![PTMSP free volume model [35] with a cube edge length of ~5 nm and each next section spaced by ~0.3 nm from the preceding one (order of sections: a 1 , …, a 4 ; b 1 , …, b 4 ; c 1 , …, c 4 ; d 1 , …, d 4 ).](https://www.researchgate.net/profile/Vladimir-Volkov-9/publication/225382145/figure/fig8/AS:668345414197250@1536357372331/PTMSP-free-volume-model-35-with-a-cube-edge-length-of-5-nm-and-each-next-section_Q320.jpg)
![A plot of the pore diameter D versus time t of etching in KOH solution (0.25 mol/l; 61°C) for a PET film irradiated by Xe ions to a fluence of N = 2 × 10 9 cm –2 : (1) track swelling time and (2, 3) region of reduced etching rate for a pore diameter of ~10 and 25 nm, respectively [2].](https://www.researchgate.net/profile/Vladimir-Volkov-9/publication/225382145/figure/fig3/AS:393624366927879@1470858770687/A-plot-of-the-pore-diameter-D-versus-time-t-of-etching-in-KOH-solution-025-mol-l-61C_Q320.jpg)
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656
ISSN 1995-0780, Nanotechnologies in Russia, 2008, Vol. 3, Nos. 11–12, pp. 656–687. © Pleiades Publishing, Ltd., 2008.
Original Russian Text © V.V. Volkov, B.V. Mchedlishvili, V.I. Roldugin, S.S. Ivanchev, A.B. Yaroslavtsev, 2008, published in Rossiiskie nanotekhnologii, 2008, Vol. 3, Nos. 11–12.
1. INTRODUCTION
Modern membranes are advanced technological
products used in various fields of industry and science,
and they are one of the necessary prerequisites for the
development of these fields and for solving important
social and ecological problems. It is difficult today to
find a field of human activity that can exist without
membranes. For this reason, membrane science and
technology has always been given much attention at the
federal level in the former USSR and now in Russia. In
the USSR, research and development in this field was
coordinated by the Interdisciplinary Scientific-Techno-
logical Corporation “Membranes,” and now these
activities are supported on a competitive basis by the
Federal Agency for Science and Innovations in accor-
dance with the State Program of Research and Develop-
ment “Industry of Nanosystems and Materials,” within
the subprogram “Technology of Membranes and Cata-
lytic Systems.” It should be emphasized that membrane
technologies were in high demand even during the
industrial crisis of the 1990s, and numerous small and
medium level businesses engaged in the production of
membranes and related devices. Presently, more than
50 domestic companies operate on the Russian membrane
market, many of which (e.g., Voronezh-Akva Co. [1],
GRASYS Co. [2], Aspekt Co. [3], and Ceramic Filter
Co. [4]) have been created and managed by high-qual-
ity specialists of former military enterprises.
Among the domestic manufacturers of membrane
gas separation devices, GRASYS Co. [2] is also the
most successful competitor on the international market,
being one of the biggest suppliers of this equipment in
Europe. The products from this company are fully
adapted for use in all climate belts of Russia. The com-
pact design of the gas separation units makes it possible
to combine them with compression stations on the plat-
forms of freight vehicles or in special containers. Sta-
tionary and mobile membrane nitrogen liquefaction
plants are employed at oil and gas deposits and used to
help extinguish fires in the coal, chemical, and petro-
chemical industries, and in various other fields (muse-
ums, exhibitions, archives, banks, computer centers,
rooms with advanced electronic equipment, etc.).
Since the beginning of the 1990s, the conversion
projects of the Keldysh Research Center (Moscow) [5],
which is among the oldest enterprises of the aerospace
industry, have been partly directed toward developing
technologies for membrane purification of natural and
waste waters and related equipment. In particular, this
institution developed a special technology for mem-
brane desalination of Caspian seawater, which is being
successfully employed at the Mangistauss Desalination
Plant (Aktau, Kazakh Republic), the largest enterprise
of this kind in the CIS countries [6, 7]. Modern mem-
brane technologies have also been developed at the
Leipunsky Physical and Power Engineering Institute
and some other enterprises of the former-USSR mili-
tary-industrial complex.
At present, Russian enterprises produce track mem-
branes, ion-exchangers, micro-, ultra-, and nanofiltra-
tion membranes, as well as reverse-osmosis and gas-
separation membranes. Unfortunately, the prolonged
economic crisis of the 1990s hindered the development
of research in this field. Hence, only a small number of
domestic membranes can compete in performance
characteristics with the best foreign analogs. Some
membranes are in demand due to the reasonable ratio of
price and quality. On the other hand, some original
technological solutions allowed a number of Russian
companies and enterprises to win in competition on the
domestic market and even enter the international mar-
ket for special membranes and membrane technologies.
The present review considers products of domestic
advanced technology that have been implemented, as
well as promising membrane projects for nanotechnol-
ogy. The most important direction in developing
advanced technologies in the nearest future is related to
catalytic membrane reactors. Membrane catalysis was
Membranes and Nanotechnologies
V. V. Volkov
a
*, B. V. Mchedlishvili
b
, V. I. Roldugin
c
,
S. S. Ivanchev
d
, and A. B. Yaroslavtsev
e
a
Topchiev Institute of Petrochemical Synthesis, Russian Academy of Sciences, Moscow, 119991 Russia
b
Shubnikov Institute of Crystallography, Russian Academy of Sciences, Moscow, 119333 Russia
c
Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences, Moscow, 119991 Russia
d
Boreskov Institute of Catalysis, Russian Academy of Sciences, St. Petersburg, 197198 Russia
e
Kurnakov Institute of General and Inorganic Chemistry, Russian Academy of Sciences, Moscow, 119991 Russia
*e-mail: vvvolkov@ips.ac.ru
Received May 12, 2008
DOI:
10.1134/S1995078008110025
REVIEWS
NANOTECHNOLOGIES IN RUSSIA
Vol. 3
Nos. 11–12
2008
MEMBRANES AND NANOTECHNOLOGIES 657
conceived in the 1960s by Gryaznov [8, 9]. Recent
reviews on catalytic membrane reactors [10, 11] ana-
lyzed in sufficient detail the available literature on the
subject and discussed promising domestic R&D
projects for commercially important processes (incom-
plete methane oxidation, methanol oxidation to formal-
dehyde, oxygen reduction in aqueous media, carbon
monoxide to dioxide conversion) employing catalytic
membrane reactors. For this reason, such reactors and
related investigations are only briefly mentioned in the
sections devoted to metallic membranes and asymmet-
ric membrane transport.
