes a support whose role is to
simultaneously provide hydrophilic channels that continuously
wick water via capillary forces to the hot layer to generate steam.
Nanocellulosic materials have been used as supporting
substrates in these systems.
860
3.3.2.4.4. Solar Energy Harvesting Devices. Solar energy-
harvesting devices require high surface area and good charge
transport properties so that photons can be absorbed and
converted into electrical energy. CNF 3D mesoporous
structures oer a very large surface area and present good
mechanical properties.
861
Therefore, CNFs can be an attractive
template/matrix for processing structures used in photo-
electrochemical (PEC) electrodes.
862
PEC water splitting is a
promising strategy for directly converting solar energy into
hydrogen fuels.
863
Rapid charge generation and separation, large
surface area, and broadband light absorption are important
aspects of PEC development
862,864
and nanocellulose based 3D
structures can oer a medium for these technologies, due to their
water absorption and water retention capacity.
865−868
The
hydrophilic mesopores within the cellulose film could serve as an
ideal host matrix for the embedment of the nanoparticle catalysts
with minimized agglomeration.
866
The mesoporous cellulose
films can be used as a framework for photosynthetic and
photoactive material and to organize the redox couples. The
water retention capacity of the mesoporous 3D cellulose
nanostructures plays a crucial role in the reactions and processes
in these systems because water is the medium for these
devices.
867,869
3.3.2.4.5. Supercapacitors. An important application of
CNC-derived porous carbon materials is supercapacitors that
are closely connected to the interactions of nanocellulose and
water. In the first step, self-assembly of CNCs in an aqueous
environment results in the development of a chiral nematic
phase. This structure can be further carbonized to form chiral
nematic mesoporous carbon.
147,870,871
MacLachlan and co-
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AG
workers synthesized composite films with chiral nematic
structures by the evaporation-induced self-assembly of CNC
with silica precursors. After pyrolysis and etching of the silica,
freestanding films of chiral nematic mesoporous carbon were
obtained, which were used in symmetrical capacitor with H2SO4
as the electrolyte.
872
CNCs and CNFs can be integrated
together to form porous carbon materials for supercapacitors as
well.
873
3.3.2.4.6. Flexible Devices. Nanocellulose can be easily
manipulated to make stable physical or chemical bonds with
other cellulosic materials such as fibers used in textiles. This
characteristic has been used to design flexible electronic devices.
Hu et al. has constructed breathable structures with decoupled
electrolyte and oxygen gas pathways for Li-oxygen batteries
using carbon nanotube-coated nanocellulose cotton textile as a
flexible substrate and electrolyte reservoir.
874
Other than Li-
oxygen batteries, nanocellulose macrofiber-based textiles have
also demonstrated great potential in constructing flexible
supercapacitors and lithium-ion batteries.
875,876
Water is almost
always used as the dispersion medium for nanocellulose
materials that would be filtered to form positive or negative
electrodes in the preparation of flexible electrodes.
848,877−881
In
a typical process to prepare conductive polymer/nanocellulose
hybrids by in situ polymerization, the nanocellulose particles are
dispersed in a mixture of acid and water to produce a suspension.
Oxidant, initiator, and monomers of conductive polymers are
then added to the suspension and the polymerization occurs
under mild stirring.
882−890
Water also plays an important role in
full recyclability of the nanocellulose used in the conductive
flexible material. In the first report of a flexible, transparent, and
metal-free triboelectric nanogenerator that is naturally degrad-
able after 60 min stirring, the aluminum-doped zinc oxide/CNF
paper was completely dispersed into the water. The final
dispersion was clear and was used to form a CNF gel with the
small amount of Zn and Al ions coming from the aluminum-
doped zinc oxide coating. This coating can be redispersed in
water and further concentrated and filtered to produce CNF film
again, which indicates an excellent recyclability.
891
Despite the
positive roles of water in these devices, one of the main
detrimental eects of water on the energy related applications of
nanocellulose is also observed in the case of wearable devices.
The washability of these materials exposes them to large
amounts of water that can threaten their integrity and
function.
842
3.3.3. Powders, Aerogels, and Foams. Water is usually
partially or mostly removed from colloidal dispersions or
hydrogels of nanocellulose to produce new diverse products
such as powders, aerogels, and foams, to extend the shelf life of
the material, to omit the intrusive eect of water on
characterization techniques, or simply to facilitate easier and
cheaper transportation (Figure 14).
21,299
The extremely high
surface area present in nanocellulose in comparison to other
cellulosic materials can be lost easily during removal of water
from the water−nanocellulose systems. Thus, an entire family of
novel dewatering techniques have been developed especially for
the nanocellulose−water systems to minimize this eect.
3.3.3.1. Tuning Dried Nanocellulose Morphology by Water
Removal Techniques. As mentioned before, CNCs can be
freeze-dried, spray dried, supercritically dried, oven-dried, or
freeze-spray dried, and the drying techniques are known to have
an eect on the properties of the product (refs 315,321,323,
327,554,565,892−897). However, the diculty of removing
water from nanocellulose suspensions, while retaining the
nanoscale properties of the nanocellulose is a substantial
challenge. Lavoine et al. have reviewed the methods to prepare
nanocellulose-based foams and aerogels, exhaustively (Figure
15a).
320
In a review by Sinquefield et al. on the current state of
nanocellulose dewatering and drying methods,
299
the authors
present SEM images showing the representative morphology of
CNCs and CNFs dried by oven, spray drying, and freeze-drying.
Figure 15b illustrates the eect of water removal technique on
the morphology of dried nanocelluloses.
For both CNCs and CNFs, oven drying produced the largest
agglomerates. Samples of oven-dried CNC and CNF retained
some nanoscale surface textures but resulted in a lower surface
area than the other drying methods. Spray drying results in
aggregates with a wide range of sizes in both CNFs and CNCs.
The CNF spray-dried particles were larger than the CNC
particles. The nanoscale surface textures of both CNF and CNC
spray-dried particles appeared similar.
299
Wang et al. used
supercritical drying to preserve the original gel structure and
network for CNC aerogel preparation and showed that the
produced aerogels have a nanoporous network structure and
high specific surface area.
899
The products of freeze-drying are
usually networked multiscalar structures.
315,377
Freeze-dried
CNFs maintain at least partially the nanoscale features with
reduced fibril agglomeration relative to oven-dried samples.
The morphology of freeze-dried CNCs can be templated by
ice formation during the freezing process which can be adjusted
by freezing rate, initial dispersion concentration, and including
additives. On the other hand, aerogel preparation with uniform
pore size is a formidable challenge (Figure 16a,b).
320
Han et al.
studied the self-assembling behavior of both CNFs and CNCs
during freeze-drying. Within a certain range of concentration,
the fibrils self-aligned into a lamellar-structure foam composed
of aligned membrane layers. The authors described the
mechanism of these assemblies, again, as a result of ice crystal
growth. When the stable nanocellulose suspension was frozen,
ice crystals gradually grew in the same preferred direction and
created a lamellar structure oriented in the direction of the
freezing front. Nanocellulose was expelled and separated by ice
crystals and formed a morphology templated by ice crystals
(Figure 18a).
321
Huang et al. reported that when the solid
content in the original suspension of CNF varied between 4 and
Figure 14. Summary of nanocellulose−water interactions in dewatering process.
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AH
10 wt %, the morphology of the aerogel transformed from the
membranous to fibrillar network, increasing its specific surface
area and redispersibility, while further dehydration was
detrimental. They attributed this phenomenon mainly to the
entanglement of cellulose fibrils in the cellulose network, which
suppressed the growth of ice crystal during the process. The
Figure 15. (a) From nanocellulose to nanocellulose-based foams and aerogels: terminology and processing.
320
(a) Adapted from ref 320 under the
terms of CC_BY. Copyright 2017 The Royal Society of Chemistry. (b) Eect of drying method on the morphology of nanocellulose materials. This
figure is produced with permission from ref 898. (b) Adapted with permission from ref 898. Copyright 2020 American Chemical Society.
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AI
interaction between intra/intermolecular hydrogen bonds was
also responsible for the variation of the cellulose morpholo-
gies.
894
The ice templating eect during freeze-drying was also
reported by Deville, and the mechanism of ice templating and
nanocellulose behavior during freezing was studied in detail,
322
although their study did not include the drying step. Lewis et al.
used cyclic physical confinement of CNCs between growing ice
crystal domains to promote aggregation of CNCs. Freeze
thawing (FT) cycling was employed to form larger aggregates of
CNCs without changing the surface chemistry or ionic strength
of the suspensions.
900
They showed that the rheology of CNC
suspensions can be tuned by FT cycling for suspensions with 4%
or more concentration. The complex modulus of 4 wt % CNC
suspensions after each FT cycle shows a significant increase,
reaching a plateau finally. SEM images of the freeze-dried CNC
aerogel formed by this method demonstrated the formation of
an interconnected, porous cellulosic sheet network.
900
In the case of BC aerogels, Gromovykh et al. investigated the
eect of culturing conditions on the structure and anisotropy of
the produced bacterial cellulose aerogels and found out that ice
crystal templating is not the only mechanism for formation of
layered morphology of BC aerogels. They suggested apart from
the dewatering techniques, geometry, source of carbon and
nitrogen, and oxygen availability of the aqueous culture
influence the anisotropy and structure of the produced bacterial
cellulose network and subsequently result in dierent
morphologies in the formed aerogels.
901
Some eorts have been taken to decrease the detrimental
eects of water crystal growth on dispersion stability, and
subsequently, the structural uniformity of the aerogel. For
example, freeze-drying from water dispersions can result in
significant CNF aggregation. If the aggregation is not desired,
especially for dispersibility or surface area reasons, solvent
exchange to tert-butanol prior to freeze-drying can be help-
ful.
492,903,904
Saito et al. converted CNF aqueous suspensions
into hydrogels, which were afterward solvent exchanged to tert-
butanol and freeze-dried in order to obtain aerogels with less
aggregations.
905
Peng et al. investigated the eect of drying method on thermal
stability and crystallinity of the dried products. Supercritical-
drying produced CNFs with the least thermal stability and the
lowest crystallinity index. Air-drying or spray-drying produced
CNFs which were more thermally stable compared to freeze-
dried CNFs. The dierent drying methods resulted in various
char weight percentages at 600 °C for the dried CNFs or CNCs.
The dried CNFs are pure cellulose I, while the dried CNCs
consist of cellulose I and II.
327,901
3.3.3.2. Dewatering of Nanocellulose for Characterization
and Preservation. 3.3.3.2.1. Eect of Water Content on the
Results of Characterization Methods. As mentioned briefly in
section 3.3.2.1, a very important challenge in the field of
nanocellulose characterization is the dierence in the analytic
data based on the dierence in water content. It is important to
compare the qualities of dierent nanocelluloses reported in
dierent studies, only with the same dewatering/drying history.
Nanocellulose materials hold residual moisture contents of
approximately 2−5 wt %
327
in their powder form, which can
greatly increase the diculty of analyzing results for many
characterization techniques, such as specific surface area
measurements and some processing conditions like melt
compounding, which imposes serious issues in their applica-
tions.
21
By annealing at 100 °C in vacuum, most moisture can be
removed but nanocellulose readsorbs water immediately upon
coming into contact with the atmosphere.
330,331
In practice,
characterization techniques such as DVS of celluloses in general,
but especially of nanocelluloses, are greatly influenced by the
treatment and drying history of the material, e.g., whether air-
dried from water or from another solvent,
126
by freeze-drying,
318
or by supercritical drying.
21
The structural changes aect the
moisture uptake and its retention in the material, and therefore
special care should be devoted to the method of sample
preparation to obtain relevant information on its water uptake
properties.
3.3.3.2.2. Dewatering for Preservation. As mentioned in
section 2.2.2, drying of the cellulose source material before
nanocellulose isolation procedure, results in hornification and
can result in CNFs or CNCs, with dierent characteristics than
the ones produced from never-dried cellulose sources.
906
More
critically, drying the already produced CNFs and CNCs
irreversibly alters their characteristics mainly due to pronounced
hornification, as stated on several occasions in this review.
However, keeping large amounts of water in the nanocellulose
samples causes many problems such as transportation
diculties, storage problems, and vulnerability toward fouling.
Dewatering methods (not fully drying) are a compromise to
minimize the complexities of having large quantities of water in
the system without completely drying the samples. Nano-
cellulose is typically stored and transported as a gel with a
nominal solid content of up to 5 wt % to avoid interfibril
hornification, which means a large amount of water should
remain in the gel. There are strategies to reduce the volume of
nanocellulose gels for preservation and transport. For example,
Reverse dialysis in poly(ethylene glycol) (PEG) has been shown
to be a safe dewatering method via osmotic dehydration, without
causing irreversible aggregation and sample heterogeneity.
907
Santmarti et al. used low molecular weight PEG as a replacement
for the water phase in nanocellulose aqueous gels. These gels
Figure 16. (a) Schematic diagram of directional ice-templating of CNC
dispersion. (a) Adapted from ref 320 under the terms of CC-BY.
Copyright 2017 The Royal Society of Chemistry. (b) SEM images of
the horizontal cross-section of nanocellulose foams obtained by (i)
homogeneous freezing and (ii) unidirectional ice templating. (b)
Adapted with permission from ref 902. Copyright 2016 American
Chemical Society.
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AJ
had solid contents of up to 70% without interfibril horn-
ification.
554
Despite such reports, the majority of research
laboratories and industries report preservation of nanocellulose
in water. Some studies have focused on accelerating the
dewatering process by changing the pH and ionic strength of
the dispersion.
908
According to the suggestions for best practices
in storing nanocellulose materials, all nanocellulose in wet state
should be stored in the refrigerator or small amounts of sodium
azide or toluene should be added to the suspensions. Dried
powders should be stored under low temperature and humidity
conditions. However, if the nanocellulose materials are intended
to be used in toxicity testing or biomedical applications the use
of antimicrobial agents is prohibited, and dewatering, refriger-
ation, or a combination of both are the only secure ways to store
the materials.
21
3.3.4. Reinforcing Nanofillers in Composites. One of the
major potential industrial applications of nanocellulose particles,
CNFs and CNCs, is their use as reinforcing fillers for
nanocomposites due to the inherently high mechanical strength
of crystalline cellulose. One of the critical parameters for
polymer composites, which governs the properties of the
material, is the compatibility of the interfaces between
constituting components. The interface is particularly important
for nanoscale components because of their immense surface area
(refs 12,119,247,344,430,449,452,467,470,473,474,477,
480,483,486,540,551,909−924). In this section, we are not
delving into the vastly researched topic of how to disperse
nanocellulose fillers as eciently as possible within the
continuous matrix in polymer composites. Rather, we
investigate the interactions of water and nanocellulose
reinforcing fillers, which have been used in polymer nano-
composites and describe the eect of these interactions on the
applications (Figure 17). For the full review on strategies on how
to solve the issues in nanocellulose applications in polymer
nanocomposites, the readers are referred to numerous available
reviews.
64,470,477,925−935
3.3.4.1. Water Sensitivity in Nanocellulose Composites/
Blends. There are many potential applications where adding an
easily dispersible reinforcing nanomaterial to polymer matrices
results in nanocomposites, with the advantages such as
improved mechanical properties.
485
These nanocomposites
can be manufactured in 1D (fibers), 2D (films, nanopapers,
fabrics), and 3D (aerogels, foams, and hydrogels) forms tailored
toward dierent applications (Figure 18).
835
Most polymers
used in nanocomposites are more hydrophobic than CNFs or
CNCs and, in consequence, there are usually compatibility
issues between the components. Besides the compatibility
problems, the main drawback in application of nanocellulose in
Figure 17. Water in nanocellulose-reinforced polymer composites.
Figure 18. Schematic illustrations of various structures of nanocellulose composites (including 1D fiber, 2D film, paper, and fabric, and 3D aerogel,
sponge, and hydrogel).
398,835,937−939
Adapted with permission from ref 835. Copyright 2019 John Wiley and Sons. SEM figures were originally
published in refs 398,937−939. Reproduced with permission from ref 398. Copyright 2015 John Wiley and Sons. Reproduced with permission from
ref 937. Copyright 2018 Elsevier. Reproduced with permission from ref 938. Copyright 2017 John Wiley and Sons. Reproduced with permission from
ref 939. Copyright 2017 Elsevier.