At present, it is commonly accepted that the class of
nanodimensional objects includes those with character-
istic dimensions ranging from the molecular to the cel-
lular level, that is, within 1–100 nm [12]. Such nanodi-
mensional systems are typical objects in colloidal
chemistry. According to the classification adopted in
colloidal chemistry [13], ultradisperse systems consist-
ing of particles with characteristic dimensions within
1–100 nm constitute a special subgroup. In turn, nano-
and ultrafiltration were successfully used for a long
time for the membrane separation of colloidal solu-
tions, which offer an illustrative example of nanosys-
tems. Thus, on the one hand, membranes are objects of
nanosystems and, on the other hand, they are instru-
ments for solving many tasks in nanotechnology.
Based on the average pore diameter in selective lay-
ers of filtration membranes, the baromembrane pro-
cesses involving these media can be subdivided into the
following classes (pore size):
(i) reverse osmosis (0.3–1 nm) [14];
(ii) nanofiltration or low-pressure reverse osmosis
(1–10 nm);
(iii) ultrafiltration (10–100 nm) [14];
(iv) microfiltration (100 nm–10
μ
m) [14].
The same range—from fractions of a nanometer to
several nanometers—contains the average size of ion-
exchanger membranes [15]. As to gas separation and
pervaporation membranes, these media separate com-
ponents on a molecular level via the dissolution–diffu-
sion mechanism. The above classification of filtration
membranes with respect to the average pore size has a
conditional character, since a very important role in
baromembrane processes is played by the surface inter-
actions on the side at which the membrane is entered
(selective layer).
The mechanisms of membrane filtration processes
in most cases are related to the special structure of liq-
uid layers immediately in contact with the walls of capil-
laries in these porous media. The existence of liquid
boundary layers with the special structure was the central
point of the investigations by Academician B.V. Derjaguin
(see, e.g., [16, 17]), the most authoritative specialist in
surface phenomena. Recently, Filippov and Starov
[18], students of and successors to Derjaguin’s scien-
tific schoo, maintained their opinion that baromem-
brane processes should be described using a common
approach, but with allowance for the specific features
of surface interactions involved in each particular pro-
cess.
In this context of the above considerations, the most
widely used classification of baromembrane processes
is based on the functional characteristics of the process,
that is, on the size or molecular mass of the retained
components of the separated mixture (Fig. 1).
The main task of membrane technologies is to sepa-
rate components at the expense of minimum consumed
energy. Modern membranes competitive on the market
are typically composed of several layers of materials,
each layer possessing a certain structural organization
on the micro- and nanolevel. This structure provides a
given complex of technological characteristics of the
whole membrane, including high transport and separat-
ing properties and the ability to regeneration in the
course of fouling.
The next stage necessary for the successful use of
any membrane in a technological process is the creation
of a high-efficiency separation module, which ensures
realization of the aforementioned technological param-
eters. These separation modules are the key components
in the design of membrane setups, the productivity of
which can readily be scaled. The membrane setups are
frequently calculated, designed, and manufactured as
“key-ready” using an assortment of membrane separa-
tion modules available on the market. Each manufac-
turer possesses patented technical solutions and know-
how for every stage in the creation of a gas separation
setup. For this reason, below we will also consider
some engineering solutions used in advanced mem-
brane separation processes.
2. NANOPOROUS POLYMER GLASSES,
MEMBRANE GAS SEPARATION
AND PERVAPORATION
2.1. Membrane Gas Separation
Using High-Permeability Polymer Glasses
The first representative of the class of highly perme-
able polymer glasses was poly(vinyl trimethylsilane)
(PVTMS), which was synthesized in 1962 at the
Topchiev Institute of Petrochemical Synthesis (Mos-
cow) [19, 20]. The unexpectedly high permeability
coefficients of PVTMS led to a revision of then com-
monly accepted notions of polymer glasses as low-per-
meability materials [21, 22]. The joint USSR–France
R&D activities resulted in the creation and commercial
implementation of the PVTMS membrane in the begin-
ning of 1970s. A commercial technology for the syn-
thesis of VTMS and PVTMS was developed within
5 years on the chemical plants at Redkino and Kus-
kovo. The commercial production of asymmetric mem-
branes with a 200-nm-thick selective layer was devel-
oped by the Ron-Poulenc Co. [23] and NiiKhimMash.
Thus, the USSR was a pioneer in the field of membrane
separation [24]. The first PVTMS membrane setups
658
NANOTECHNOLOGIES IN RUSSIA
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2008
VOLKOV
et al.
were manufactured at the Kriogenmash Corporation in
the 1970s and used for air separation (with a selectivity
coefficient of
α
[O
2
/N
2
] = 3.1–3.5 and an output flux of
J(O
2
) = 0.5 m
3
/(m
2
h bar)) and hydrogen removal from
gas flows in the chemical industry (ammonia synthesis)
and petrochemistry (pyrolysis).
The next breakthrough in the field of high-perme-
ability polymer glasses was related to the synthesis of
poly(1-trimethylsilyl-1-propyne) (PTMSP), the most
permeable polymer, which was originally obtained at
Kyoto University in 1983 [25]. Despite the large num-
ber of investigations devoted to the synthesis of poly-
mer materials for membranes, PTMSP is still the most
permeable polymer. For example, the coefficient of
oxygen permeability for PTMSP and PVTMS amounts
to 9000 and 45 Barrer, respectively (1 Barrer =
10
10
cm
3
cm cm
–2
s
–1
cmHg
–1
).
The unique properties of PTMSP as a polymer glass
are also manifested by the high diffusion coefficient of
small molecules and low selectivity of diffusion and
permeability (e.g.,
α
[O
2
/N
2
] = 1.5 –1.7. At the same
time, PTMSP as a membrane material exhibits unique
gas separation and pervaporation properties. For exam-
ple, the permeability of PTMSP for butane is higher
than for methane [26], and the butane selectivity
increases with the hydrocarbon mixture permeability
[27]. During the pervaporation separation of aqueous–
organic mixtures such aqueous ethanol solutions,
PTMSP exhibits selectivity with respect to organic
components [28, 29]. Thus, the pervaporation and other
separation properties of PTMSP are determined by the
selectivity of dissolution rather than by that of diffusion
[30].
The unique membrane properties of PTMSP are
related primarily to the structural organization of the
free volume of this polymer glass, which is a microhet-
erogeneous material containing regions of increased
and decreased density. The PTMSP structure is charac-
terized by a very high (20–26%) fraction of nonequilib-
rium (unrelaxed) free volume that is formed by a sys-
tem of interconnecting nanocavities [31, 32]. Thus,
with respect to the laws of mass transport, PTMSP
membranes are close to microporous carbon and hydro-
phobic zeolite (silicalite) membranes, rather than to
membranes based on the typical glassy polymers such
as polysulfone.
The notion, according to which PTMSP is considered
as a nanoporous material, is now commonly accepted.
Russian science has considerable contributed to the
development of this approach, which is objectively
reflected in the most comprehensive review devoted to
the synthesis and properties of PTMSP by Nagai et al.
[33] (see references therein, including about 500 publi-
cations and patents) and in the review by Yampolsky [34]
Molecular mass
Reverse osmosis
Sugars
Colloids
Size of
component,
nm
Retained
of component
Metal
components
Herbicides
ions
Bacteria
Pesticides
Viruses
Tobacco smoke
Latexes/emulsions
Membrane
separation
process Nanofiltration
Ultrafiltration
Microfiltration
1 10 100 1000 10000
100 200 1.000 10.000 20.000 100.000 500.000
Fig. 1.
Filtration spectrum.
NANOTECHNOLOGIES IN RUSSIA
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MEMBRANES AND NANOTECHNOLOGIES 659
devoted to the probing techniques. The results of Molec-
ular dynamics simulations of PTMSP show evidence for
the presence of interconnected nanocavities (free volume
elements) with dimensions on a level of 1 nm (Fig. 2)
[35], which agrees with the data of positron annihilation
spectroscopy.
A leading position in Russia and in the world in the
field of investigations into the synthesis of record high-
permeability polymer glasses based on polyacetylenes
(PAs) and the creation of PA-based separation mem-
branes is occupied by the Topchiev Institute of Petro-
chemical Synthesis (Moscow), the most successful
developments are the PTMSP and poly(4-methyl-2-
pentin) and poly(1-trimethylgermyl-1-propyne). In
addition, methods have been developed (jointly with
Yarsintez Corp. and Yaroks Co.) for the synthesis of
monomers of the acetylene series on the basis of
domestic raw materials [36, 37] and for the production
of related polymers and membranes [38, 39].
At present, the membrane science and technology of
Russia meet challenges that require understanding new
trends in the development and production of mem-
branes, membrane modules, and competitive gas sepa-
ration setups. It should be emphasized that the modern
membrane gas separation technology is based mostly
on the use of membranes in the form of hollow fibers
and only about 20% of setups employ flat membranes.
The main advantages of hollow-fiber membranes are as
follows:
(i) A much greater specific surface of membranes in
a module (on the order of 10
4
m
2
/m
3
) that allows the
size (and, hence, cost) of gas separation setups to be
significantly reduced (the spiral wound and plate-and-
frame modules are characterized by specific areas that
are smaller by 1–2 orders of magnitude).
(ii) A much lower cost of the membrane modules as
compared to that of other types, which is additionally
related to the fact that the production of hollow fibers is
5–50 times cheaper than the equivalent (per unit area)
amount of flat membranes.
(iii) A high mechanical strength, which makes pos-
sible stable operation of the fibers in contact with gases
at high pressures (up to 70 bar or above).
The high mechanical and film-forming properties of
materials based on the PA-based high-permeability
polymer glasses (primarily, PTMSP) developed at the
Topchiev Institute of Petrochemical Synthesis (Mos-
cow), make possible the fabrication of ultrathin layers
of composite separating membranes with flat and hol-
low-fiber configurations. This opens way to the creation
of promising multipurpose nanoporous polymer mem-
branes for large-scale-production facilities in many
industrial fields, from oil and gas production to nano-
12 3 4
a
b
c
5 nm
d
Fig. 2.
PTMSP free volume model [35] with a cube edge length of ~5 nm and each next section spaced by ~0.3 nm from the pre-
ceding one (order of sections:
a
1
, …, a
4
; b
1
, …, b
4
; c
1
, …, c
4
; d
1
, …, d
4
).
660
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VOLKOV
et al.
and optoelectronics, employing advanced technologies
[39, 40–44].
2.2. Pervaporation Membrane Bioreactor
for Production of Alcohols
The usage of bioethanol and biobutanol as an alter-
native fuel offers some ecological advantages to oil
fuels, the main of which is a significant decrease in the
volume of nonutilizable carbon dioxide wastes. The
application of pervaporation membrane bioreactor is
considered as a promising pathway to economic pro-
duction of bioethanol and biobutanol [45, 46]. In a sim-
ple variant such a membrane bioreactor, the fermenter
and membrane module are combined in a single circuit,
where the fermentation mixture is continuously
pumped through the module [47]. An optimized
scheme of the membrane bioreactor allows two prob-
lems to be simultaneously solved, namely, to concen-
trate alcohol in the permeate and reduce its content in
the fermenter (i.e., decrease the inhibiting effect of
alcohol on the microorganisms).
In order to realize such a process, it is necessary to
create organophilic membranes possessing increased
affinity toward organic substances and higher perme-
ability for the target organic components than for water.
Since the size of alcohol molecules is much greater than
that of water, the choice of organophilic membrane
materials is very restricted. The most promising mate-
rials for bioalcohol synthesis are the membranes based
on PTMSP [45], which ensure both a high sorption
selectivity of alcohols and their preferential permeabil-
ity via membrane as compared to that of water.
A joint Russia–United States group including
researchers from the Topchiev Institute of Petrochemi-
cal Synthesis (Moscow), National Renewable Energy
Laboratory (US Department of Energy, Golden, CO),
and the University of Nebraska (Lincoln, NE) per-
formed a series of investigations [48–51] devoted to the
pervaporation of model and real mixtures of the ethanol
and acetone–butanol–ethanol (ABE) fermentation via
PTMSP samples synthesized under various conditions
using three catalytic systems (TaCl
5
/n-BuLi,
TaCl
5
/Al(i-Bu)
3
, and NbCl
5
). It was demonstrated that
PTMSP samples, synthesized on TaCl
5
/Al(i-Bu)
3
and
NbCl
5
catalysts under conditions limiting the branching
processes, possessed high and stable characteristics of
pervaporation for the separation of a model fermenta-
tion mixture during 450-h laboratory tests. As an exam-
ple, Table 1 presents data on the initial mixture and per-
meate compositions for the pervaporation of a multi-
component model mixture on PTMSP-5 (TaCl
5
/Al(i-
Bu)
3
catalyst) and PTMSP-8 membranes (NbCl
5
cata-
lyst) [51]. Both membranes ensure a high degree of
concentration for ethanol, butanol, and methyl acetate.