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AK
polymeric matrices is their sensitivity to water, which has a
profound eect on dispersion, wetting, interfacial adhesion,
matrix crystallization, water uptake, and hydrothermal stability
(see sections 3.1.1 and 3.1.2). Nanocellulose surfaces are rich in
hydroxy groups, which absorb significant amount of water under
moist conditions. Surface water molecules weaken the
cellulose−cellulose interactions, act as plasticizers, and reduce
the network stiness and strength.
444
Apart from compatibility
issues water could be also detrimental to the processing and
product performance in some techniques such as melt
compounding for nanocomposites.
21
Addition of CNFs or
CNCs to nanocomposites may also aect the glass transition
temperature (Tg) of the material. This eect, although not
reported for all nanocellulose nanocomposites, is shown
especially in the case of moisture-sensitive systems. Plasticiza-
tion eects of water, whose concentration can be increased by
the presence of nanocellulose, and the strong interaction
between water and the matrix can be the main reasons for this
eect. Change in Tghas a potentially important eect on some
applications of these nanocomposites that require thermal
stability,
74
and nevertheless, it is an issue that must be taken into
account. Therefore, a large number of studies aim at improving
the outcome of nanocellulose applications in nonaqueous
polymeric systems and the key to these approaches is usually
surface modification (Figure 19a)
936
and cross-linking (Figure
19b).
444
3.3.4.1.1. Reducing Water Sensitivity of Nanocellulose
Composites/Blends. Several attempts have been taken to
modify CNCs and CNFs in composites physically or chemically
to address water sensitivity. Models have been developed to
investigate the eect of modifications on water−matrix
interaction to predict the final materials performance of a
Figure 19. Strategies to minimize the detrimental eect of water in nanocellulose reinforced composites. (a) Surface modification: Illustration of the
synthetic route and structure of natural rubber (NR)/mercaptoundecanoyl-modified CNC (m-CNC) nanocomposites.
936
(a) Adapted with
permission from ref 936. Copyright 2015 American Chemical Society. (b) Cross-linking: Schematic of the steps to graft phenyl glycidyl ether (PGE)
on CNF, where PGE monomers were impregnated in a CNF network and the reaction was initiated thermally. The free oligomers formed at this stage
were removed by extensive washing with acetone.
444
(b) Adapted with permission from ref 444. Copyright 2016 Elsevier. (c) Emulsification: (i)
Images obtained by confocal fluorescence microscopy for emulsions containing poly(styrene) (PS) in toluene and CNFs in the aqueous phase and
emulsified at a PS:CNF dry mass ratio of 90:10, (ii) SEM images of electrospun nanofiber mats prepared from double emulsions containing PS and
CNF (90:10) with surfactant mixture concentration of 3.
940
(c) Adapted with permission from ref 940. Copyright 2016 The Royal Society of
Chemistry. (d) Using hydrophilic polymers: (i) Illustration of fiber processing to optimize the microstructure of CNC/poly(vinyl alcohol) (PVA)
composite fibers: (1) Coagulation of spinning dope (CNC and PVA) by injection into a coaxial flowing stream of coagulant; (2) hot-drawing of the
fiber under tension at high temperature (150 °C). Comparative mechanical strength of all CNC/PVA composite fibers and the pure PVA control in
textile and materials units. The strength reached 880 MPa, exceeding the properties of most other nanocellulose based composite fibers previously
reported.
941
(d) Adapted with permission from ref 941. Copyright 2016 American Chemical Society.
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AL
nanocomposite structure. Lyubimova et al. performed MD
simulations of neutral and negatively charged sulfated CNC in
water and employed a statistical mechanical molecular theory of
solvation to evaluate the solvation structure and thermody-
namics of the relaxed CNC in ambient aqueous NaCl solution.
This model predicts molecular recognition interactions in
solution and can be used to improve the compatibilization of
CNC with matrix polymers to enhance the CNC loading levels
in composites. The method was shown to be able to accurately
predict the degree and type of CNC surface modifications
necessary to achieve a good dispersion in polymer solutions
while preserving the desired crystallinity and mechanical
properties.
942
Chemical and physical surface modification,
counterion change, and emulsification are among approaches
that have been used to improve the qualities of nanocellulose−
polymer composites. Wei et al. used computational approaches
to obtain understanding of water adsorption and interfacial
mechanics of modified CNC surfaces to address the issues
regarding the response to moisture. They found both
experimentally and theoretically that methyl(triphenyl) phos-
phonium (MePh3P+)-exchanged CNCs have lower water uptake
than Na-CNCs due to the disruption caused by the bulky ionic
structure of MePh3P+. The adsorbed water accumulates near the
cations and is oriented by electrostatic interactions as well as
water−water hydrogen bonding. Traction−separation behavior
of these interfaces is highly dependent on the surface chemistry.
MePh3P+cations serve to change the interface in a way that it
exhibits hydrophobic behavior, such as formation of capillary
bridges and preservation of mechanical properties upon wetting.
The researchers showed that chemical surface modification is a
viable option for changing the adsorption and traction−
separation behavior of CNC as an important first step toward
the design of moisture-tolerant CNC−polymer nanocompo-
sites.
943
Carrillo et al. proposed double emulsion systems for the
compatibilization of aqueous dispersions of CNFs with a
nonpolar polymer matrix (Figure 19c). Nonionic surfactants
were used in CNF aqueous dispersions equilibrated with an
organic phase. This method of CNF integration within
hydrophobic polymers removed the need for drying or
solvent-exchanging of the CNF aqueous dispersion prior to
processing, proving double emulsion systems as a novel,
ecient, and scalable platform for CNF coprocessing with
nonpolar systems in nanocomposite preparations.
940
3.3.4.2. Hydrophilic Nanocellulose Composites. Due to the
hydrophilic character of nanocellulose, the simplest polymer
systems that incorporate nanocellulose are water-based systems.
In these systems, water plays a positive role. Nanocellulose
dispersions can be simply mixed with aqueous polymer solutions
or dispersions (both natural and synthetic polymers).
944
Although these systems suer from limited applications and
are only appropriate for water-soluble or dispersible polymers
such as latexes,
74
they can be very useful in the scope of colloids,
emulsions, hydrogels, films, membranes, 3D printable, and
responsive materials, as discussed in section 3.3.1,3.3.2, and
3.3.3 (refs 9,584,641,642,654,675,705,727,732,733,
945−949). For example, cross-linked CNF/poly(acrylic acid)
(PAA) composites have been prepared in order to improve the
material properties in humid environments.
950,951
Nano-
cellulose/PVA composites have similarly been popular.
941
In
fact, Lee et al. managed to prepare a fiber composite that bears
one of the highest reported tensile strength values of all
nanocellulose-based fiber composites, ca. 900 MPa (Figure
19d).
941
But such materials with all-hydrophilic components are
usually susceptible to moisture and water, which possesses an
intrinsic constraint for their usage.
4. ANALYTICAL TOOLS TO PROBE
NANOCELLULOSE−WATER SYSTEM
4.1. Computational Methods to Uncover
Water−Nanocellulose Interactions
MD simulation is an appropriate tool for performing atomistic
computer simulations of moleculear interactions within material
itself, with other molecules, and with solvents under given
thermodynamic conditions. Both water and cellulose have been
studied by MD simulations. For both of these materials, some
parallels appear in simulation studies and experimental results.
Simulations allow the creation of hypothetical measurements
and the predicted results for these measurements. These
simulated test results are then compared with real experimental
results, and the results of other simulated measurements with
dierent assumptions and conditions, to both understand the
real structure of the material, and the origin of the experimental
signal.
The first atomistic MD simulations of cellulose−water
interface were performed already in the late 1990s by Andreas
Heiner and Olle Teleman, who found that the interactions with
water gave rise to structural disorder in the first cellulose layer,
with respect to the crystal lattice parameters.
228,952
Since then,
the importance of using computational modeling for the
understanding of cellulose and cellulose−water interactions
has grown rapidly. Specifically, developments in molecular
modeling methods such as MD and quantum mechanical
density functional theory (DFT) have been instrumental for this
advancement and has recently been reviewed by Bergenstråhle-
Wohlert and Brady,
953
Zhang et al.,
954
and by Buehler and co-
workers.
955
While MD is based on classical physics and treats atoms as
point particles that are interacting pairwise through empirical
potentials, DFT takes the electronic configuration into account
and is therefore considered very close to an ab initio method.
This has consequences for its practical use. While DFT is more
precise, it is only ecient in systems consisting of a few
molecules, whereas MD currently can treat systems of millions
of atoms and reach millisecond simulation times. In addition,
there has been considerable eort in developing coarse-grained
potentials for cellulose simulations, in which atomistic details are
sacrificed for the benefit of significantly extending available time-
and length-scales.
956,957
Therefore, in these models, the details
such as hydrogen bonding and water molecular orientation are
averaged out, and they are less suitable for specific investigations
of cellulose−water interaction on the molecular scale.
There exist several empirical force fields for atomistic MD that
are specifically designed to treat carbohydrates in aqueous
solution. Among the most popular are CHARMM (Chemistry
at Harvard Macromolecular Mechanics),
958,959
GLYCAM
(developed and maintained by Complex Carbohydrate
Research Center at the University of Georgia in Athens
GA),
960
GROMOS (GROningen MOlecular Simulation
computer program package),
961
and OPLS-AA (Optimized
Potentials for Liquid Simulations-All Atom) force field
(developed and maintained at Purdue and Yale universities),
962
which are all frequently used for simulation studies of cellulose.
The potentials are developed in close connection with a water
model, which thus can be considered part of the force field. The
GROMOS potentials uses the SPC (simple point-charge) water
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AM
model,
963
while CHARMM, GLYCAM, and OPLS-AA are
developed with the TIP3P (a 3-site rigid) water model.
964
In
spite of the relative simplicity of these models, they have proven
to be accurate enough for average properties related to hydrated
cellulose systems. In fact, one study which used a more complex
water model that included electronic polarizability reported only
minor dierences compared to static models.
965
For a detailed
comparison between the performances of dierent water
models, the reader is referred to the literature.
966
4.1.1. Cellulose Is Insoluble in Water. From a
fundamental perspective the nature of cellulose is ambiguous.
Owing to its large number of hydroxyl functionalities, it is
considered a hydrophilic molecule. On the other hand, it is
completely insoluble in water at ambient conditions (Figure 5).
It has been suggested that this phenomenon is due to the many
intermolecular hydrogen bonds of the aggregated structure. This
view was contested by Lindman and co-workers,
967,968
who
argued that cellulose is an amphiphilic molecule and that
aggregation is a consequence of hydrophobic assembly. Indeed,
early MD simulations had shown that the contribution from
hydrogen bonding to the dissolution free energy is an order of
magnitude smaller than from dispersion interactions and the
hydrophobic eect.
969,970
The latter was later explicitly shown to
originate in a large entropic penalty of the water in the first
hydration shell.
971
The entropic cost was however significantly
reduced at elevated temperature and pressure and as a
consequence cellulose was soluble in simulations of water at
supercritical conditions.
972
The entropic cost associated with
water can also be mitigated by addition of cosolvents such as
urea,
973,974
which is exploited for enhancing cellulose dis-
solution in cold alkali
256
(Figure 5).
4.1.2. Cellulose Twist in Water. The fiber/fibril twist of
cellulose is an intriguing and significant observation of
morphological changes in model cellulose crystals solvated
with explicit water molecules with bulk water proper-
ties,
154,155,975−983
although from an experimental perspective
the mechanisms behind its occurrence, and even its existence,
are still a debated subject.
152,984−986
To a certain extent, the
twist is speculated to cause the inability of individual microfibrils
to coalesce into merged fibrils over long distances. Until now,
MD studies have simulated microfibrils with either a rectangular,
diamond, or hexagonal cross-section made up of up to 100
cellulose chains and of length between 10 and 50 nm, and a
right-handed twist of a few degrees per nanometer of length has
been found. Furthermore, the twist has been found to scale
inversely with the cross-sectional area yet be independent of
fibril length.
154,976,987
The molecular origins of the twist were
suggested to be the result of a combination of intra- or
intermolecular hydrogen bonding, electrostatic interactions, van
der Waals forces, and the chirality of cellulose chains. Isolated
glucan chains in water deviate from the strict 2-fold symmetry
assumed in the crystal structure of cellulose rendering them a
slight right-handed twist. This is a common feature of all β-1−4
linked oligosaccharides and has been shown to be caused mainly
by steric eects with negligeable contribution from intra-
molecular hydrogen bonding.
988,989
It is likely that this single-
chain twist propagates up within the structural hierarchy giving
twisted microfibrils.
155
This conclusion is reinforced by a study
where the microfibril twist is shown to be insensitive to
intrachain hydrogen bonding.
990
Some other studies have also
suggested that hydrogen bonding may not influence the twist at
all.
981,987
The microfibril twist in turn progresses to the next
hierarchical level of the fibril bundle. However, the twist results
in an increase in conformational disorder from the microfibril
core toward the surface in the presence of both water and
vacuum, causing a longitudinal variation in the bundle structures
that prevents cocrystallization and opens up routes for the
diusion of water molecules into said bundles (Figure 20a,b).
991
The amount of interfibrillar water was found to be roughly half
of that of extrafibrillar surface water in MD simulations of
microfibril bundles, which correlates well with the experimental
analysis of bleached hardwood pulp.
991
The role of water or solvation for the mechanisms behind
fibril twisting is not clear. It has been shown, however, that the
physical presence of water significantly mitigates the extent of
twisting due to the ability of water to form hydrogen bonds with
the crystal surface.
978
On the other hand, Conley et al. found
that water only has a limited eect on the twist unless it disrupts
the hydrogen bonds across the glycosidic bonds.
154
Further,
adding an excess of water beyond the first solvation shell does
not change the degree of twisting, but this is dependent on the
water model used; when using TIP5P water resulted in a
noticeable reduction of the twist compared to the TIP3P model,
for example.
978
4.1.3. Eect of Hydration on Cellulose Dynamics. The
structure of cellulose at the surface is significantly dierent from
the crystalline bulk structure, as is evidenced by the distribution
of the hydroxy groups and torsional angels.
228,952
These angles
are strongly aected due to changes in the hydrogen bonding
potential in the presence of water at the cellulose surface.
Moreover, the dynamics of surface polymers is distinctly
dierent from those inside the crystal. MD simulations of
cross-polarization and magic angle spinning (CP/MAS) solid-
state 13C nuclear magnetic resonance (NMR) longitudinal
relaxation enabled quantitative interpretation of experimental
NMR data (explained in detail in section 4.2.2).
994
The
experimental data along with the simulations demonstrated a
direct correlation between dynamic and structural heterogeneity
at atomic resolution, enabling the understanding of structure−
function relationships in controlled hydration conditions. It was
found that the doublet at resonance ca. 84 ppm for C4 atoms was
due to the nonequivalence of accessible surfaces located on top
of dierent crystallographic planes. Within hydrated cellulose
fibril aggregates,
995
localized cellulose macromolecular dynam-
ics have been deconvoluted into contributions from distinct
molecular sources within the aggregated CNFs: (i) the hydroxy
groups in the core of CNFs, (ii) the less accessible and accessible
surface regions, and (iii) within structurally dierent surface
groups. Upon hydration, this leads to an increased disorder in
the hydroxy group conformation at the cellulose surface.
996
As
found in MD simulations, and confirmed with neutron
reflectivity experiments, cellulose hydrated to 10% w/w with
water is more rigid than dry cellulose as a result of one water
molecule forming two or more hydrogen bonds with cellulose to
bridge cellulose chains.
996
4.1.4. Water Structure and Dynamics at Cellulose
Surfaces and within Fibril Aggregates. The solvation of
cellulose is also of importance in understanding the reactivity of
cellulose as it relates to its interfacial properties. Understanding
the role of water at cellulosic surfaces and within fibril aggregates
will aid the discovery of the underlying process not only for
deconstructing cellulose but also for designing functional,
chemically modified cellulosic materials for targeted applica-
tions.
The solvation structure is determined by asymmetry of
crystallographic cellulose surfaces due to the topographical and
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AN
chemical heterogeneity of each surface. As a result of the
heterogeneity resulting form of C−H and O−H rich regions,
respectively, the local water density in dierent hydration layer
diers considerably, as illustrated in (Figure 20c,d). Around the
O−H rich regions, a high water density leads to a dense first
solvation shell, which has been hypothesized to slow down the
diusion of other molecules toward the cellulose surface.