For example, the content of ethanol increases from 6 to
41–42%, while acetic acid can be classified into low-
permeability components.
The selectivity and stability of operation of the per-
vaporation membranes can be significantly increased
by adding small amounts of poly(dimethylsilmethyl-
ene) (PDMSM), a compound of the class of high-elas-
ticity poly(silicon hydrocarbons). It has been shown
that there is a very narrow region of compositions
(within 4–5 wt % PDMSM in PTMSP) in which the
pervaporation selectivity of the composite is about two
times that of PTMSP [39]. For example, the selectivity
of butanol separation from dilute aqueous solution
grows from 70 to 130, along with a significantly
increased stability of membrane operation with the
time. This is a record high result achieved so far.
It should be noted, however, that commercial imple-
mentation of this process requires solving a number of
problems encountered in investigations [45, 49, 50].
2.3. Production of High-Purity Volatile Substances
Another example that proves the high, by no means
exhausted potential of flat gas-separating membranes,
is the method of separating high-purity volatile sub-
stances, which was developed at the Nizhni Novgorod
State Technical University. It is clear that the creation of
modern advanced technological processes must be
based on high-purity substances of the new generation,
among which the leading positions are occupied by
high-purity volatile substances. Despite a small-scale
character of the production of such compounds, their
cost is quite high and an additional increase in the
purity by only one order of magnitude increases the
price severalfold in the case of using the traditional
energy-consuming methods of purification (distillation,
adsorption).
It should be noted that most of these substances,
especially the volatile inorganic hydrides and halides,
are rather toxic and explosive and, hence, the problems
of industrial and ecological safety in their production
must be solved on the basis of new, in particular, mem-
brane technologies.
Based on the domestic flat polymer membranes
(such as, e.g., MDK series available from Vladipor
Company [52]), original technological processes and
devices have been developed for membrane gas separa-
tion (Fig. 3a), which allowed the selectivity of separa-
tion to be increased by a factor of 100–10 000 (for the
Table 1.
Pervaporation characteristics of PTMSP mem-
brane samples for the separation of a model ethanol fermen-
tation mixture (wt %): ethanol, 6; butanol, 0.2; acetone, 0.2;
methyl acetate, 0.5; water, 92.1 [54]
Membrane
Content of organic component in permeate, wt %
Ethanol Butanol Acetone Methyl
acetate Acetic
acid
PTMSP-5 42.1 5.2 6.4 19.8 0.6
PTMSP-8 40.4 5.7 6.1 20.9 0.4
NANOTECHNOLOGIES IN RUSSIA
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MEMBRANES AND NANOTECHNOLOGIES 661
initial membrane selectivity not exceeding 4–10). This
membrane gas-separating technology was imple-
mented at the Horst Company [53] for the production of
monosilane with a purity of 99.9995–99.9999% (Fig. 3b),
which is now delivered for all enterprises of the micro-
electronics and electrical engineering industry of Rus-
sia and Belarus.
The success of this R&D project was based on the
results of profound theoretical investigations [54–57],
which allowed an engineering computational complex
to be developed for the simulation of membrane sepa-
ration processes involved in the deep purification of
substances, for example, in a radial membrane module
with gas counterflow [56, 57], a continuous membrane
column ensuring high values of the separation coeffi-
cient and the degree of recovery of a high-purity prod-
uct (95–98%) [58], and a membrane module with a
feeding reservoir, which allows the separating effect to
be increased by two orders of magnitude even for
poorly permeating impurities [59]. Good prospects are
also related to the use of membranes based on a PTMSP
nanoporous polymer glass with a record high perme-
ability in a radial gas counterflow module [44], which
allows the productivity of the separation of high-purity
volatile hydrides and halides to be increased tenfold as
compared to the existing membranes. In addition, a new
hybrid technology based on the given radial module
design has been developed, which combines absorption
and membrane gas separation processes (absorption
pervaporation) [60, 61]. The new technology was suc-
cessfully tested for the production of high-purity
ammonia (99.9999%).
2.4. Membrane Gas Absorption
at Elevated Pressures
Another example of a promising hybrid technologi-
cal process, where key elements are membranes based
on nanoporous polymer glasses, is the gas absorption at
elevated pressures. Within the framework of coopera-
tion of the Russian Academy of Sciences and the Neth-
erlands Organization for Applied Scientific Research
(TNO), a new technology has been developed for the
economic separation of CO
2
from natural gas and/or
synthesis gas, followed by its utilization or storage
[41]. These are the so-called gas–liquid contactors
operating at elevated pressures (up to 50 bar for synthe-
sis gas, and above 100 bar for the natural gas). The can-
didate absorbing liquid media are the traditional CO
2
absorbers such as aqueous ethanolamine solutions. The
membrane must ensure the maximum transport charac-
teristics with respect to CO
2
at the absence of leakage
of the absorbing liquid through the membrane under
elevated pressures, which excludes the use of porous
membranes.
Using the membrane gas absorption–desorption
systems, it is possible to significantly reduce the dimen-
sions of apparatuses and, in contrast to the traditional
absorption column, change the spatial orientation of a
separating setup without decreasing the operation effi-
ciency, which is especially important, for example, for
oil-production facilities occurring on a sea shelf.
Investigations showed that one of the most promis-
ing membrane materials in this case are nanoporous
polymer glasses, primarily, PTMSP [41]. In particular,
laboratory samples of flat and hollow-fiber PTMSP-
based composite membranes have been obtained with
separating layers of asymmetric structure obtained by
means of phase inversion. These investigations are per-
formed within the framework of the Russia–Nether-
lands cooperation and the joint DECARBit Project
(www.DECARBit.com), which is the first large inte-
gration project of the EC Seventh Framework Program
(FP7) involving industrial partners and sponsored by
these parties.
The membrane gas absorption–desorption at pressures
close to atmospheric is carried out using hydrophobic
porous membranes, which provide high mass exchange
characteristics. However, nanoporous PTMSP mem-
branes have been also studied as candidate media for
these processes [62], since they may offer some advan-
tages as compared to the porous membranes, especially
in cases where the mass exchange characteristics a lim-
(a)
(b)
Fig. 3.
Radial polymer membrane module of MDK series
with a diameter of 250 mm: (a) internal view; (b) general
view of a setup for monosilane production.
662
NANOTECHNOLOGIES IN RUSSIA
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VOLKOV
et al.
ited by the chemical reaction of CO
2
binding in the liq-
uid adsorbent.
2.5. Nanofiltration of Organic Media
Another promising field of application of the mem-
branes based on nanoporous polymer glasses is the
nanofiltration of nonaqueous media, which is used to
separate nanosized components from low-molecular-
weight organic solvents. The latter must, in turn freely
permeate through the membrane. The most extensively
studied directions of in the nanofiltration of organic
media are the homogeneous catalysis and the extraction
processes in petrochemical, chemical, and food indus-
tries [63]. The absence of phase transitions during
nanofiltration ensured low energy consumption during
this process as compared to the traditional distillation
separation methods. In the case of the homogeneous
catalysis, nanofiltration makes possible the separation
of expensive catalysts from the reaction medium and
to return it back to the reactor without regeneration
and deactivation and without the loss of pressure in
the system.
Recently, PTMSP polymer glasses have been used
for the first time to demonstrate that membranes for the
nanofiltration of organic media can be created on the
basis of such polymers with a high fraction of unre-
laxed free volume [42]. The unique nanoporous struc-
ture of PTMSP provides at a least a tenfold increase in
the solvent permeability for a dense PTMSP layer as
compared to the selective layers of membranes based on
silicon rubbers [64], representing a traditional class of
membrane materials for the given application field [65].
2.6. Metal Membranes
for High-Purity Hydrogen Production
At present, pure hydrogen is widely used in micro-
and nanoelectronics, the production of pure materials
(tungsten, rare-earth metals, silicon, and single crystals
possessing unique magnetic and electrical properties,
including anisotropy), reductive metallurgy, etc. The
pure hydrogen (>99.999 vol %) consumption for various
applications varies from several dozen nm
3
/h in micro-
and nanoelectronics to several dozen millions nm
3
/h in
hydrogen power engineering.
A comparison of various methods used for the puri-
fication of hydrogen and for its recovery from gas mix-
tures shows that membrane separation techniques are
characterized by minimum capital expenditures, time
of setting into operation, and working costs [66]. Sepa-
ration processes using polymer membranes are highly
effective in the production of technical purity hydrogen
(<95–98%). The development of hydrogen power engi-
neering gave rise to the demand for high-purity hydro-
gen (>99.9999%), the recovery of which from various
hydrogen-containing gas mixtures is possible using
palladium alloy based membranes. The main character-
istics of palladium membranes are their unique hydro-
gen permeability, strength, plasticity, stability under
conditions of thermo-baro-concentration expansion
(dilatation) in hydrogen atmosphere and resistance with
respect to aggressive components of industrial hydro-
gen-containing gas mixtures in a broad range of tem-
peratures (200–800
°
C) and trans-membrane pressure
drops (up to 10 MPa) [67].
Specialists of the Topchiev Institute of Petrochemi-
cal Synthesis (Moscow), Baikov Institute of Metallurgy
and Materials Science (Moscow), and Sinplaz Com-
pany developed the membrane elements and modules
based on flat disk membranes with diameters of 50–
150 mm made of a 30- to 70-
μ
m-thick palladium alloy
foils (Fig. 4) [68]. A module of membranes based on a
93.5% Pd–6.0% In–0.5% RuO alloy was tested over
more than two years for the separation of pure hydro-
gen (>99.9999%) from various gas mixtures, including
those containing components such as hydrocarbons (up
to 18 vol %), hydrogen sulfide (up to 1.5 vol %), car-
bon monoxide (up to 15 vol %), carbon dioxide (up to
30 vol %), nitrogen (up to 25 vol %), and chlorine-con-
taining compounds at temperature of up to 600
°
C and
pressures of up to 10 MPa. The results showed evidence
for stability of the device characteristics under such
conditions. The working area of the membrane surface in
a unit module varies from 0.007 to 2 m
2
, which provides
for the pure hydrogen productivity from hundreds nl/h to
1000 nm
3
/h. The search for optimum compositions of
Pd/Y alloys is in progress [67].
The joint R&D project of the Institute of Problems
of Chemical Physics (Chernogolovka), Moscow Plant
for Processing of Special Alloys, All-Russia Research
Institute of Experimental Physics (Sarov), and Khim-
Fist-Splav Company succeeded in creating thin sepa-
rating membrane layers using approaches underlying
the commercial production of tinsel [69]. The main
component in the proposed composite membrane with
an ultrathin diffusion layer of palladium layer is a foil
Fig. 4.
General view of a filter unit for a commercial hydro-
gen purification module employing FEL-150 elements with
a productivity of 120–1000 m
3
/h of pure hydrogen
(>99.9999%).
NANOTECHNOLOGIES IN RUSSIA
Vol. 3
Nos. 11–12
2008
MEMBRANES AND NANOTECHNOLOGIES 663
with a thickness of 10
μ
m and below. The flux of hydro-
gen through such a membrane at a temperature of
550
°
C, a partial pressure of hydrogen in a mixture of
0.6 MPa, and an output hydrogen pressure of 0.15 MPa
amounts to ~100 nm
3
/(m
2
h). The next stage of this
project consists in the design and development of diffu-
sion elements and modules.
3. NANO-, ULTRA-, AND MICROFILTRATION
The most authoritative school of Russian specialists
in the field of baromembrane technologies for the sep-
aration of liquid media was constituted by Yu.I. Dytner-
skii more than half a century ago at the Mendeleev Uni-
versity of Chemical Technology (Moscow) [70]. At
present, the Chair of Membrane Technologies of this
University (headed by G. G. Kagramanov) occupies
leading positions in Russia in this field of knowledge
and specialist training [71].
The purification of liquids by means of micro-,
ultra- and nanofiltration through separating membranes
is a multiparametric process. In order to solve each of
these tasks, high-productivity membranes with the cor-
responding nominal transport pore size are necessary.
However, this is only a necessary but not sufficient con-
dition for the development of an effective competitive
membrane separation process, since the filtration is
accompanied by the accumulation of retained compo-
nents on the membrane, which leads to blocking of the
pores and a decrease in the filtration rate to zero. The
most technological solution of this problem is the cre-
ation of membranes with low adhesion to deposits and
the development of effective processes for the cleaning
of membranes (removal of deposits) in the course of fil-
tration without disassembly of the membrane module.