155,992
Due to the lack of anity toward the C−H rich regions, water
molecules form strong hydrogen bonds with each other, causing
small-scale hydrophobic eects.
997
Along the longitudinal
direction of microfibrils, water molecules near the C−H rich
surface, i.e., the (200) and (100) planes of the cellulose Iβcrystal,
and the (110) and (110) planes of the cellulose Iαcrystal, have a
higher compressibility than those in other solvation shells
(Figure 20b).
992
Miyamoto et al. focused their simulation
specifically on the orientational structuring of water molecules
over the (100) crystal plane of cellulose Iβ.
998
In terms of
orientation, they showed that water molecules can approach
hydrophilic troughs between cellulose chains and form hydro-
gen bonds to hydroxy groups to form interchain hydrogen
bonding. The hydrophobic strips of cellulose crystals are
suciently narrower in comparison to the fibril size.
998
In general, the behavior of the bound water to cellulose
surface and fibrils is thermodynamic-driven. With confirmations
from quasi-elastic neutron scattering (QENS) studies on
deuterated bacteria cellulose on a picosecond scale, O’Neill
applied MD simulations to elucidate two unambiguous
populations of water (explained in detail in section 4.2.7). As
explained in subchapter section 2.4.2.2, “nonfreezing bound”
water gradually becomes mobile with increasing temperature in
the vicinity of cellulose surface. The second population is
“confined water”, which can be attributed to water accumulation
in the narrow spaces between microfibrils.
273
Langan et al. found
that fibril aggregation during thermochemical pretreatment with
increased temperature up to 160 °C would cause core water
expelling among the fibrils. An induced increase of fibril
crystalline domain indicating fibril coalescence was also
experimentally observed by Kuribayashi et al.
999
However,
water steadily stays between fibrils when the applied temper-
ature is maintained below 76 °C and Chen et al. showed that this
population of water is in thermodynamic equilibrium as opposed
to being kinetically trapped.
1000
This means that interfibrillar
water lowers the free energy of the bundle and thus acts as an
adhesive. On the other hand, interfibrillar water lowers the
friction between fibrils facilitating shear deformation,
1001
contributing to the ductility of cellulose materials.
1002
Surface
charge was also shown to interfere with cellulose−water
interactions.
274
Paajanen et al. presented an informative
understanding of the water interactions between fibrils of
TOCNF regarding its rheological, aggregation, and disintegra-
tion properties using MD simulations.
219
4.1.5. Wetting and Water Sorption. Cellulose Iβcrystals
with the existence of both hydrophilic surface (e.g., (010),
(110), and (110) surface with exposed −OH) and hydrophobic
surface (e.g., (100) plane with buried −OH and exclusively
−CH moieties) show a featured hygroscopic property. The ab
initio studies of interactions between (100) plane and single
water molecule were carried out using dispersion corrected DFT
method.
1003
It was concluded that water adsorption on the Iβ
(100) plane is depending on the adsorption size on the plane
(Figure 20e,f). Hydrogen bonds could be formed with more
accessible CH moieties protruding out of the plane than oxygen
atoms of the equatorial hydroxys. Wetting of two cellulose
surfaces, (010) and (100), was carried out by MD simulations of
contact angle using the native Iβmodel.
993
In the simulation, a
nanodroplet of around 3 nm TIP1P/2005 water was placed near
the surfaces. The calculated contact angle in the simulation was
around 16°for the (010) plane and around 23°for (100) plane
Figure 20. (a) Illustration of water diusion into a bundle at the cross-
section of fibril bundle along its longitudinal axis. Color legend:
cellulose in microfibrils (gray), disordered cellulose (dark red), and
water (orange). (a) Reproduced from ref 991 under the terms of CC-
BY. Copyright 2019 Spring Nature. (b) Solvated water layer on the
microfibril cross-section. Both Iβand Iαare shown next to their
corresponding faces with the cvector as orthogonal to the plane of the
image. (b) Reproduced with permission from ref 992. Copyright 2010
American Chemical Society. Water density of the solvated water at
dierent surfaces of Iβ(c) and Iα(d) with microfibrils represented in
the interior. (c,d) Reproduced with permission fromref 992. Copyright
2010 American Chemical Society. (e,f) Side view of water wetting
behavior of cellulose Iβ(010) plane. (g,h) Side view of water wetting
behavior of cellulose Iβ(100) plane; 2-D density profile of water
molecules on Iβ(010) plane (i) and on Iβ(100) plane (j). (e−j)
Reproduced with permission from ref 993. Copyright 2019 Elsevier.
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AO
(Figure 20i,j), exhibiting hydrophilic behavior. Similarly,
Nawrocki et al. found a contact angle of 25°for TIP3P water
with cellulose (100) plane, predicted in their MD simulations
using CHARMM force field.
1004
In a comprehensive study using
the CHARMM force field Trentin et al. studied the spreading of
water nanodroplets on the dierent crystallographic planes in
several cellulose polymorphs. It was found that all surfaces were
hydrophilic (even those commonly termed “hydrophobic”) with
static contact angles ranging from <5°(complete wetting) up to
48°. Interestingly, the highest contact angle was found for the
“hydrophilic” (001) surface of cellulose Ia. The dierences in
wetting were correlated with the conformation and accessibility
of surface hydroxymethyl groups.
1005
Karna et al.
1006
conducted
contact angle simulations between water and cellulose surface at
the presence of an external electric field. The application of an
electric field with varied direction and magnitude would tune the
Figure 21. (a) The scale of hierarchical structure of the primary cell wall of plants with respect to cellulose and tools enabling characterization of
cellulose in each level of magnitude.
1010−1013
Adapted from ref 1011 under the terms of CC-BY. Copyright 2019 Frontiers. Reproduced with
permission from ref 1010. Copyright 2002 American Chemical Society. Reproduced with permission from ref 1012. Copyright Elsevier. Reproduced
with permission from ref 1013. Copyright 2015 Elsevier. (b) Overview of experimental techniques for direct and indirect analysis of nanocellulose−
water interactions: AFM, atomic force microscopy; CA, contact angle; DMA, dynamic mechanical analysis; DS, dielectric spectroscopy; DSC,
dierential scanning calorimetry; DVS, dynamic vapor sorption analysis; EM, electron microscopy; IGC, inverse gas chromatography; IR, infrared
spectroscopy; IS, inelastic scattering; NMR, nuclear magnetic resonance spectroscopy; OM, optical microscopy; QCM-D, quartz crystal microbalance
with dissipation monitoring; SAS, small angle scattering; SE, spectroscopic ellipsometry; SPR, surface plasmon resonance spectroscopy; TLW, thin-
layer wicking; TOL, tritium oxide labeling; WBT, Washburn techniques; WVTR, water vapor transport rate.
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wettability of cellulose surface, which could be beneficial to the
dewatering process of nanocellulose related products. Albeit the
wetting behavior is rather similar for both (010) and (100)
planes, the solvent molecular organization interacting with each
plane is substantially dierent. It is observed that water
penetrating in the interstices between cellulose molecules in
the (010) plane forms hydrogen bonds with the exposed surface
−OH groups due to high surface roughness. However, on top of
the (100) plane, water is inclined to form water structure at the
interface along with a characteristic gap on top of the apolar ring
(Figure 20g,h).
1007
Water molecules also prefer to occupy the
specific positions, particularly between the glycosidic oxygen
atom and the adjacent O-2 and O-3 hydroxy groups.
155
A phenomenon related to wetting is the formation of capillary
bridges. Ogawa et al.
1008
used MD to study capillary eects
during drying of model CNF. As water was gradually removed a
meniscus formed, exerting a capillary force large enough to
plastically deform the fibrils. At the end of the simulated drying,
the two fibrils had partially fused, although some water still
remained trapped within the aggregate.
4.1.6. Simulation of the Interactions of Functionalized
Cellulose with the Environment. In general, MD simulation
studies can be an appropriate tool to understand the role of
water in functionalized nanocellulose surface and composite in
terms of tailoring interfacial behaviors and tuning compatibilities
in composite materials on both bulk and molecular level. As
discussed in section 3.3.4, the presence of water at nanocellulose
surface detrimentally weakens the interaction between hydro-
philic cellulose and hydrophobic polymer matrices, which
becomes a problem in cellulose nanocomposite materials. A
cellulose Iβ(110) surface was grafted with caprolactone of
dierent degrees of substitution by Bergenstråhle et al. The
hydroxy groups in the native cellulose surfaces interact with
other medium such as caprolactone not limited by present water.
Surfaces with modified O6H6 and a DS of at least 50%
interacted with the surrounding medium mainly through the
grafted monomer unit instead of hydroxy groups. However, the
adhesion between grafted surfaces and surrounding polymer
medium was still prohibited by the presence of water. Increasing
the degree of polymerization of the grafts was suggested to
diminish the eect of surface water.
994
Simulations of the
interactions between functionalized cellulose surfaces and
surrounding medium leading to increased understanding of
the self-assembly of nanocellulose would help the development
of novel materials. Bouchard et al.
1009
studied the interaction
and adsorption of water and electrolyte on the cellulose
nanocrystal pristine surface and its modified surfaces, i.e.,
carboxylic and sulfate groups, by DFT quantum chemical
calculations. The cellulose Iβ(110) surfaces were more repulsive
toward each other, possessing a slightly more negative
electrostatic potential map than (110) surfaces. The negative
surface functionalities impart a greater CNC surface hydro-
philicity, while hydrogen-bonding network within cellulose was
restructured in the presence of positive electrolyte ions.
Recently, Chen et al. investigated the influence of topochemical
modification of cellulose surface on its interaction with water
and among the modified nanocellulose particles. It was
concluded that acetylation in the C6 position led to hydro-
phobization of cellulose fibrils and the decrease of the work of
adhesion between the acetylated model surface and water. Most
interestingly, the acetylation was found to greatly increase the
dispersibility of nanocellulose.
376
4.2. Experimental Methods to Uncover
Nanocellulose−Water Interactions
This section introduces the key analytical techniques that are
currently used to investigate cellulose−water interactions and
gives a few selected examples of published works from each that
specifically measure the eect of water on nanocellulose-based
materials. The overview of analytical methods is not exhaustive
and is limited to the key analytical techniques that are currently
used to investigate cellulose−water interactions. For each of the
discussed techniques, we provide selected examples of works
that specifically measure the eect of water on nanocellulose-
based materials.
The wide variety of analytical techniques and protocols
developed to probe nanocellulose−water interactions reflects
the complexity of these interactions and the need to examine
them from dierent perspectives. In paper making processes,
well-established technical process-orientated standard methods
such as freeness value (Schopper Riegler (°SR) test), water
retention value, and other online paper web moisture measure-
ments reflect cellulose−water interaction including runnability
and processability in a macroscale level. However, most of these
techniques are not directly applicable for studies of nano-
cellulose materials, unless further development takes place. This
section introduces the key analytical techniques that are
currently used to investigate cellulose−water interactions and
gives a few examples of published works from each that
specifically measure the eects of water on nanocellulose-based
materials. These techniques and their analytical focus are
summarized in Figure 21a,b to guide the reader to the
appropriate analytical technique for their research needs.
4.2.1. Microscopic Methods. Microscopy, including
electron microscopy (EM) and optical microscopy can be
used as an indirect analytical tool to shed light on water related
phenomena in nanocelluloses, mostly with respect to drying and,
in some cases, also to swelling processes.
4.2.1.1. Electron Microscopy (EM). Conventional scanning
electron microscopy (SEM) and transmission electron micros-
copy (TEM) have been used to observe the morphology of
CNCs and CNFs, to determine their dimensions,
297,759,1014,1015
and to study the impact of dierent treatments and drying
conditions on nanocellulose structures.
299,444,525,760,900,1016
While an excellent tool to study nanocellulose, EM measure-
ments are carried out in vacuum, and so sample drying (e.g.,
ambient drying, freeze-drying, heated drying) has an important
and sometimes irreversible eect on the morphology of the
structures,
1017
demanding careful image interpretation. More-
over, SEM imaging often involves sputtering of the samples with
thin conductive layers which can distort sample features,
particularly at the nanoscale. In contrast to conventional SEM,
environmental SEM (ESEM) does not require as high a vacuum,
thus enabling measurements to be carried out with some
residual moisture and/or relative humidity.
1018
However,
controlling the imaging conditions is dicult, and the limited
resolution of the microscope may explain the restraint in using
this technique to explore nanocellulose−water interactions.
Nevertheless, a few examples highlighting the analytical value of
SEM and ESEM are briefly described below.
Natarajan et al. used SEM to quantitatively investigate the
morphology and chiral nematic structure of sulfated CNC films,
whereby the CNCs were neutralized with dierent cations and
dried under controlled conditions.
1016
SEM images revealed
that faster evaporation rates caused a disruption in the chiral
nematic liquid crystal ordering of the film due to vitrification.
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The images were also used to quantify the nominal pitch (i.e.,
the helical modulation length) distribution from fractured film
cross-sections using a novel image processing method (Figure
22a,b). The calculated pitch distributions were in good
agreement with UV−vis−NIR total reflectance spectra. While
Gaussian fitting the pitch distribution from SEM helped to
obtain peak positions and peak widths, the measured pitch and
standard deviation values showed an identical dependence on
the evaporation rate of water. For example, greater sample
uniformity upon slow evaporation gave a much smaller pitch
(i.e., a tighter chiral nematic twist) and smaller standard
deviation. Moreover, the SEM was sensitive to the multidomain
structures of the fast-dried films. In addition to pitch variations
related to dierent water evaporation rates, a gradient in pitch
was observed in going from the top to the bottom of each film
down the cross-section, which was larger for films and dried
more quickly. Finally, the SEM images showed that the shape of
moisture inclusions and water confinement in the CNC films
varied with the dierent counterions investigated.
1016
Lewis et al. used SEM to investigate the morphology of freeze-
dried hydrogels formed by cyclic freeze−thawing of CNC
suspensions
900
and confirmed the formation of an intercon-
nected, porous cellulosic sheet network. Ansari et al. used
fractography studies in SEM images of nanopapers from phenyl
glycidyl ether oligomer-grafted CNFs to relate property
dierences to structural changes.
444
Moreover, dierences in
swelling could be observed from the SEM micrographs. Rapid
dewatering of nanocellulose suspensions was also a key point in
the study of O
sterberg et al., who used hot pressing to prepare
robust, solvent-resistant, CNF films. SEM revealed a signifi-
cantly increased film density after extensive pressing (2 h).
760
In
contrast to SEM studies of dried nanocellulose structures, ESEM
enabled Gelin et al. to obtain information about the
nanostructure of BC with respect to water content at dierent
relative humidity values.
1018
ESEM images taken at 77% relative
Figure 22. Nanocellulose−water interactions revealed by microscopic techniques: (a) SEM images of the cross-section of pristine CNCs (i) fast-dried,
(ii) slow-dried, (iii) slowest-dried, and MePh3P-modified CNCs (iv) fast-dried, and (v) slow-dried. Scale bars = 5 μm.
1016
(b) Nominal pitch
distributions measured from SEM images of films (i−v).
1016
(a,b) Adapted with permission from ref 1016. Copyright 2017 American Chemical
Society. (c) AFM height image of a dry CNC film (i) which was scratched (ii) for cross-section height analysis (iii) to determine the film thickness.
284
(d) Change in CNC film thickness in water determined by AFM and SPR (solid line for eye guidance).
284
(c,d) Adapted with permission fromref 284.
Copyright 2009 The Royal Society of Chemistry. (e) Scheme representing CNC film moisture sorption. (i) moisture diusion into the film, (ii) 3D
scheme of the CNC film for moisture diusion analysis by cross-polarized microscopy, (iii) interspace between the CNCs increasing upon water
penetration, and (iv) simulation contour of moisture diusion after 15 h.
331
(f) Hygroscopic strain, obtained by digital image correlation, as a function
of the RH for self-organized CNC films for axial (black) and transverse (red) directions.
331
(e,f) Adapted with permission from ref 331. Copyright
2017 American Chemical Society.
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AR
Figure 23. Application of methods to measure surface energy and mass transport in investigating nanocellulose−water interactions. (a) The operating
principle of sessile drop goniometry.
1022
(b) A liquid droplet on a nonideal solid surface: (i) apparent and real contact angles on a rough surface, (ii)
apparent and real contact angles on a heterogeneous surface.
1029
(a,b) Adapted with permission from ref 1029. Copyright 2018 Springer Nature. (c,d)
Comparison of the dispersion component of surface energies of CNFs dried with dierent methods: (c) CNFs and (d) CNCs.
255
(c,d) Adapted with
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AS
humidity showed that the inner side of the BC gel tubes
contained between 10 and 40% water.
Even though TEM has been widely used to investigate the size
and shape of nanocelluloses either frozen (cryo-TEM) or dried
from water and other solvents, the technique has not been
directly used to investigate nanocellulose−water interactions.
This is likely due to sample preparation and imaging limitations
despite the nanometer spatial resolution. Nevertheless, TEM has
been used to corroborate the results obtained from other
methods. Gelin et al. coupled the information from freeze-
fractured samples imaged by TEM with the magnitude of
diusion coecients extracted from dielectric spectroscopy,
supporting the theory that free (or bulk-like) water present in
BC gels is confined as “lakes” rather than forming a continuous
phase throughout the gel structure.
1018
4.2.1.2. Atomic Force Microscopy (AFM). AFM, a scanning
probe microscopy technique, is another visualization tool to
investigate nanocellulose. Its versatility is based on the dierent
possible measurement modes and experimental setups including
imaging modes (e.g., contact mode or alternating current/
tapping mode), and surface force measurement modes (e.g., to
determine precontact attractive/repulsive/steric, adhesive, and
frictional forces, as well as mechanical properties), e.g., AFM
measurements can be done in ambient conditions, under
controlled temperature/humidity, or samples can be fully
submerged in liquid all while ensuring nanoscale and
piconewton resolution. For example, Reid et al., studied the
swelling behavior of CNC thin films in the presence of water, by
imagining the height profiles of scratches made into the film
before and after exposure to water (Figure 22c,d).
284
Ahola et al.
used colloidal probe AFM to measure forces between a cellulose
sphere, glued to a tipless cantilever, and a CNF surface.
1019,1020
They studied the eect of nanofibril charge density, ionic
strength, and pH on the swelling and surface interactions of
CNF model films; AFM and QCM-D were used in conjunction
to infer that both steric and electrostatic forces were present in
water and that steric forces dominated between cellulose
surfaces under low pH and high ionic strength condi-
tions.
1019,1020
4.2.1.3. Optical Microscopy. The limited resolution of
optical (or light) microscopy compared to other techniques
has restricted its application as a visualization tool for
nanomaterial characterization.
299
Nevertheless, there are
examples of indirectly using optical microscopy to investigate
nanocellulose−water interactions. For example, Shrestha et al.
used a contrast enhanced microscopy digital image correlation
technique to characterize the dimensional changes induced
during hygroscopic swelling of self-assembled and shear-
oriented CNC films (Figure 22e,f).
331
The authors applied the
distinct microstructure and birefringence of CNC films to
explore the in-plane hygroscopic swelling by correlating cross-
polarized microscopy images at relative humidity levels ranging
from 0 to 97%. The in-plane coecient of hygroscopic swelling
of the CNC films was thereby determined by optically tracking
humidity-driven strain fields in a contact-free manner.
4.2.2. Surface Energy and Mass Transport Methods. As
discussed earlier, surface energy together with the surface
roughness, determines the wettability of a surface by a liquid.
Among the most common ways to characterize the surface
energy of cellulosic materials are inverse gas chromatography
(IGC), thin-layer wicking (TLW), and dierent types of contact
angle measurements,
479
which will be outlined in the following
section. Moreover, as elucidated in sections 2.4.1.2 and 2.4.2.4,
surface energy and wettability are intimately linked with mass
transfer, the phenomenon of a net movement of species from
one location to another. Here, we will only consider methods to
characterize the mass transfer of water in cellulosic materials and
make a distinction between the transfer of liquid water and water
vapor. The mass transfer of liquid water can be characterized
using TLW experiments using the Lucas-Washburn eq (eq 1),
introduced in section 2.4.2.4. Moreover, the water mass transfer
across, for example, cellulose nanopapers can be measured
gravimetrically by dierent flow tests (gravitational or
pressurized)
1021
or even by diusion cells equipped with tritium
oxide labeling and radioactivity monitoring.
525
In the case of
water vapor transfer over cellulosic films, the vast majority of
literature report either the water vapor transmission rate
(WVTR) or its close variant water vapor permeability (WVP)
using a gravimetric method described in the following.
4.2.2.1. Contact Angle. Among the most common ways to
analyze the contact angle is sessile drop goniometry, where a
high-speed camera captures a video of a surface wetted by a
droplet of liquid. In the simplest setup, this is done on a
horizontal surface (Figure 23a),
1022
although other possibilities
also exist.
1023−1026
The captured images are then analyzed by
computer by numerically fitting a function to the images,
1027,1028
yielding an estimate for the contact angle at each moment of
time. The time-dependency of the contact angle varies a lot: for
example, a water droplet will wet a rough, high surface energy
solid almost instantly, whereas hydrophobic surfaces will resist
wetting by the droplet and the droplet will retain its shape until it
slowly evaporates.
509
It is therefore common to record not just
initial (or static) contact angle but also to observe how stable the
droplet is over time. One of the limitations of static contact angle
measurements is that they are not always reproducible, but one
way to overcome this is to record advancing and receding
contact angles, which tend to be more reliable.
1022
A dynamic extension of the static sessile drop technique
allows the detection of advancing and receding contact angles by
increasing and decreasing the volume of the deposited droplet
Figure 23. continued
permission from ref 255. Copyright 2013 Elsevier. (e,f) Water vapor uptake studies to investigate the eect of surface modification: (e) Water vapor
uptake isotherms of the initial and C14-modified CNF films (black squares and red circles, respectively), plotted together with data from comparable
CNF samples. (f) The qualitative dierence in wetting behavior of the two film types 20 s after being exposed to methylene blue dyed water droplets.
The hydrophobic C14-modified film is on the left, the untreated control film is on the right.
297
(e,f) Adapted with permission from ref 297. Copyright
2017 Springer Nature. (g) Permeation of water under ambient pressure and at room temperature for unmodified CNF nanopapers and modified CNF
nanopapers by hot pressing and lignin content. Hot pressing and lignin reduce the permeation of water in ambient conditions, significantly.
1021
(g)
Adapted under the terms of CC-BY from ref 1021. Copyright 2019 American Chemical Society. (h) Water flux of fabricated support layers for
enhanced adsorption of metal ions with and without sludge microfibers/cellulose nanofibers (CNFSL). Layered fabricated membranes indicate a
decrease in water flux but in situ TEMPO oxidation has no significant eect on water flux.
1030
(h) Adapted under the terms of CC-BY from ref 1030.
Copyright 2017 The Royal Society of Chemistry.
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AT
(Figure 23b).
1022
The dierence between these angles is
referred to as contact angle hysteresis,
1026
which provides an
indication of surface roughness, possible surface contamina-
tions, etc.
1026,1031
In the case of cellulosic materials, contact
angle hysteresis can also infer tendency of the material to absorb
moisture.
1032
The more pronounced the hysteresis, the more
firmly the droplet is attached to the surface and the more
wettable the surface is by the liquid.
1022,1026
To determine surface energies by contact angle measure-
ments, multiple probe liquids of varying polarities are used,
including ethylene glycol, formamide, toluene, glycerol, and
diiodomethane.
1033,1034
Surface energies can then be deter-
mined using the Fowkes method, where the surface energy is
divided into the dispersive and the polar component, combined
by a geometric mean approach.
1035
As contact angle measurements are sensitive to surface
roughness, they are at their most accurate on smooth and rigid
surfaces. Nanocellulosic surfaces are not considered smooth or
rigid, and as such this limitation should be kept in mind even
though contact angle analysis, especially by the static sessile drop
technique, is commonplace within the cellulosic materials
research literature. Spin-coating and multilayer Langmuir−
Blodgett/Schaefer deposition of nanocellulose films can be used
to obtain smoother films than those from solvent casting, but the
degree of aggregation of nanocellulose in liquid prior to film
preparation can also aect surface roughness and porosity; AFM
is a good method to quantify these film properties. Owing to its
operational simplicity, contact angle analysis has most often
been used in a semiquantitative fashion, where the wettability by
a static droplet of liquid water on a series of samples is
compared.
253,509,755
At the very least, significant dierences in
wettability can be detected by this simple approach, allowing for
a quick check of, for example, the success of a surface
hydrophobization treatment.
The multiple probe liquid approach has been applied to
calculate the surface energies of cellulose nanopapers,
784
CNC
thin films,
1032
ultrathin films of amorphous cellulose,
245
etc.
Moreover, through hysteresis analysis, cellulosic surfaces have
been shown to be sensitive to moisture uptake; some water
molecules inevitably remain on the surface upon retraction of
the droplet, further enhancing their wettability by water.
1032
It
should also be noted that cellulose readily swells and curls under
water during contact angle measurements, and these dynamic
changes caused by wetting and drying may significantly alter the
perceived surface energies of cellulosic surfaces.
232,245,253,254
These eects complicate the interpretation of contact angle
values,
1036
so an alternative strategy employing the Washburn
technique may be considered.
4.2.2.2. Washburn Technique and Thin-Layer Wicking.
Closely connected with contact angle analysis, the Washburn
technique (or the capillary rise technique) is based on the
spontaneous flow of a probe liquid through a column filled with
the porous material to be analyzed.
1037
It enables the
simultaneous determination of the pore radius, contact angle,
and surface energies of porous samples. The uptake of the probe
liquid can be detected either gravimetrically or by observing the
change in the height of the liquid layer.
1038
Very similar in
operational principle, TLW is an example of dynamic wetting
that is based on liquid penetration into a porous solid.
254,1039
All
of these techniques rely on a phenomenon called wicking, i.e.,
the spontaneous spreading of a liquid into a porous material
caused by the pressure dierence resulting from the spherical
meniscus of the wetting liquid.
254
To determine the surface
energy components of a solid by TLW, a multiple liquid
approach is used, much in the same way as in the contact angle
measurements.
254
At least three probe liquids of known
dispersive and polar components are required, and their contact
angle, vapor adsorption isotherm, or the penetration rate should
be measured.
1040
Operating under a set of assumptions,
1040
a
modified form of the Lucas−Washburn equation relates the rate
of penetration to the surface energy components:
1041
=xRt G
2
2
(2)
where xis the distance traveled in time t,ηis the liquid viscosity,
Ris the eective capillary radius, and ΔGis the Gibbs energy
change accompanying liquid penetration into the solid. ΔGmay
then be converted to the surface energy by dierent means,
depending on the type of the liquid−solid system in
question.
1041
TLW has been applied to measure the surface energy of
microcrystalline cellulose (MCC)
239
and cotton fabrics,
1042
but
its role in the cellulose materials science has remained much
more peripheral than the omnipresent contact angle measure-
ments. The same applies to other wicking-based techniques,
including capillary intrusion that has also been utilized to
measure the surface energy of MCC.
114
As we noted in the
context of contact angle analysis, the adsorption of water vapor
on nanocellulose surfaces has an influence on the wicking
behavior of water.
254
This is a relevant concern for an extremely
hygroscopic material like cellulose that is in practice covered by a
layer of adsorbed water molecules at all times.
1043
4.2.2.3. Inverse Gas Chromatography (IGC). In contrast to
contact angle and wicking-based experiments that are sensitive
to sample roughness and porosity, IGC can be used to
characterize the surface energies of particles. The basic principle
of IGC is similar to gas chromatography (GC) in the sense that
there is a gaseous substance passing through a column of a
stationary phase. In IGC, the sample to be investigated acts as
the stationary phase, whereas in traditional GC, the sample
would be carried by an eluent gas.
1044
IGC analysis can be run at dierent relative humidity and
temperature and can yield information on, for example, the
specific surface area, degree of crystallinity, solubility, and
thermodynamic interaction parameters, glass transition temper-
atures, and the dispersive and polar components of surface
energy.
1044,1045
IGC surface energy analysis is run at infinite-
dilution conditions utilizing a number of dierent probe
molecules, usually a series of alkanes (e.g., hexane, heptane,
octane, etc.) in combination with polar probes (e.g., dichloro-
methane, acetone, acetonitrile, and ethyl acetate).
1045,1046
The
retention times and surface coverages are then recorded and
used to calculate the dispersive component, polar component,
and total surface energies. This requires the assumption that all
probe liquid retention is due to adsorption solely to the sample,
and that the interactions between the probe molecules are
negligible.
1047,1048
For cellulose and nanomaterials made thereof, IGC has been
used to assess the surface energies of wood-based and cotton
fibers,
106,237,1049
MCC,
114,1050
CNFs,
236,242,255
and CNCs,
534
nanostructured cellulose II particles,
243
and even amorphous
cellulose,
241
although the reliability of the analysis of amorphous
samples may be aected by probe diusion into the material.
1050
In the context of cellulose−water interactions, the possibility
to determine the surface energy components at dierent relative
humidity values oers insights into the rearrangements that take
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place when cellulose is subjected to moisture (Figure 23c,d).
Dierences in cellulose reactivity have been reported, depending
on the utilized drying method, and it is likely that the measured
surface energies are also sensitive to such eects, as evidenced by
XPS and contact angle measurements.
232
4.2.2.4. Gas Permeability Studies. Several standardized tests
exist for the permeability of various gases through a number of
polymeric materials. Here, we will only consider water vapor
permeability measurements of cellulosic films, although the
permeability of other gases through cellulose nanopapers also
Figure 24. Application of gravimetric methods in the investigation of nanocellulose−water interactions: (a) An example of a gravimetric water
retention analysis apparatus.
1068
(a) Adapted under the terms of CC-BY from ref 1068. Copyright 2014 North Carolina State University. (b) The water
retention values (g/g) of the CNFs produced from semichemical pulps obtained from neutral sulfite (NS) pulping (with or without the addition of
anthraquinone (AQ)), as a function of specific energy consumption (kWh/kg) in the process.
1069
(b) Adapted under the terms of CC-BY from ref
1069. Copyright 2020 Springer Nature. (c) Typical behavior exhibited by a lignocellulosic material by DVS, when desorbing moisture from a fully
water-saturated state and when desorbing moisture from a nonwater-saturated state. Hysteresis between the adsorption and desorption isotherm is
shown.
1066
(c) Adapted with permission from ref 1066. Copyright 2009 John Wiley and Sons. (d) Sorption hysteresis of four nanocellulose samples
(both CNC and CNF), measured by DVS.
215
(d) Adapted under the terms of CC-BY from ref 215. Copyright 2017 Springer Nature. (e) Change in
frequency (Δf) and dissipation (ΔD) as a function of time in stepwise increasing relative humidity (% RH) as detected by QCM-D water vapor
adsorption measurements for TEMPO-oxidized CNF (TOCNF) thin films (dierent overtones are indicated with colors).
282
(f) (i) Mass (pink),
optical thickness (blue), and thickness (green) fractions of water in TOCNF thin films due to water vapor uptake as a function of relative humidity.
Thickness and optical thickness fractions of water are deduced from data collected by SE. Mass fractions of water are based on QCM-D measurements;
(ii) schematic illustration of the water vapor uptake of a TOCNF thin film in dry air (RH < 10%), in humid air (10−75% RH) and at high humidity
levels (RH > 75%).
282
(e,f) Adapted with permission from ref 282. Copyright 2017 American Chemical Society.
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AV
increases at relative humidity values of approximately 70−
80%.
745,1051
The simplest way to assess the water vapor
permeability of cellulose nanopapers is gravimetrically, by the
so-called wet cup method,
1052
although corrections have been
proposed to the method.
1053,1054
In the common wet cup setup,
a sealable cup is filled with a constant volume of distilled water,
covered tightly by the tested material, and placed on the top of a
balance that records weight loss as a function of evaporation
time.
253,1055,1056
Relative humidity inside the cup is assumed to
be 100%, and the relative humidity outside the cup can be
controlled by the use of saturated salt solutions, for
example.