This is most frequently achieved by applying a pulse of
reverse pressure from the side of the permeate (liquid
phase penetrated through the membrane). The detached
material must be effectively carried away from the
module by the retentate (liquid phase not penetrated
through the membrane).
Microfiltration of water removes colloidal particles
with dimensions of 100 nm and above. Manufacturers
of the membrane separation setups believe that mem-
brane elements of the cartridge type based on the
domestic polymer microfiltration membranes corre-
spond to the world level of performance. An example is
offered by filter elements of the Tekhnofiltr Company
[72], which are made of membranes based on Capron,
fluorinated polymers. cellulose esters, and regenerated
cellulose.
Ultrafiltration removes nanodimensional compo-
nents, including macromolecular organic substances,
with particle dimensions no less that 10 nm. Dissolved
salts can be removed from water by means of nanofil-
tration and reverse osmosis.
3.1. Nanofiltration for Drinking Water Production
The market of services related to the production of
high-quality drinking water rapidly grows. The most
widely used membrane method for the removal of dis-
solved salts from water is reverse osmosis. However, in
many cases (e.g., for obtained high-quality sound
drinking water) nanofiltration technology is advanta-
geous to the reverse osmosis with respect to both capi-
tal expenditures and working costs. This is related to
the fact that modern nanofiltration membranes and
membrane assembly elements can differently retain
salts consisting of mono- and polyvalent ions. For
example, the degree of retention of sulfates, carbonates,
and phosphates reaches 95% and above, while that for
chlorides, bicarbonates, and nitrites is 50–70%. Salts of
calcium, magnesium, iron, manganese, and heavy met-
als are retained much better than salts of sodium, potas-
sium, and lithium. The reverse osmosis removes all
(both harmful and useful) dissolved salts from the initial
water. The filtrate can be used as drinking water only upon
adding salts necessary for the human organism.
Of course, the production of drinking water from sea
water requires using high-pressure reverse osmosis with a
membrane selectivity of no less than 99.5%. However, at
a total salt content of up below 2 g /l, it is also expedient to
use nanofiltration instead of reverse osmosis.
A series of competitive composite nanofiltration
membranes occupying an intermediate state between
the ultrafiltration and reverse-osmosis membranes and
possessing a moderate selectivity of 50–70% with
respect to sodium chloride at a rather high selectivity
(above 90%) with respect to hardness salts were devel-
oped [73] by the Akvapor Company in cooperation with
the Vladipor Company [52]. These membranes effec-
tively remove organic substances with a molecular
mass above 100 Da and can be used for the partial
desalination of water. Table 2 presents the performance
characteristics of a nanofiltration setup for drinking
water production from surface sources (40 such setups
are operating in Almet’evsk.
A large number of nanofiltration elements are used
in the food industry for assembling membrane separa-
tion setups of water conditioning for alcoholic and
alcohol-free drinks production and for the technologi-
Table 2.
Performance characteristics of a nanofiltration
plant for drinking water production from surface sources
Characteristic Dialysis Initial
water Purified
water
Productivity,
10 m
3
/day Total hardness, mg-
eq./l 3–5 1–2
Working pressure,
10–12 bar [ ], mg/l 20–30 5–10
Alkalinity, mg-eq./l 3–4 1–2
Filtrate output,
70% Coloration, deg
≤
30 0
Turbidity, mg/l
≤
15 0
Total iron, mg/l
≤
10
SO4
2–
664
NANOTECHNOLOGIES IN RUSSIA Vol. 3 Nos. 11–12 2008
VOLKOV et al.
cal quality control of products with respect to microbi-
ological criteria [74].
3.2. Ultra- and Microfiltration
A recent growth in the application of micro- and
ultrafiltration is related to the appearance of the new
types of membranes, which possess increased produc-
tivity with respect to the filtrate, are stable under condi-
tions of washing with various chemical solutions, and
produce drinking water of very high quality. The puri-
fication of natural water by means of the micro- and
ultrafiltration allows all suspended matter to be
removed, significantly decreases the turbidity and col-
oration (especially, in combination with coagulation)
and allows water to be conditioned for the subsequent
processing on reverse-osmosis membranes.
New types of competitive microfiltration mem-
branes include, in particular, the polymer track mem-
branes that are considered below (Section. 4). Below
we consider some examples of domestic inorganic
micro- and ultrafiltration membranes.
3.3. Ceramic and Combined Membranes
Liquids are frequently purified using ceramic fil-
ters, which are capable of reliable service in micro-
and ultrafiltration processes involving pressure drops
up to 10–15 bar in the direct (working regime) and
reverse (regime of washing fouled membranes)
directions.
Specialists of the Keramikfiltr Corporation [4],
which was previously involved in the military aero-
space Buran project, have developed ceramic micro-
and ultrafiltration membranes with α-Al2O3 supports
and a selective layer based on single crystal nanofibers
of β-SiC, which were bound to the support via ZrO2
based ceramic binder [75–77]. The fibers appear as fil-
amentary single crystals with diameters from 10 nm to
1 μm and lengths from 3–5 to 10–15 μm. The nanofiber
structure of the selective layer ensures its high porosity
(90–95 vol %) and, accordingly, accounts for a high
productivity of membranes (the initial distilled water
productivity, up to 10 m3/(m2 bar)) and increased crack-
ing resistance of membranes in the course of manufac-
ture and operation.
By varying the support grain size and nanofiber
packing density, a series of micro- and ultrafiltration
membranes (single and multichannel) with various
retention thresholds (from 50 nm to several microns)
have been obtained. According to the performance
specification, the membranes are stable in aggressive
media with pH 0–13 at temperatures up to 200°C.
These characteristics determine, in particular, the abil-
ity of membranes to withstand commercial steriliza-
tion, for example, in the dairy industry, pharmaceutical
production, etc.
Specialists of the Institute for Physics and Power
Engineering (Obninsk) developed a technology of plas-
machemical synthesis based on the cathode ion bom-
bardment, which provides bottom-to-top self-assembly
of high-efficiency tubular ultrafiltration membranes
with inorganic selective layers possessing a nanodi-
mensional structure [78, 79]. The main feature of this
technology of plasmachemical synthesis of ultrafiltra-
tion membranes is that the transitions from solid to
plasma state and back proceed as a whole unified pro-
cess.