253,1057
An analogous but reversed setup (i.e., the dry
cup method) also exists, where a desiccant material is used on
one side of the material (Figure 23e,f).
1054
This diusion-driven process is sensitive to changes in
temperature and relative humidity (and thereby partial
pressure) on both sides of the nanopaper,
291
and meaningful
comparisons of mass transfer rates can only be made when these
factors are kept constant. Obviously, properties of the
nanopaper such as thickness,
292
crystallinity,
293
porosity, and
pore size as well as pore structure,
295
and hydrophilicity,
294
also
influence mass transfer rate, and it is usually necessary to
characterize these properties in combination with the measured
WVTR values to truly understand the material behavior. WVTR
can be calculated from the steady-state region of the mass
loss:
298
=
×
× ×WVTR m
A t g m d( )
2 1
(3)
where Δmis the change in sample mass, Ais the evaporation
area, and tis time.
Obviously, the underlying phenomena in WVTR are related
to mass transport as elaborated in section 2.4.2.4. As the
foremost application potential of cellulose nanopapers is as
barrier films, the focus research in the field has largely been on
improving their barrier properties in humid condi-
tions,
745,755,767,768
rather than maintaining ecient mass
transfer across the nanopaper.
297
As the transport of gases and
liquids proceed dierently, and it is easier to block the flow of
liquid water with its high cohesion and surface tension than that
of individual water molecules.
4.2.2.5. Liquid Permeability Studies. In the nanocellulose
literature, liquid water mass transfer values are often omitted
from the experimental setup, even if dierent iterations of
contact angles and WVTRs are frequently reported. The ease of
dewatering during cellulose nanopaper preparation has received
far more attention
784,1051,1058,1059
than the permeability of liquid
water through pre-existing cellulose nanopapers, although some
accounts do exist (Figure 23g,h).
1021,1030
For relatively water-permeable nanocellulose films, dierent
flow tests have been applied which rely on either atmospheric
pressure or an added pressure gradient for mass transfer,
1021,1060
and the permeated water is recorded as either volume or mass.
Despite their tendency to swell and lose their gas barrier
function in water, cellulose nanopapers can eciently slow down
the transport of liquid water. To characterize the mass transfer
that is too slow to be meaningfully determined by gravimetric or
volumetric methods (due to evaporation), an alternative
strategy of employing tritium oxide labeling may be consid-
ered.
525,1061
In such a setup, a dual chamber is divided by the
tested film and filled with deionized water simultaneously on
both sides to avoid any pressure dierences. Then, a small
quantity of tritiated water (tritium oxide) is added on the donor
side of the chamber, and the radioactivity of water on the
acceptor side is monitored at regular time intervals (the
measurement is carried out under constant stirring).
525,1061
This technique is more cumbersome than gravimetric flow tests
and requires special facilities suitable for working with
radioisotopes.
4.2.3. Gravimetric Methods. Several gravimetric methods
exist for the analysis of water retention and water uptake capacity
of pulp and paper materials,
332,1062−1067
and many of them have
been applied to the analysis of nanocelluloses as well. In this
section, we will move from more crude techniques such as water
retention value (WRV) and Cobb tests to those of higher
sensitivity such as DVS and QCM-D.
4.2.3.1. Water Retention Value and Cobb Test. The
WRV
1063
and the Cobb test
1064
provide slightly dierent
perspectives on the water retention and water uptake capacity of
cellulosic materials. As the swelling ability of cellulosic materials
depends on the pH and ionic strength of the solvent, and the
charge group and counterion of the nanocellulose, these should
be kept constant when running these types of analyses in order
to yield comparable data.
134,285
WRV can be used for the
quantification of the liquid water that remains in a saturated
cellulose sample after a controlled centrifugation cycle (i.e., a
capillary swelling test) (Figure 24a,b), whereas in a Cobb test, a
preconditioned cellulose sample is wet with a constant volume
of water for a given time, followed by a rapid removal of any
excess water.
The quantity of retained (WRV) or absorbed (Cobb test)
water is then determined through gravimetric analysis. For
WRV, the quantity of retained water is expressed as g of water
per g of cellulose and calculated by eq 4:
1063
=WRV
m
m
1
1
2
(4)
where m1is the mass of the centrifuged wet test pad and m2is the
mass of the test pad after oven drying.
The Cobb test result, or absorptiveness, is expressed as g of
water per m2cellulose, and calculated by eq 5:
1064
= ×m mabsorptiveness ( ) 100
w c
(5)
where mwis the mass of the wet sample, mCis the mass of the
conditioned sample, and the multiplication factor 100 is used for
the standard specimen area of 100 cm2.
Both WRV and the Cobb test have been applied with slight
modifications for the analysis of microfibrillated cellulose and
CNFs,
1058,1068−1074
whereas WRV has been used to characterize
BC.
1075
However, the characterization of CNCs using these
means it is not advisible due to the risk of film disintegration
and/or material loss during the experiments.
4.2.3.2. Dynamic Vapor Sorption Analysis (DVS). DVS has
been widely applied to study the adsorption−desorption
behavior of gases on solids. In relation to cellulose−water
interactions, it has been utilized extensively to study wood,
1076
various plant fibers,
1066
MCC,
1065
CNFs,
1077
and CNCs.
215
In
its most simple application, DVS can be used to quantify how
much a given mass of material “takes up” water vapor at a given
relative humidity and temperature (i.e., its equilibrium moisture
content) (Figure 24c,d). The measuring device is essentially a
high-precision mass balance that measures the weight of the
sample at dierent relative humidity values (or partial pressures
of gases other than water vapor) and temperatures. DVS can also
be used to study the kinetics of the water uptake.
215
The rate of
adsorption and desorption of water from nanocellulose
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structures has been associated with the accessibility of cellulose
hydroxy groups, another parameter frequently researched using
DVS, often in conjunction with deuterium exchange.
205,1078
Moreover, cellulose hydroxy accessibility has been found to
correlate well with the equilibrium moisture content of the
material.
106,205
When the relative humidity is increased and decreased
gradually, a sigmoidal adsorption−desorption isotherm is
obtained, the shape of which provides indirect information on
material properties such as pore size, cumulation mechanism of
the vapor (i.e., monolayer or multilayer formation), and capillary
condensation.
215,1066,1079
It should also be noted that the
adsorption and desorption isotherms are dierent; this
phenomenon is referred to as hysteresis, reminiscent of the
dierence between the advancing and receding contact angles.
Part of the reason may lie in that the mechanisms of adsorption
and desorption are dierent (sorption on an initially dry surface
as opposed to evaporation from a capillary meniscus). Another
proposed explanation is that the adsorption and desorption take
place from dierent material states (a dry/collapsed vs a swollen
one).
1065,1066
The sorption hysteresis of freeze-dried CNFs has
been reported to be larger than that of CNCs, which is
interpreted to be the result of the higher portion of disordered
cellulose in CNFs.
215
DVS data has been compared with that
extracted from techniques such as WRV
1078
and thermopor-
ometry,
1065
during both the initial building of a hydrated layer
and after all of the material pores have been filled with water (i.e.,
saturation). DVS and thermoporometry data from macroscopic
cellulose fibers by Driemeier et al. suggested that both the
hysteresis behavior and wet porosity are the result of the
suprafibrillar organization of cellulosic material
1065
and that
there may be an “ink bottle” type of eect that would further
explain the diculty of water removal upon desorption.
1066
The topic of sample preparation is crucial, especially when
applied to nanocellulose. Usually, DVS experiments are carried
out for predried samples that are wetted during the measure-
ment, although it is in principle possible to dry the sample in situ
while monitoring the weight loss of the sample. As discussed in
sections 2.4.2.5,3.1.1, and 3.2.1, all cellulosic materials undergo
structural collapse upon drying, and this is especially true for
CNFs that change from a swollen hydrogel state to a rigid film-
like material. This structural collapse has an influence on the
swelling ability and accessibility of nanocelluloses, as is evident
from experiments attempting to redisperse dried CNFs
1080−1083
or modify them chemically.
333,413
In practice, DVS analysis of
celluloses (and especially of nanocelluloses), is influenced by the
treatment and drying history of the material (e.g., air-dried from
water or another solvent,
126
freeze-dried,
318
or supercritical
drying
1080
). The structural changes aect the moisture sorption
and retention in the material, and therefore special care should
be devoted to the method of sample preparation to obtain
relevant information on its water uptake properties. The main
limitation of DVS is that it only provides quantitative assessment
of the vapor uptake of a material, with no information about the
structural changes that take place during the process such as
swelling or changes in viscoelastic properties as is the case with
QCM-D.
4.2.3.3. Quartz Crystal Microbalance with Dissipation
Monitoring (QCM-D). QCM-D is a microgravimetry technique
which simultaneously measures changes in the resonance
frequency and energy dissipation of an oscillating piezoelectric
quartz crystal sensor as a result of changes in the mass of the
crystal.
1084
Experiments can be carried out in both liquid and
vapor environments, yielding quantitative information on the
adsorption of, for example dissolved polymers
1085
or water
vapor, respectively.
271
QCM-D is a highly sensitive micro-
balance that detects extremely small changes (<1 ng/cm2) in
mass of the sensor as a change in the oscillation frequency
(Δf).
1084
For uniform and rigid layers, the change in mass of the
sensor (Δm) is proportional to Δf, according to the Sauerbrey
equation:
1086
=mC f
n
(6)
where Cis the sensitivity constant of the device (typically C≈
0.177 mg m2Hz−1), and nis the measurement harmonic number
(n= 1, 3, 5, ...).
1087
The simultaneous monitoring of energy
dissipation when the sensor stops oscillating provides a measure
of the viscoelastic properties of the film. The dissipation of
energy is defined through eq 7:
=D
E
E2
lost
stored
(7)
where Elost is the dissipated energy during one cycle of oscillation
and Estored is the total energy stored in the oscillator.
QCM-D therefore provides quantifiable information on the
mass of adsorbed molecules onto an ultrathin model film and
also yields information on the changes in the physical properties
of the film as a result of adsorption. Indeed, a number of studies
have been carried out on model films of regenerated
cellulose,
245,1088
CNCs,
271,279
and CNFs
280,281,1089
to better
understand the swelling of these materials (Figure 24e,f). It is
important to note that the method of film deposition onto the
quartz sensors greatly influences film morphology and thereby
also its water uptake capacity.
280
For cellulose solutions, the
main techniques of ultrathin film preparation on QCM sensors
are spin-coating
1090,1091
and Langmuir−Schaeer deposi-
tion,
1092,1093
the former being much faster but yielding films
that are not in thermodynamic equilibrium, whereas Langmuir−
Schaeer films, are deposited slowly (one monolayer at a time),
allowing for the organization of the cellulose into stable films.
Spin-coating has been widely used for the preparation of CNF
281
and CNC surfaces
271
but accounts on their adsorption and
electrophoretic deposition exist too.
280
QCM-D has been used
in multiple studies to determine the water adsorption capacity of
the cellulose following a solvent-exchange protocol, whereby
H2O is exchanged for D2O (originally on regenerated
cellulose).
276
Delepierre et al. used the same technique to
measure the bound water in CNC films and reported that a
decrease in remnant surface oligosaccharides of dierent grades
of CNCs corresponded to an increase in the water binding
capacity.
1094
Niinivaara et al. surface modified CNCs with
oligosaccharides and used the same technique to show that
oligosaccharide coated CNCs demonstrated slightly lower water
adsorption capacities
528,1094
4.2.4. Spectroscopic Methods. 4.2.4.1. Nuclear Magnetic
Resonance Spectroscopy (NMR). 13C solid-state NMR has long
been recognized as viable tool to determine the degree of
crystallinity of celluloses by taking advantage of variations in
chemical shifts and 13C spin−lattice relaxation time constants in
ordered and disordered molecular regions.
1095
Consequently,
the amorphous and crystalline signals of the C4 and C6 carbon
appear at a dierent chemical shift, and their ratio can be used to
calculate degree of crystallinity with sucient accuracy.
1096,1097
Besides providing information on the molecular order of
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Figure 25. Analysis of cellulose−water interactions by NMR (a−c), IR (d), and dielectric (e,f) spectroscopy, and by spectroscopic ellipsometry (SE, g-
i) and surface plasmon resonance spectroscopy (j-l): (a) Solid echo 2H NMR spectra of dried and wet cellulose at 210 and 300 K. Transition of the
typical Pake pattern (slow 2H exchange) to narrow Lorentzian peaks (fast 2H exchange).
1102
(a) Modified with permission fromref 1102. Copyright
1996 American Chemical Society. (b) 2H MAS NMR spectra of hydrated and redried MCC in full scale (top) and magnified (bottom) illustrating the
signals arising from mobile water (intense central peak) and from the other deuterium-containing sample fractions (SSBs spread over a broad
frequency range).
270
(b) Modified under the terms of CC-BY from ref 270. Copyright 2017 Royal Society of Chemistry. (c) Comparison of
experimental and simulated distributions of 13CT1 relaxation times, normalized to the same value, for C6.
995
(c) Reproduced with permission from ref
995. Copyright 2019 American Chemical Society. (d) Static FT-IR spectra of spruce cellulose at increasing relative humidity with deuterium vapor.
204
(d) Modified with permission from ref 204. Copyright 2006 Springer Nature. (e) Dielectric site model of a polysaccharide repeating unit. (f) Principal
structure of a dielectric loss spectrum of a wet polysaccharide.
1104
(e,f) Adapted with permission from ref 1104. Copyright 2001 Elsevier. (g−i) SE
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AY
cellulose, dierent 1D and 2D solid-state NMR spectroscopy
techniques are also used to characterize specific cellulose−water
interactions. For instance, signals of surface crystalline domains
observed through 13C CP/MAS NMR provide qualitative
insights into the (irreversible) structural changes upon drying,
as a consequence of cellulose microfibril aggregation (i.e.,
hornification).
105,1098
As with many of the analytical techniques
discussed, sample preparation is of particular importance here to
ensure minimal changes to the cellulose crystallinity due to, for
example, previous drying, milling, or chemical treat-
ments.
1098,1099
For example, Newman et al. were able to
demonstrate changes within the surface crystalline domains of
softwood kraft pulp simply due to wetting and drying cycles.
105
Both 1H and 2H (after a hydrogen−deuterium exchange)
solid-state NMR experiments can be used to probe water
interaction dynamics with nanocelluloses and gain an under-
standing of the state of the water present (i.e., free or bound
water). 1H MAS NMR has also been used to investigate water
mobility as a function of the moisture content. Vittadini et al.
showed that with decreasing relative humidity (<12%), the 1H
spectra of cellulose show a sharp Lorentzian signal typically
observed for the free water component, and a wide Gaussian
contribution, which dominated the spectrum at low relative
humidity and was attributed to the rigid protons (i.e., low
mobility) of the cellulose backbone and bound water.
1100
High-resolution 2H NMR has significant advantages over 1H
NMR for identifying dierent water populations in a cellulose
sample as it is (i) selective toward accessible hydroxy groups and
(ii) based on the quadrupolar coupling of intramolecular and
single-spin properties, thus favoring intramolecular interac-
tions.
270,1101
Combined, these features enable the identification
and quantification of distinct water populations. Less mobile
molecules (i.e., those with a slow 2H-exchange) give broad, Pake
patterned 2H NMR spectra, while narrow Lorentzian peaks are
typically observed for highly mobile molecules (i.e., those with a
rapid 2H-exchange) (Figure 25a).
270,1102,1103
2H single-pulse
experiments are known to result in broad spectral components,
making the interpretation of immobile domains at low water
contents speculative.
1100
On the other hand, 2H static
quadrupolar echo (QE) NMR provides a high-resolution
information by refocusing the large quadrupolar broadening,
which enables the observation of slow molecular dynamics.
1102
However, the experiment is restricted by a low signal-to-noise
ratio (SNR), which complicates the identification and
quantification of the dierent water populations.
270
Furthermore, 2H MAS NMR has been used to confirm the
existence of an immobile and a mobile adsorbed water phase in
hydrated cellulose, and to quantify the fraction of each
population.