Figure 5 shows a schematic diagram illustrating the
production of tubular ultrafiltration membranes by
means of particle deposition from erosion plasma onto
a porous support surface. This technology was imple-
mented on the basis of a commercial setup for the elec-
tric-arc evaporation of various cathode materials. A
high-current (90–130 A) electric arc initiated between
the cathode and anode ensures the evaporation of mate-
rial (Ti, Zr, Al) from the cathode surface. Under the
action of applied electromagnetic fields, the flow of
erosion plasma particles emitted from the cathode sur-
face is focused and (if necessary) accelerated. Then, the
plasma flow enters the working chamber, where porous
supports are situated. The chamber can be either evac-
uated or filled with a gas (nitrogen, oxygen, argon, acet-
ylene, etc.), which is necessary for plasmachemical
reactions with erosion plasma ions. For example, if
nitrogen is present in the working chamber, its chemi-
cal reactions with erosion plasma ions lead to the for-
mation of nitrides of the cathode material (metal).
The technology of plasmachemical synthesis of the
tubular nanostructural ultrafiltration membranes on the
surface of a porous support sintered from a powder of
ultra-high-molecular-weight polyethylene (UHMWPE)
was implemented on a commercial production scale.
The formation of a nanostructural membrane on the
surface of a porous UHMWPE support is complicated
by a large difference in thermal properties of the grow-
ing membrane and support (the ratio of thermal con-
ductivity coefficients of the nanostructural membrane
and support amounts to 105; the UHMWPE softening
temperature is 95–105°C; plasma temperature,
>1000°C). The deposition of plasma particles on a
porous polymer support is accompanied by the compet-
itive processes of polymer degradation and membrane
synthesis. By changing the parameters of erosion
plasma, it is possible to provide conditions where a
functional membrane tightly bound to the polymer is
formed on the surface. Due to a nanodimensional struc-
ture, the inorganic membrane exhibits a sharp increase in
the plasticity and acquires the properties of flexible
ceramics. The average thickness of the nanostructural
membrane was 7–12 μm and its volume porosity is 10–
12% (the volume porosity of the support was 40–55%).
Figure 6 shows micrographs of the typical structure
of a macroporous UHMWPE support, nanostructural
selective layer (titanium nanomembrane), and the so-
NANOTECHNOLOGIES IN RUSSIA Vol. 3 Nos. 11–12 2008
MEMBRANES AND NANOTECHNOLOGIES 665
called dynamic membrane (i.e., a growing layer of
retained nanoparticles) formed in the course of ultrafil-
tration [80]. The titanium membrane structure pre-
sented in Fig. 6b suggests that the filtration apparently
proceeds by liquid phase permeability through the
boundaries of grains and subgrains (the average size of
the latter ranging from 3–5 to 10–15 nm). The X-ray
diffraction analysis of nanostructural membranes
showed evidence of the absence of a clearly pro-
nounced diffraction pattern. Investigation of the cross
section of the membrane showed that the erosion
plasma particles penetrating into the polyethylene sup-
port produce fusion of the pores to a depth equal to the
membrane thickness (7–12 nm). The resistance of the
whole membrane to the airflow increases no more than
by 15% as compared to the support.
Another interesting example of membranes with
selective layers based on plastic ceramics is offered by
the products of Aspekt Company [3]. Thin tubular and flat
two-layer metalloceramic membranes of the Trumem
(Trade Mark) type with a thickness of ~250 μm and a
~15 μm-thick ceramic layer are produced according to
the RF, EC, and USA Patents. The ultra- and microfil-
tration membranes are characterized by pore sizes in a
range from 100 nm to 5 μm and withstand a transmem-
brane pressure drop of up to 10 bar. The two-layer
membranes cover a range of the average pore sizes
from 100 nm to 1 μm and typically comprise a stainless
steel support with a thin ceramic layer of TiO2,
TiO2/Al2O3, ZrO2, and SiO2. The flux of distilled water
varies from 2.8 to 18 m3/(m2 h) at a transmembrane
pressure drop of up to 2 bar. These membranes can be
regenerated by means of washing in a back flow, heat-
ing at 300°C in air or up to 800°C in the inert or reduc-
tive atmosphere), and treating in sharp steam, and most
of sterilization solutions. The Aspekt Corporation also
developed a pilot production technology of metallocer-
amic nanostructural membranes Trumem, including a
unique technological complex for the application of
ceramic layers on a moving ribbon representing a
porous metal support. The deposited layers can be
made of different materials, with each element being
heat-treated and shear-strained according to a preset
schedule, which ensures the formation of selective lay-
ers based on a plastic ceramics. An important advantage
is the creation of a porosity gradient in any of the
deposited layers. This is manifested, in particular, by
the asymmetry of gas transport and catalytic reactions
[81] as is discussed in detail below.
4. TRACK MEMBRANES
Track membranes represent thin crystalline layers,
metal foils, or films (typically, 5- to 25-μm-thick poly-
mer), in which a system of pores is formed using irra-
diation of the initial nonporous materials by high-
energy particles (typically, accelerated multicharge
ions or fission products, fluxes of high-velocity nano-
and microparticles, or a beam of synchrotron radiation)
followed by etching of the latent tracks of particles,
penetrating through the target, in an etching solution
until obtaining through pores of required diameter. The
main distinction of track membranes from the tradi-
tional membranes is the regular pore geometry, the pos-
Rotary drive
Electric arc
Arc-igniter
Stabilization,
focusing
electrode
1
2
34
5
Fig. 5. Schematic diagram of the technology of plasmachemical deposition of tubular ultrafiltration membranes by means of particle
deposition from erosion plasma onto a porous support surface: (1) working chamber; (2) polymer or inorganic porous support;
(3) electromagnetic coils; (4) cathode; (5) cathode-anode space.
666
NANOTECHNOLOGIES IN RUSSIA Vol. 3 Nos. 11–12 2008
VOLKOV et al.
sibility of controlling their number per unit membrane
surface, and narrow pore size distribution. The sizes of
pores in track membranes used in science and technol-
ogy range from 1 nm (primary channel of the track of a
high-energy particle) to several hundred nanometers
(ultra- and microfiltration membranes). Thus, accord-
ing to the modern classification [12], track membranes
belong to the typical nanomaterials.
Track membranes are widely used in medicine, for
example, in the purification of drugs and viral suspen-
sions (vaccines), preparation of blood plasma (plasma-
pheresis), bacteriological control of the quality of food
products and water. In the middle of 1980s, above
40 million doses of influenza virus vaccine were pro-
duced in Russia using domestic track membranes. In
technology, track membranes are used for the purifica-
tion of air and liquids (e.g., clean-room technology and
drinking water production) and in analytical monitor-
ing of substances.