270
Through magic angle spinning, orientation
averaging can be used to remove chemical shift anisotropy and
produce spectra resembling that of liquid water with an
improved SNR. Such experiments rely mainly on the
interpretation of spinning sidebands (SSBs) (i.e., the contribu-
tion of immobile water and cellulose 2HO) (Figure 25b), which
become visible if the sample is spun at a rate less than the
magnitude of the anisotropic interaction. The ratio of integrals
and line width of the SSBs compared to the central peak (i.e.,
mobile water) at dierent relative humidity, thus providing
direct quantitative information on the dierent adsorbed water
phases after subtracting the contribution of cellulose deuteroxys.
Furthermore, it provides information on the dierent locations
of immobile and mobile adsorbed water on/within the
microfibril aggregates. Moreover, 2H MAS NMR can provide
information on the state of the cellulose hydroxy groups and site
specific 1H−2H exchange.
203
In addition to 1D NMR experiments, 2D heteronuclear
1H−13C wide-line separation (WISE) NMR can provide
information on water localization and organization in cellulose
and cellulose−polymer blends.
1102
This technique is based on
the correlation between a high-resolution 13C spectrum with a
wide line 1H spectrum, where the proton line width refers to the
polymer chain dynamics.
1105
Moreover, the insertion of a spin
diusion time enables direct observation of dipolar couplings
between mobile water and cellulose protons, where the spin
diusion coecient is related to the distance between cellulose
and water.
1102
Another approach to characterize cellulose−water interac-
tions is through the acquisition of spin−lattice (i.e., longitudinal,
T1) and spin−spin (i.e., transverse, T2) relaxation times using
NMR relaxometry.
1101,1106,1107
The relaxation behavior of
nuclei after excitation is a fingerprint in molecular dynamics
that can provide information on, e.g., water adsorption. Terenzi
et al. used 2H transverse and 13C longitudinal NMR relaxation
measurements to assess the hydration of CNF/xyloglucan
nanocomposites.
1108
The length and width of T2as measured by
2H NMR provided information on the orientational molecular
mobility of water molecules present in the nanocomposites, in
addition to water molecule distribution and characteristic length
scales, respectively. They demonstrated that a short T2can be
attributed to slow water mobility and, thus, strong poly-
saccharide−water interactions.
1108
On the other hand, T1, as
measured by 13C CP/MAS, was used to quantify the dierence
in molecular mobility between ordered and disordered
(including surface) regions in CNF as a function of relative
humidity. Chen et al. compared molecular dynamic simulations
and experimental relaxation data to quantify nanoscale
structure−function relationships of CNFs in a highly hydrated
and aggregated state.
995
This approach enabled the deconvo-
lution of experimental 13C NMR T1distributions into
contributions from both accessible and inaccessible regions of
the aggregated CNFs and to distinguish between local C−H
dynamics at dierent carbon sites (i.e., C1, C4, and C6) (Figure
25c). In addition to 13C and 2H, 17O relaxation data has also
been used to study cellulose−water interactions.
1101
Similar to
NMR relaxometry, low-field time domain (LF-TD) NMR can
also be used to study the dynamic properties of polymer chains
Figure 25. continued
adsorption isotherms of (g) 5, (h) 10, and (i) 20 g L−1CNC thin films. Hysteresis of swelling is a measure of the dierence between the integrated areas
below the adsorption and desorption isotherms.
271
(g−i) Adapted with permission from ref 271. Copyright 2015 American Chemical Society. (j)
Swelling profiles of highly sulfated CNC (HS-CNC) films measured by SPR. Curves are normalized to the angular shift immediately following NaCl
solution addition. (k) Calculated volume of water within the CNC films (LS-CNC, low sulfate content CNC; CAT-CNC, cationic CNC) in various
concentrations of NaCl after 30 min of swelling determined from SPR profiles. Film porosity (the volume of air in the films initially) is taken as 20%. (l)
Diusion constants calculated for water in CNC films measured from SPR swelling experiments in NaCl solutions.
283
(j−l) Adapted with permission
from ref 283. Copyright 2017 American Chemical Society.
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AZ
from the relaxation of 1H spins. Felby et al., for instance, used
FT-TD NMR to study cellulose−water interactions during
enzymatic hydrolysis.
1109
4.2.4.2. Fourier Transform Infrared Spectroscopy (FTIR).
FTIR spectroscopy is commonly used to elucidate cellulose−
water interactions. Particularly, it is sensitive toward dierences
in OH conformation and the hydrogen bonds of dierent
cellulose polymorphs,
1110
even though the exact assignment of
the strongly overlapping bands of cellulose hydroxy groups, and
the dierent water populations is a matter of debate.
206
For
example, when observing cellulose−water interactions, strongly
bound water is associated with the 3200 cm−1band (−OH
stretching range) in the cellulose spectrum, whereas the 3600
cm−1band is associated with loosely bound water that bridges
via another water molecule to cellulose.
610
Moreover, the main
spectral regions aected by water adsorption are between 3700
and 3000 cm−1(mainly −OH stretch), 1740−1618 cm−1(−OH
bending), and 1190−1139 cm−1(C−O−C asymmetric stretch
of the glycosidic linkage).
1110,1111
Thereby, the signal at ca. 1640
cm−1is very often loosely used as a reference for adsorbed
water.
1112−1114
Dierent IR instrumentation setups oer various possibilities
to characterize cellulose−water interactions. Micro-FTIR is
equipped with a light microscope, which allowed for the
combined analysis of changes in sample morphology and in situ
spectral analysis during water sorption.
1111
Polarized IR, on the
other hand, relies on the polarization dependence of specific IR
absorptions of oriented molecules and can be used to unravel the
origins of the −OH signals found in the vibrational spectra of
crystalline cellulose.
1115
Hofstetter et al. used dynamic FTIR,
which couples dynamic mechanical analysis with step-scan FTIR
and has better spectral resolution compared to static FTIR, due
to the mechanical extension to investigate the interactions
between cellulose and moisture.
204
Moreover, the authors
conducted a controlled 1H2O−2H2O (H2O−D2O) exchange,
which provided direct evidence on the accessibility of the
dierent cellulose domains
204,1116
(Figure 25d).
203,204,206
4.2.4.3. Dielectric Spectroscopy (DS). DS is a powerful tool
to examine molecular motions of polymeric systems in an
extended time span (0.1 ns to 100 s) by probing the dielectric
properties of the material.
1117
Hence, the central prerequisite of
this method is the existence of mobile molecular units with a
permanent dipole moment or the presence of free mobile
charges creating space charges at interfaces.
1118,1119
In the case
of cellulose, for example, only the dipolar sites (Figure 25e) in its
specific orientation are of interest.
1119
The application of DS in
cellulose research dates back over a century, and its principles,
parametrizations, and experimental aspects are presented in an
excellent way by Einfeldt et al.
1104
In brief, DS measures the response of the dielectric properties
of a material in an alternating electric field with a frequency
range typically between 10−6and 1012 Hz (i.e., broadband
dielectric spectroscopy (BDS))
1117
The latter is of particular
relevance for cellulose,
1104
and in fact cellulose has often been
characterized in the low-frequency range, i.e., by dielectric
relaxation spectroscopy (DRS), in which the dielectric response
is largely dominated by structural eects.
1119
Consequently,
sample preparation for DRS analysis is highly important, and the
dielectric response is highly sensitive to water content.
1104
The response of the sample’s electric dipole moment (i.e.,
electric polarizability) is directly measured as complex
impedance (Z*), giving the real (ε′, dielectric store coecient)
and the imaginary (ε″, dielectric loss coecient) part of the
complex permittivity (ε*(f,T)).
1104,1117
Both parameters, ε′
and ε″, are central for the interpretation of cellulose−water
systems and are typically recorded and interpreted as a function
of the field frequency (f) and/or the temperature (T).
1018,1120
Isothermal measurements at varied frequencies are thereby
favored because of their possible quantitative interpretation.
1104
Moreover, if the sample has a noticeable conductivity (e.g., wet
samples), conductivity spectra (i.e., σ*(f,T)) are necessarily
discussed.
1018
As explained by Einfeldt et al., the macroscopic dipolar
moment (Μ) considers all molecular groups with a permanent
dipole moment, which in the case of cellulose is three types of
dipolar and movable sites: (i) the pyranose ring (orientational
motions around the glycosidic linkage), (ii) the C2/C3 side
groups (rotational mobility around the C−O linkage), and (iii)
the C6 side group (mobility around the C5−C6 linkage and the
C6−O linkage).
1104
Water bound to cellulose hydroxy groups
increases the dipolar moment of the side groups aecting the
main polymer chain mobility and produces an additional
relaxation process.
1118
Moreover, the polymeric relaxation,
expressed by Μand ε*, is aected by cross-correlation functions
of dierent dipolar moments within the same chain and between
neighboring chains, as a consequence of the complex hydrogen
bonding network.
Parts e and f of Figure 25 show the dierent secondary
relaxation processes for wet cellulose in the form of a dielectric
loss (ε″) spectrum, in which βand βwet relaxations are the most
relevant processes. They are observed below the glass transition
temperature and in the low frequency range. βrelaxations have
been related to local motions of the main chain at low
temperatures, whereas βwet relaxations are observed in wet
samples in the room temperature range
1121
and are attributed to
orientational motions of cellulose and water in a mixed
phase.
1118,1119
The latter disappears completely in dry samples
and must be distinguished from σrelaxations which are observed
for well-dried polysaccharides in the high temperature range.
1104
The relaxation strength of βwet relaxations decreases with lower
water contents, accompanied by a shift of βwet to lower
frequencies, which has been related to the eect of water on
the activation energy of local polymer reorientational
motions.
1104,1118
The presence of water in a cellulose sample increases the
electric conductivity, which superimposes the dielectric
processes of the loss spectrum. This has been associated with
the ionization of water, leading to an increase in the number of
OH−and H+ions. Thus, the dielectric loss spectrum must be
corrected by a simultaneous measurement of the conductivity
(σ0).
1018,1122
As consequence of the drastic eect of water on the
electric conductivity of cellulose, the investigation of cellulose−
water interactions and mixed-phase dielectric processes using
DRS is limited to low water contents (max 12−15 wt %).
Seemingly, low water contents (<6 wt %) do not aect the high
frequency ε′of cellulose, which is related to bound water that
does not contribute to the dielectric response.
1123
4.2.4.4. Sum Frequency Generation Vibration Spectrosco-
py (SFG-VS). SFG-VS is a versatile method for in situ study of
molecular arrangement at surfaces and interfaces. SFG is a
second-order nonlinear optical process where a tunable pulse
infrared (IR) laser beam is mixed by spatial and temporal overlap
with a fixed wavelength visible (VIS) laser beam to generate a
sum frequency output beam. Because SFG at the phase-
matching condition require noncentrosymmetry, it is highly
sensitive and specific to surfaces and interfaces as long as the
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BA
bulk phase is amorphous or centrosymmetric.
1124,1125
Crystal-
line cellulose bulk has a noncentrosymmetrical environment,
which makes it dicult for SFG to confidently distinguish the
OH groups of the surface vs bulk. However, regenerated
cellulose models (amorphous cellulose)
1091,1126,1127
would be a
promising substrate for circumventing this problem with
cellulose−water interface studies using SFG.
4.2.4.5. Spectroscopic Ellipsometry (SE). SE is an optical
technique which can be utilized to measure the thickness of thin
films based on changes in the polarization of light as it is reflected
through the film. A beam of single wavelength or spectroscopic
light is passed through a polarizer, which is then reflected o the
surface of a thin film. Traveling through the film, the light
undergoes a change in its elliptic polarization dependent on the
thickness (and porosity) of the film.
1128
The magnitude of the
change in polarization can directly be used to model thin film
properties such as thickness, refractive index, and extinction
coecient. For cellulosic materials, the classic Cauchy
model
1129
is typically applicable to nonabsorbing transparent
films, and as such is commonly used to model cellulose thin
films.
280
SE is a well-established and nonintrusive technique
with very high precision and accuracy reaching a resolution in
the order of 1 Å for optically uniform and transparent films.
However, it is worth mentioning, that ellipsometry data is most
meaningful when used in conjunction with complementary
characterization techniques such as AFM, QCM-D, or surface
plasmon resonance (SPR).
1130
In terms of nanocellulose−water interactions, SE can be used
to monitor changes in the film upon water vapor sorption in situ
at the nanoscale (Figure 25g−i).
1128
Furthermore, because
swelling upon water sorption in nanocellulose thin films is
restricted only in the lateral dimension, SE can also be used to
carry out volumetric analysis.
1131
For example, Niinivaara et al.
unveiled that upon hydration with water vapor, each CNC in a
spin-coated film become enveloped in a 1 nm thick layer of
adsorbed water (or three monolayers of water molecules),
resulting in a two nanometer increase in film thickness for every
layer of CNCs within the film as measured by SE.
271
Hakalahti et
Figure 26. Using DSC to uncover nanocellulose−water interactions. (a,d) Characteristic DSC melting curve for CNCs and CNF. (b,e) Split peak
fitting curve of CNCs-0.5% and CNF-0.5%. (c,f) Distribution histograms of various water components in dierent solids of CNCs and CNF. FW, BWf,
BWnf, and TBW stand for free water, freezing bound water, nonfreezing bound water, and total bound water, respectively.
332
(g,h) TBW and zeta
potential of redispersed nanocellulose dispersion after dehydration. (a−h) Adapted with permission from ref 332. Copyright 2019 Elsevier. (i)
Schematic DSC curves of water on dierent cellulose samples: free water (peak I) and freezing bound water (peak II).
272
(i) Adapted with permission
from ref 272. Copyright 1981 Sage Publications.
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BB
al. used SE (and QCM-D) for determination of thickness
fractions of water in the TOCNF thin films in dierent relative
humidity values. Thickness fractions of water in the TOCNF
thin film were calculated by dividing the thickness increase due
to water vapor sorption at a given point along the % relative
humidity spectrum by the total thickness of the TOCNF thin
film at the same % relative humidity point under the assumption
that all changes in thickness or optical thickness occurred due to
water vapor uptake. Hence, this method describes the added
thickness due to sorption of water, but it does not dierentiate
between actual thickness of the water molecules and thickness
changes in the film due to structural changes like swelling or
reconfiguration of individual fibrils or hemicellulose.
282
4.2.4.6. Surface Plasmon Resonance Spectroscopy. SPR is a
surface sensitive optical technique used to measure the
adsorption/desorption of molecules or swelling of a thin film
deposited onto a surface plasmon active metal substrate. P-
polarized light is focused on the surface of the substrate resulting
in the generation of a plasmonic wave at the substrate−film
interface, the refractive index of which dictates the angle at
which the light is coupled into the sensor to excite the plasmons.
When a molecule binds (or adsorbs) to the surface of the
substrate, or the film swells, the refractive index in the space
Figure 27. Application of rheology and mechanical measurements in probing nanocellulose−water interactions. (a,b) The influence of shear on the
dewatering of high consistency CNF furnishes: (a) Development of the ηduring the measurement of 5% consistencies CNF-furnish without cellulose
fibers, subjected to the rotation rate of 200 s−1during controlled shear rate cycle. (b) Solid content increase Δφ, for dierent dewatering schemes
versus shear rates in controlled shear rate intervals 0, 40, and 200 s−1and the flow index (K) for all furnished.
307
(a,b) Adapted with permission from ref
307. Copyright 2013 Springer Nature. (c,d) Eect of water on mechanical properties of TOCNF/PVA films measured by DMA: (c) Storage moduli
and (d) equilibrium water contents of pure TOCNF films and TOCNF−PVA films containing PVA at dierent RH levels.
759
(c,d) Adapted with
permission from ref 759. Copyright 2015 Elsevier. (e−g) Using DMA to investigate dynamic mechanical properties of stimuli responsive CNC−
polymer nanocomposites inspired by sea cucumber dermis. (e) Natural model and bioinspired design of chemomechanical nanocomposites. Pictures
of a sea cucumber in relaxed (left) and stiened (right) state demonstrating the firming of dermal tissue in the vicinity of the contacted area.
687
(f)
Schematic representation of the architecture and switching mechanism in the artificial nanocomposites with dynamic mechanical properties. In the
“on” state, strong hydrogen bonds between rigid, percolating CNCs maximize stress transfer and therewith the overall modulus of the nanocomposite.
The interactions are switched “o” by the introduction of a chemical regulator that allows for competitive hydrogen bonding.
687
(g) Time-dependent
modulus decrease of neat poly(vinyl acetate) (PVAc) and a 12.2% v/v PVAc/CNC nanocomposite upon immersion into artificial cerebrospinal fluid
and increasing the temperature from 23 to 37 °C.
687
(e−g) Adapted with permission from ref 687. Copyright 2008 The American Association for the
Advancement of Science.
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BC
where the plasmonic wave propagates is changed, as is the
surface plasmon resonance angle.
1132,1133
Multiparameter
surface plasmon resonance spectroscopy (MP-SPR) acquires
spectra which are dependent on the angle of the incident light in
various media
284
(i.e., gas or liquid) at dierent wavelengths
(i.e., 670 and 785 nm) during a single experiment. From the
data, one can deconvolute film thickness and eective refractive
index based on optical modeling (using the Fresnel
equations).
1134
Typically, the refractive index of nanocellulose
is reported to range from 1.51 to 1.62.
276,284,1135
To examine the swelling dynamics of CNC thin films, Reid et
al. utilized SPR to monitor changes in film thickness as a
function of CNC surface chemistry and charge density in various
solvents and aqueous environments with dierent ionic
strengths (Figure 25j−l).
283
The authors found that while the
total water uptake capacity of each film was similar due to the
restrictions imposed be van der Waals forces between particles,
the rate of swelling was highly dependent on CNC surface
charge and solvent ionic strength.
283
SPR has also been
frequently used in the studies that include surface modification
of nanocellulose films to tailor the interactions between
nanocellulose and water. However, in these studies, SPR is
mostly used to show the adsorption of the polymer on the
surface. For example, Ahola et al.
1136
and Eronen et al.
1137
used
SPR to surveil the adsorption profile of multiple synthetic and
natural polymers on CNF films for the purpose of changing
water binding capacity of the material.
4.2.5. Thermal Analysis. 4.2.5.1. Dierential Scanning
Calorimetry (DSC). DSC is a thermoanalytical technique that
measures the energy transferred to or from a sample undergoing
a chemical or physical change as a function of temperature. The
heat flow profiles attained through a DSC measurement (i.e.,
DSC curves) can be interpreted to provide detailed information
on the enthalpies of transition of a material. When utilized on
wet samples at low temperatures, DSC can yield direct
information on the pore size distribution of the sample with a
method called thermoporosimetry where the Gibbs−Thomson
relation yields the relationship between melting point
depression and capillary pore diameter. In practice, the sample
is frozen, and an increasing temperature ramp detects the
melting water in pores of varying size. In terms of cellulose−
water interactions, DSC has been intensively investigated for its
ability to quantitatively evaluate the dierent water populations
in celluloses, i.e., the contents of free and bound water (section
2.4.2.2).
100,274
The DSC curves of free water within cellulosic materials have
similar profiles to that of pure water, indicating similar transition
temperatures and enthalpies. Bound water, on the other hand,
shows a lower transition temperature than that of pure water due
to the restriction caused by the hydroxy groups of cellulose. The
enthalpy of melting of freezing bound water adsorbed to
cellulose can be quantified by integrating the area under the
endothermic DSC curve (Figure 26a−f).
100
In the case of
celluloses, two exothermic peaks of crystallization of adsorbed
water are observable during the cooling step: (i) a sharp peak at
ca. 255 K for bound water (peak I) and (ii) a broad peak
between 230 and 250 K (peak II), indicating free water and
freezing bound water in cellulose (Figure 26i).
272
The melting
enthalpy can be plotted against the total water content in the
cellulose sample (g water/g dry cellulose sample). Theoretically,
the points should fall on a straight line with a slope equal to the
heat of fusion of bulk water (ΔHf= 79.7 g cal−1)
1138
and an x-
intercept showing the nonfreezing water content (ΔH= 0).
1139
DSC can yield indirect information on the crystallinity and
fiber structure of cellulosic materials,
272,1140
as well as the
accessibility of the cellulose hydroxy groups (Figure
26g,h).
103,1017
As such, DSC can also help to garner an
understanding on the eects of nanocellulose hornification as a
result of removal of water.
332
For instance, Ding et al.
implemented DSC to demonstrate a collapse in the mesoporous
structure of nanocellulose fibers and a subsequent loss in
hydroxy group accessibility of redispersed nanocellulose upon
the removal of water.
332
Furthermore, DSC can also be used to
attain information on the pore size distribution of a material
such as a wet cellulose nanopaper.
784
Thermoporosimetry is
typically carried out after isothermal heating where temperature
is kept constant until the enthalpy of melting has stabilized. This
technique assumes that water contained within pores is
subjected to elevated pressures and has a higher melting
temperature than free water. Hence, the dierent isothermal
melting points obtained during the measurements provide
information about the number of pores and their respective size,
as reported by Rojo et al.
784
However, a clear limitation of DSC-
based thermoporosimetry are nanocavities smaller than several
Å, which cause the formation of nonfreezing water and might
hinder an accurate size measurement.
1141
4.2.6. Rheological and Mechanical Testing.
4.2.6.1. Rheology. The rheological behavior of aqueous
nanocellulose suspensions has been studied to determine the
eects such as size or morphology, surface chemistry, and aspect
ratio, which have been discussed in multiple comprehensive
reviews.
3,21,102,219,580,1142
Furthermore, the flow behavior of
aqueous nanocellulose suspensions has been studied in dilute,
semidilute, and concentrated regimes, all providing information
related to nanocellulose−water interactions. Simply put, the
dilute regime typically provides information on the aspect ratio
of nanocelluloses and their particle−particle interactions, while
the semidilute and concentrated regimes shed light on the
viscoelastic properties and structure of suspensions as a function
of the shear rate. Additionally, rheological measurements can
provide in situ information on the dewatering of nanocellulose
suspensions (Figure 27a,b). Dimic-Misic et al. studied the
relationship between shear stress-induced dewatering of CNFs
and microfibrillated (MFC)-containing furnishes and their
rheological behavior.
307,310
Dynamic dewatering as a function
of shear was studied under vacuum in an immobilization cell
combined with a rheometer. Dewatering led to an increase in the
elastic modulus and viscosity of the suspension as a function of
the increase in solids content. The shear-thinning behavior of
the cellulosic materials was thereby found to be the central
feature influencing shear-induced dewatering and was related to
the creation of dewatering channels.
307
Moreover, the work
showed the surface charges of both MFC and CNFs aect both
rheological and dewatering properties, which was attributed to
fibrillar swelling and nonremovable surface-bound water.
310
4.2.6.2. Dynamic Mechanical Analysis (DMA). DMA is a
characterization technique used to measure the viscoelastic
properties of a material under an oscillating load as a function of
oscillation frequency and temperature. While DMA has mostly
been used to investigate the eect of water on the performance
of (mostly) nanocellulose nanocomposite materials, it can also
provide insights into the reversible water-response dierent
types of materials.
687,689
Utilizing DMA, Hakalahti et al. compared the mechanical
properties of TOCNF/poly(vinyl alcohol) (PVA) films in both
the dry and wet state to demonstrate that PVA significantly
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BD
improved the wet stability of the films (Figure 27c,d).
759
In a
dierent study, Capadona et al. also used DMA to understand
the reversible water response of sea cucumber-like CNC
reinforced polymer composites (Figure 27e−g).
687
DMA
Figure 28. Scattering techniques to elucidate nanocellulose−water interactions. (a−c) Structural properties of CNC dispersions. Time-resolved SAXS
data of drying dispersions of (a) CNC-Li and (b) CNC-H over a total time span of 60 and 20 min. (c) Time-dependent change of the center-to-center
CNC separation distance dwith increasing CNC concentrations during evaporation-induced assembly (●, CNC-H; ■, CNC-Li). The solid curves
describe a power law relation with exponents of −1/3 and −1.
264
(a−c) Adapted with permission from ref 264. Copyright 2018 American Chemical
Society. (d−f) Analysis of the structure and morphology of dry and hydrated bacterial cellulose by SANS. (d) SANS profile of dry bacterial cellulose.
The experimental SANS (green), the power-law fit (red), and the deviation from the power-law behavior (black).
273
(e) SANS profiles of hydrated
bacterial cellulose. The color scheme of the curves is the same as in (d).
273
(f) Powder X-ray diraction pattern of bacterial cellulose hydrated in H2O.
Experimental data (blue), background (green), and peak positions were obtained from data fitting (red).
273
(g) Using QENS to probe the dynamics of
water associated with cellulose. Elastic intensity scans of dry and hydrated deuterated cellulose. The data curves with blue squares and red circles
represent the hydrated and dry samples, respectively. The dashed lines denote inflection points in the curves at 220 (nonfreezing bound water) and 260
K (freezing bound water) in the hydrated cellulose sample.
273
(d−g) Adapted with permission from ref 273. Copyright 2017 Spring Nature. (h) Using
INS to study hydroxy accessibility in cellulose. The neutron spectra of amorphous cellulose and the eect of the low-energy dynamics by deuteration of
polar (OH) groups. The lower curve (filled circles) is the dierence (OH −OD), which essentially reflects the dynamics of OH groups. In this
disordered material, all hydroxy (OH) groups that are not completely saturated by hydrogen bonds will be aected by water molecules. Due to the
penetration of water molecules, the sample swells and distances between cellulose molecules increase, additionally perturbing the hydrogen bonds
between cellulose molecules. This process finally makes almost every hydroxy group accessible to water molecules. Hence, swelling and washing of
amorphous cellulose in D2O will lead to an exchange of 30% of all hydrogens in the cellulose molecule. On removing heavy water by drying the sample,
OD groups are preserved as long as the sample is protected against atmospheric humidity or other protonated polar solvents.
211
(h) Adapted with
permission from ref 211. Copyright 2000 American Chemical Society.
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BE
analysis showed that upon contact changes in composite
modulus, elongation at break, and tensile strength were the
result of “switching o” the CNC−CNC interactions through
water adsorption. Conversely, in the dry state, strong hydrogen
bonding between the CNCs maximized the stress transfer and
improved the modulus of the nanocomposite. Zhu et al. used
DMA to understand the mechanisms behind the shape-memory
and the water sensitivity of a CNC/TPUs (thermoplastic
polyurethane) nanocomposite. They also evaluated the
reversibility of the change in modulus by repeated drying−
wetting cycles.
689
4.2.7. Scattering Techniques. 4.2.7.1. Small-Angle
Scattering (SAS). Small-angle X-ray scattering (SAXS) and
small-angle neutron scattering (SANS) are able to provide
information on the structure and morphology of nanocellulose
on a multiscale length,
49,1143−1145
although data interpretation
can be complex. One of the major benefits of small-angle
scattering techniques is that changes in sample properties can be
measured in real time when exposed to an external force. For
example, the alignment of CNCs in aqueous suspension in a
relatively weak (0−1.2 T)
1146
and strong (up to 6.8 T) magnetic
field has been monitored in situ using SAXS and SANS,
respectively. Furthermore, Håkansson et al. utilized SAXS to
detect nanocellulose hydrodynamic alignment in filaments
produced using a flow focusing device. This approach gave
very detailed information on the dynamics and parameters
aecting the aligned assembly of CNF and CNC using such a set
up.
1147,1148
Lastly, the self-assembly of sulfated CNCs upon
drying and rewetting was measured in situ by Liu et al. using
time-resolved SAXS (Figure 28a−c). They were able to
quantitatively follow the nanoscale assembly of CNCs and the
reswelling of the dry structure upon hydration as a function of
the counterion of the sulfate half ester charge group (hydrogen
or lithium).
264
Using SAXS, Guccini et al.
855
assessed the influence of
atmospheric moisture content on the structure of carboxylated
CNF membrane as a function of surface charge and counterion.
Strips of the membranes were introduced to a humidity-
controlled cell, while the scattering pattern was recorded. The
team was able to track the formation of pores and water channels
within the nanofiber network in a range of 55−95% relative
humidity and concluded that the amount of water absorbed and
the swelling increases with the surface charge density of the CNF
and decreases with the substitution of sodium with hydrogen
ions. Others have also used small-angle scattering to understand
the liquid crystalline properties of CNC suspensions as a
functions of solids content, total surface charge, and ionic
strength,
120,1149,1150
while fewer reports are available regarding
the nematic ordering of CNF suspensions using SAS.
121,266
Using grazing incident SANS (GISANS), Brett et al. studied
the morphological features of ultrasmooth carboxylated CNF
thin films as a function of relative humidity. They observed that
the size of the three-dimensional CNF aggregates were not
aected by an increase in humidity, but their average distance
increased as more water is absorbed and condensed into the
films.
1140
Compared to microscopic techniques such as TEM
and SEM, GISANS (and GISAXS) have versatile sample
environments do not require some degree of electrical
conductivity and are able to scan a bigger area with high
resolution and fast acquisition rates both parallel and
perpendicular to the sample surface. Valencia et al. monitored
the eect of water flow during a filtration process across a porous
membrane of carboxylated CNFs. Using SAXS, they were able to
monitor changes in the porosity of the membranes during
filtration and assess the antifouling performance of the
membrane.
1151
An interesting feature specific to SANS is the
possibility to contrast-match (or make components “invisible”
to the neutron beam) individual components of a multi-
component system by varying the H2O/D2O ratio, which allows
one to study individual characteristics and structural features.
O’Neil et al. used this technique in SANS to selectively highlight
the eect of hydration on BC surface and to compare the
nanostructure and bulk morphology of dry and hydrated BC
(Figure 28d−f).
273
4.2.7.2. Inelastic Scattering (IS). The dynamics of water in
nanocellulose can be directly probed as a function of
temperature using inelastic scattering methods as quasielastic
neutron scattering (QENS) and inelastic neutron scattering
(INS). The information accessible through these techniques are
characteristics of a length scale ranging from Ångstroms to a few
tens of nanometers level, and the phenomena probed are within
the time scales from a fraction of picoseconds to micro-
seconds.
1152
The results given by INS are resolved according to
the change of the kinetic energy of the sample interacting with
the neutron beam.
The applications of these techniques on nanocellulose−water
systems may not have reached their full potential as there are less
reports available compared to other techniques. Nonetheless,
Muller et al. investigate the dynamics of water within cellulose
using INS, concluding the water accessible sites correspond to
the disordered cellulose chains mostly located on the surface of
CNFs (Figure 28h).
211
The peculiarity of QENS is to
quantitatively determine the dynamical relaxation processes of
water to distinguish between localized rotational and long-range
translational motions. This characteristic has been used to
determine how the hydration level aect the water dynamics
within cellulose and its mechanical properties.
996
As mentioned
in section 4.1, O’Neil et al. detected two populations of water
within crystalline BC depending on their relaxation dynamics as
a function of temperature, one consisted of surface bound water
and the other of confined bound water (Figure 28g).
273
Guccini
et al. were able to detect two types of motions for the water
Figure 29. Future trends in the research that encompasses nano-
cellulose−water interactions.
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inside a TOCNF network by QENS: one with fast, localized
movement of water molecules and the other with slower,
diusive motion. The slower motion was independent of the
surface charge of the TOCNFs.
1153
5. FUTURE TRENDS IN NANOCELLULOSE−WATER
INTERACTIONS
Fundamentally, cellulose−water interactions have been inves-
tigated for over a century, but as new tools, such as QCM and
SPR, have been introduced aside the traditional ones, like DVS,
the understanding of how water binds to cellulose has logically
improved a great deal during the past decades. The progress in
computational methods has been equally important. Simulta-
neously, the advances in comprehending the crystalline
structure of cellulose and the true morphology of the microfibril,
as well as the intricacies of the cell wall structure, have enabled
painting a comprehensive picture of how water is incorporated
in (nano)cellulosic constructs and how the interactions with
water can be tuned in those constructs. Much of the picture is
still incomplete, as many seminal issues remain unresolved: the
shape of the cellulose crystallite cross-section, the explicit nature
of the disordered regions in the microfibril, the role of
hemicellulose in cellulose crystallite−water interactions, and
so on. The present century will no doubt answer many of these
fundamental questions, particularly considering the central role
that has been bestowed on renewable polymers in the modern
environment. Cross disciplinary approaches with constantly
developing methodologies are imperative in solving these issues.
Data from modern NMR tools, for example, have rarely been
superimposed with the other emerging methods, or traditional
methods for that matter. The combination of advanced
characterization (NMR, SANS/SAXS) with computational
modeling, such as MD, is an especially promising approach as
it enables experimental data normally recorded as averages to be
spatially resolved. The application of multiscale computational
approaches can, in addition, improve our understanding of how
phenomena on the atomic scale (hydrogen bonding, chemical
modification, etc.) aect macroscopic properties such as the
strength of nanofibril networks or the colloidal stability of
nanocellulose dispersions. Furthermore, contemporary methods
in machine learning are still underexplored in the field of
cellulose science, both from the fundamental and the materials
perspective. One can expect that within a couple of decades, the
isolation of nanocellulose and the preparation of materials
thereof, such as hydrogels or nanopaper, can be undertaken in a
type of a combinatorial approach where selection of the source
material, parameters for isolation, and assembly methods for the
nanocellulosic material constitute an algorithm that can predict
the properties of the outcome. Comprehensive, fundamental
understanding of nanocellulose−water interactions is pivotal in
the realization of this development.
As implied by the very title of this contribution, the role of
water in nanocellulosic materials is considered ambiguous.
While many regard water as a problematic component in, e.g.,
Table 5. Terms, Abbreviations, Synonyms, and Definitions in Nanocellulose−Water Interaction Research Field
term abbreviation definition
nanocellulose cellulosic materials which have at least one dimension in the nanoscale
119
cellulose nanocrystal or cellulose
nanowhisker CNC or
CNW highly crystalline, rod-shaped nanoparticles or nanowhiskers produced via hydrolysis of disordered regions of
microfibrils
160
TEMPO-oxidized CNF TOCNF cellulose nanofibrils produced via TEMPO mediated oxidation
102
cellulose nanofibrils or cellulose
nanofiber or nanofibrillated
cellulose
CNF or
NFC isolated plant cellulose microfibrils
102
fibrillated nanocellulose CNF and BC
bacterial cellulose BC cellulose produced by bacteria
150
terminal complex or cellulose synthase
complex TC cellulose biosynthesis machinery in organisms that produce cellulose
1154
cellulose microfibrils smallest supramolecular units of cellulose in nature
58
cellulose I native crystalline polymorph of cellulose with parallel chain organization
cellulose II crystalline polymorph of cellulose with antiparallel chain organization, obtained by mercerization or
regeneration of cellulose I
71
cellulose III crystalline polymorph of cellulose, obtained by exposing cellulose I or II to liquid ammonia or certain diamines
72
cellulose Iαtriclinic polymorph of cellulose I
59
cellulose Iβmonoclinic polymorph of cellulose I
59
free water free water molecules that fill any voids in cellulosic material’s structure due to capillary forces, swell the matrix,
and crystallize at 255 K in DSC
270
bound water or adsorbed water bound (adsorbed) water molecules that interact with the cellulosic material’s structure at specific sorption sites.
It includes freezing and nonfreezing bound (adsorbed) waters
270
absorbed water any water molecule that is taken up by a cellulosic matrix (free water and bound (adsorbed) water)
270
bulk water bulk water molecules that surround the cellulosic material’s structure and do not cause observable swelling in the
cellulose matrix
275
freezing bound water bound (adsorbed) water molecules that behave like bulk water and shows a crystallization peak at about
230−250 K in DSC
272
nonfreezing bound water bound (adsorbed) water molecules that do not show crystallization peak in DSC
272
surface bound water or movable
bound water or mobile bound water bound (adsorbed) bound water molecules, located at the surface of the nanofibril belonging to two dierent
agglomerates in CNF percolated fractal networks that becomes more mobile upon hydration
270
confined bound water bound (adsorbed) bound water molecules, located at the surface of the nanofibril belonging to the same
agglomerate in CNF percolated fractal networks that is marginally influenced by hydration level
270
dewatering partial removal of water from nanocellulosic matrices
898
drying removal of (almost) all the water from nanocellulosic matrices
898
hornification loss in swelling ability of cellulose upon drying
99
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BG
nanopaper or composite structures, it is in fact a necessity in
hydrogel-based structures. Much of the potential of water in, for
instance, smart materials and energy-related applications
entailing nanocellulose has not materialized, and it is the task
of the coming years to push the research further in this respect.
Perhaps the biggest quest is to merge fundamental knowledge of
nanocellulose structures and cellulose−water interactions with
design and buildup of new functional materials from nano-
cellulose. Properties like injectability, responsive behavior,
thermal and electrical conductivity, and just chemical mod-
ification of cellulose to reach a certain property are strongly
linked to water interactions.
Overall, the dominant trend in future is to adhere to the
principles of green chemistry more avidly, not just in research
but within the whole society, and water-based systems are in a
central role in those principles. Because of the vast amount of
knowledge that has been gathered over the years on cellulose−
water interactions, we know that water is compatible with
cellulose and the behavior of cellulose−water systems can be
largely, if not fully, predicted. Therefore, processing cellulose in
water will need to be the goal even in those cases when
ultimately hydrophobic bioproducts are the end goal. These
premises will dictate the future development of truly sustainable
production and usage of cellulose-based materials. Figure 29
summarizes the future outlook in the field of nanocellulose−
water interactions.
TABLE OF DEFINITIONS AND ABBREVIATIONS
Table 5 summarizes the relevant terms in nanocellulose−water
interaction research field, their abbreviations, their synonyms,
and definitions.
AUTHOR INFORMATION
Corresponding Authors
Eero Kontturi −Department of Bioproducts and Biosystems,
Aalto University, Espoo FI-00076, Finland; orcid.org/
0000-0003-1690-5288; Email: eero.kontturi@aalto.fi
Laleh Solhi −Department of Bioproducts and Biosystems, Aalto
University, Espoo FI-00076, Finland; orcid.org/0000-
0002-8625-9982; Email: laleh.solhi@medel.com
Valentina Guccini −Department of Bioproducts and
Biosystems, Aalto University, Espoo FI-00076, Finland;
Email: valentina.guccini@aalto.fi
Authors
Katja Heise −Department of Bioproducts and Biosystems, Aalto
University, Espoo FI-00076, Finland; orcid.org/0000-
0003-4105-6759
Iina Solala −Department of Bioproducts and Biosystems, Aalto
University, Espoo FI-00076, Finland; orcid.org/0000-
0002-8110-456X
Elina Niinivaara −Department of Bioproducts and Biosystems,
Aalto University, Espoo FI-00076, Finland; Department of
Wood Science, University of British Columbia, Vancouver,
British Columbia V6T 1Z4, Canada
Wenyang Xu −Department of Bioproducts and Biosystems,
Aalto University, Espoo FI-00076, Finland; Laboratory of
Natural Materials Technology, Åbo Akademi University,
Turku FI-20500, Finland
Karl Mihhels −Department of Bioproducts and Biosystems,
Aalto University, Espoo FI-00076, Finland
Marcel Kröger −Department of Bioproducts and Biosystems,
Aalto University, Espoo FI-00076, Finland
Zhuojun Meng −Department of Bioproducts and Biosystems,
Aalto University, Espoo FI-00076, Finland; Wenzhou
Institute, University of Chinese Academy of Sciences, Wenzhou
325001, China
Jakob Wohlert −Wallenberg Wood Science Centre (WWSC),
Department of Fibre and Polymer Technology, School of
Engineering Sciences in Chemistry, Biotechnology and Health,
KTH Royal Institute of Technology, 10044 Stockholm,
Sweden; orcid.org/0000-0001-6732-2571
Han Tao −Department of Bioproducts and Biosystems, Aalto
University, Espoo FI-00076, Finland
Emily D. Cranston −Department of Wood Science, University
of British Columbia, Vancouver, British Columbia V6T 1Z4,
Canada; Department of Chemical and Biological Engineering,
University of British Columbia, Vancouver, British Columbia
V6T 1Z3, Canada; orcid.org/0000-0003-4210-9787
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.chemrev.2c00611
Author Contributions
CRediT: Laleh Solhi conceptualization, investigation, super-
vision, visualization, writing-original draft, writing-review &
editing; Valentina Guccini conceptualization, investigation,
supervision, writing-original draft, writing-review & editing;
Katja Heise investigation, writing-original draft, writing-review
& editing; Iina Solala investigation, writing-original draft,
writing-review & editing; Elina Niinivaara investigation,
validation, writing-review & editing; Wenyang Xu investigation,
writing-original draft, writing-review & editing; Karl Mihhels
investigation, visualization, writing-original draft, writing-review
& editing; Marcel Kroger investigation, writing-original draft,
writing-review & editing; Zhuojun Meng investigation, writing-
original draft, writing-review & editing; Jakob Wohlert
investigation, visualization, writing-original draft, writing-review
& editing; Han Tao methodology, visualization, writing-review
& editing; Emily D. Cranston investigation, writing-review &
editing; Eero Kontturi conceptualization, funding acquisition,
investigation, project administration, resources, supervision,
writing-original draft, writing-review & editing.
Notes
The authors declare no competing financial interest.
Biographies
Laleh Solhi received her B.Sc. in Chemistry and her M.Sc. and Ph.D. in
Polymer Engineering, focusing on designing and developing bio-
materials. She spent her first and second postdoctoral fellowships at the
University of British Columbia (Canada) and Aalto University
(Finland) using sustainable chemical and biochemical tools to develop
new materials based on natural polymers. In addition to her research
interests in next-generation sustainable materials, she is passionate
about the application of data science tools in natural sciences and
engineering and has an extra education in data science and intelligent
analytics.
Valentina Guccini completed her Ph.D. studies in 2019 at Stockholm
University (Sweden). After defending her thesis “Nanocellulose Self-
assembly and Energy Applications”, she moved to Aalto University
(Finland) to work as a postdoctoral researcher. In 2022, she has been
awarded the Academy of Finland postdoctoral fellowship. Her main
research interests consist of tailoring the assembly of nanocellulose,
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BH
nanomaterials in general, and biological systems to produce functional
materials for environmental recovery, carbon capture, resource-wise
production of raw materials, and fossil-free energy conversion.
Katja Heise is an Academy of Finland postdoctoral researcher currently
working at Aalto University (Finland) in the group of Prof. Eero
Kontturi. She received her Ph.D. in 2017 from TU Dresden (Germany).
Her current research lies at the interface between nanocellulose
materials science and selective chemical modification.
Iina Solala obtained her Dr. Sc. (Tech.) degree in Forest Products
Chemistry from Aalto University (Finland) in 2015 under Prof. Tapani
Vuorinen. Part of her dissertation work on mechanochemical reactions
of lignocellulosic materials was conducted at the University of Natural
Resources and Life Sciences Vienna (BOKU, Austria) in the laboratory
of Prof. Thomas Rosenau and Prof. Antje Potthast. After her
graduation, she worked as a postdoctoral researcher at Chalmers
University of Technology (Sweden) in the group of Prof. Anette
Larsson and at Aalto University in the group of Prof. Eero Kontturi,
conducting research on cellulose−water interactions and lignin-
containing nanocelluloses. In 2019, she returned to work with Prof.
Vuorinen as a University Teacher in the multidisciplinary CHEMARTS
program that combines biobased materials research with design.
Elina Niinivaara obtained a Master’s in Science (Technology) (2012)
and a Doctorate in Science (Technology) (2016) in Forest Products
Chemistry from the Aalto University School of Chemical Technology
in Espoo, Finland. Her Master’s degree research focused on developing
a novel, amine catalyzed pulp bleaching sequence, whereas her doctoral
studies took on a more fundamental approach, investigating the two-
dimensional response of ultrathin cellulose-based films to various
external stimuli. Her first postdoctoral research fellowship (2016−
2019) was carried out at McMaster University (Hamilton, ON,
Canada), with a focus on cellulose nanocrystal/water interactions,
along with investigations into the reinforcing capacity of cellulose
nanocrystals in various composite matrices. In 2019, she was awarded
an Academy of Finland postdoctoral research fellowship (2019−2022),
during which she developed an oligosaccharide-based, cellulose
nanocrystal surface modification in collaboration between Aalto
University and the University of British Columbia (Vancouver, BC,
Canada). Currently, she holds a Research Scientist position at BC
Research Inc. (Vancouver, BC, Canada), where her research interests
lie in utilizing natural materials to tackle environmental issues resulting
from industrial processes.
Wenyang Xu received his Ph.D. in Wood and Paper Chemistry at Åbo
Akademi University (2019, Finland). He conducted postdoctoral
research on cellulose surface modification via solid-state adsorption
(2019−2020, Aalto University, Finland), on surface analysis via sum
frequency generation vibrational spectroscopy (2020, KTH Royal
Institute of Technology, Sweden), and on conductive hydrogel
fabrication via interfacial in situ polymerization with nanocellulose
(2021, Åbo Akademi University, Finland). Wenyang Xu is currently an
Academy of Finland funded postdoctoral researcher. His research
interest focuses on self-assembly of hemicellulose-based block
copolymers and their coassembly with nanocellulose systems for
functional materials, e.g., in photonic applications.
Karl Mihhels graduated as a M.Sc. in 2017 at Aalto University. Since
then, he has continued his studies towards a Ph.D. under the
supervision of Prof. Eero Kontturi. Currently, he is figuring out new
and sustainable ways to convert algal biomass into nanocellulose.
Marcel Kroger earned his Master’s degree in Chemistry (2019) from
the University of Hamburg and has since joined the Materials
Chemistry of Cellulose group at Aalto University as a doctoral student.
His current focus is on the extraction and surface interactions of highly
charged cellulose nanocrystals.
Zhuojun Meng received her Ph.D. degree in 2017 from Groningen
University, The Netherlands, under the supervision of Prof. Andreas
Herrmann, focusing on DNA hybrid materials and applications. From
2018 to 2021, she worked as a postdoctoral researcher jointly
supervised by Prof. Eero Kontturi at Aalto University, Finland, and
Prof. Tekla Tammelin at Technical Research Centre of Finland (VTT),
where her research area included water interactions in biomaterials
engineering. Currently, she is an associate professor at Wenzhou
Institute, University of Chinese Academy of Sciences (WIUCAS),
China, and her research interests lie in nanomedicine and instrument
design.
Jakob Wohlert received a Ph.D. in Physics in 2007 from KTH in
Stockholm. After postdoctoral visits in CERMAV, Grenoble and in
Cornell University, he returned to KTH and the Wallenberg Wood
Science Center in 2010. In 2020, he was appointed Docent in Fiber and
Polymer Science. His research has been focused on the application of
atomistic computer simulations to model fundamental interactions in
biopolymers with special attention paid to cellulose and hemicelluloses.
Han Tao received his Master’s degree in 2020 at Wuhan University of
Technology (China), where his research focused on the application of
cellulose nanocrystals as reinforcing components in nanocomposites.
He started his Ph.D. at Aalto University in 2021 under the supervision
of Prof. Eero Kontturi. He is currently involved in various research
projects on cellulose nanocrystals.
Emily D. Cranston received her Ph.D. in Materials Chemistry at McGill
University (Canada) in 2008 under the supervision of Prof. D. G. Gray
and was a postdoctoral fellow at KTH Royal Institute of Technology
(Sweden). She is an Associate Professor in Wood Science and Chemical
& Biological Engineering at the University of British Columbia
(Canada) and is the President’s Excellence Chair in Forest Bioproducts.
Prior to January 2019, she was an Associate Professor at McMaster
University (Canada) and the Canada Research Chair in Bio-Based
Nanomaterials. She currently holds an E.W.R. Steacie Memorial
Fellowship from the Natural Sciences and Engineering Research
Council of Canada.
Eero Kontturi earned his Ph.D. in surface science in 2005 at Eindhoven
University of Technology (The Netherlands) under the supervision of
Prof. J. W. Niemantsverdriet. After postdoctoral spells and visits at
Aalto University (Finland), UPMC Paris (France), Imperial College
London (UK), and University of Vienna (Austria), he was appointed
Associate Professor at Aalto University in 2014. His major research
interests revolve around interfacial phenomena of plant-based materials
with a particular focus on cellulose as a natural component and as a
building block for functional materials.
ACKNOWLEDGMENTS
We acknowledge the community at the Department of
Bioproducts and Biosystems at Aalto University for constructive
discussions with numerous colleagues. K.H., W.X., and V.G.
acknowledge the postdoctoral grants received from the
Academy of Finland (grant numbers 333905, 349109, and
347219, respectively). This work is a part of FinnCERES
Bioeconomy cluster, funded by the Academy of Finland.
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