Since the onset of development of the science, tech-
nology, and practical application of the domestic track
membranes in the USSR (from the middle of 1970s),
about ten enterprises for the production of track mem-
branes and related articles have been created. Each of
these enterprises had a source of high-energy particles
(cyclic accelerators, atomic reactors) capable of provid-
ing the irradiation of polymer films with a total area of
about 1 million m2/year.
At present, the track membrane science and technol-
ogy in Russia is concentrated around the following cen-
ters: Flerov Laboratory of Nuclear Reactions at the
Joint Institute of Nuclear Research (JINR, Dubna),
Institute for Physics and Power Engineering (Obninsk),
Ioffe Physico-Technical Institute (St. Petersburg),
Tomsk State Technical University (Tomsk), Research
Center for Applied Nuclear Physics (Dubna), Keldysh
Research Center (Moscow). This constellation is sur-
rounded by a number of small and medium private
companies and corporations such as Trem Company
(production of track membranes), Plazmofiltr (mem-
brane plasma filters), Trackpore Technology Company
(large-scale production of track membranes and related
plasma filters) [82].
The above information characterizes the production
of track membranes as an advanced technology directly
related to the field of nanomaterials. This industry is
provided both with a scientific basis and the powerful
production facilities, which make it innovation-promis-
ing. This is confirmed by the fact that the private Track-
pore Technology Company has invested its own cap-
ital into the design, construction, and launching a
high-power electrophysical setup based on a cyclic
accelerator of heavy ions for the production of track
membranes of medicinal quality and the develop-
ment of related business (track membranes for plas-
mapheresis).
4.1. Models of Latent Track Formation
Despite extensive research, the theory of track for-
mation in condensed media is still cannot be considered
as completed [83]. Further theoretical and experimental
200 mm
(a)
(b)
(c)
Fig. 6. Micrographs of the structure of (a) macroporous
UHMWPE support, (b) titanium nanostructural membrane
and (c) dynamic membrane formed in the course of ultraful-
tration.
NANOTECHNOLOGIES IN RUSSIA Vol. 3 Nos. 11–12 2008
MEMBRANES AND NANOTECHNOLOGIES 667
investigations of the structure of latent tracks are neces-
sary, which also provide a basic contribution to the elu-
cidation of mechanisms of the interaction of acceler-
ated ions with solids and to the principles of the tech-
nology of porous nanostructures. The first theory of the
process of track formation in a condensed medium
(microexplosion theory) was formulated by Gol’dan-
skii in 1975 [84]. Since then, our notions about the
nature of a high-energy particle track have been sub-
stantially modified.
The latent track represents a region of the medium
near the ion trajectory where changes in the properties,
structure, or phase state of the substance took place.
These changes result from the response of the medium
to a perturbation introduced by the impinging ion. Until
the moment of stabilization, the medium features a
sequence of transformations, which begins with the
event of energy transfer from the ion to the medium. At
an ion energy in excess of 10 keV/amu, the energy
losses are mostly due to inelastic collisions, in which
case the ion interacts with the electron subsystem of the
medium. This interaction results in the appearance of
either electron-excited molecules or electron–ion pairs
in the molecular medium. High-energy secondary elec-
trons formed as a result of the ionization events can also
enter a cascade of collisions, producing excited mole-
cules and generating electrons and ions. This is the pri-
mary stage of track formation, the theory of which was
formulated by Kaplan and Miterev [85–89].
At high velocities of the primary ions, the events of
energy loss are separate and simple track forms (elec-
tron–ion pairs, separate electron-excited molecules).
As the ion is decelerated, several pairs can exhibit over-
lap that results in the formation of more complicated
track forms (spurs, blobs, short tracks). Finally, when
the mean free ion range becomes shorter than the spur
size, a continuous track is formed in the medium. Such
a continuous track contains a special cone-shaped
region called the core, which is filled with the track
forms produced both by the primary ion and the sec-
ondary electrons. In investigations devoted to the visu-
alization of tracks in dielectrics by means of etching
[90], it was suggested to evaluate the core size as the
most damaged part of track, which has a radius of about
4 nm. This region absorbs more than half of the energy
transferred by the impinging ion to the medium. The
proposed core size corresponds to the minimum radius
of a cavity obtained upon etching.
The next stage in the evolution of tracks in a molec-
ular medium is related to chemical conversions and/or
structural rearrangements on the background of excess
energy transfer out of the track and the diffusion of
active intermediate particles. In a condensed medium,
the chemical stage is subdivided into the track period
(where a spatial nonuniformity in the distribution of
chemically active particles takes place) and the subse-
quent period that features uniform distribution of the
reagents. Upon the termination of this stage, a chemical
equilibrium is established in the medium. In a solid
dielectric, this stage is terminated by the formation of a
latent track.
Judging from the variety of conversion processes,
one can conclude that numerous mechanisms contrib-
ute to the defect formation in the latent track. The rela-
tive importance of a particular mechanism in the pro-
cess of defect generation depends on the structure of the
medium and the spatial arrangement of these defects. In
order to establish the mechanism responsible for the
defect generation in the latent track, it is necessary to
analyze processes in the physicochemical stage in more
detail.
4.2. Polymer Nanoporous Track Membranes
Irradiation of a polymer by high-energy ions leads
to local modification of the polymer structure, in which
the rate of damage varies in the radial direction from the
track axis to undamaged peripheral regions [91]. Theo-
retical models [89, 92] justified the notions concerning
the initial ionization, excitation of molecules, and
monotonic radial variation of the absorbed energy dose
from high values to zero. The chemical reactions initi-
ated by ions depend primarily on the chemical nature of
a polymer, ion energy, and radiation fluence. There are
various energy dissipation mechanisms, which can ini-
tiate different chemical reactions.
At present, it is widely accepted that the track con-
sists of two zones, the core and the halo, followed by
the regions of unmodified material [93, 94]. However,
some researchers consider tracks as possessing a more
complicated structure. According to Mazzei et al. [95],
there is a zone (previously not distinguished) between the
track halo and the unmodified material, which is etched at
a lower rate as compared to that in the second zone.
The track core is a strongly excited zone along the ion
trajectory, which is formed by mechanisms described
above. At present, there is no commonly accepted
notions concerning the structure of the track halo. Most
authors believe that this is either a cross-linked region of
track or a region where the competition between cross-
linking and degradation (depending on the radiation
dose) takes place [96]. Initially, it was assumed that the
cross-linking was due to the secondary-electron-induced
radical formation [97]. However, the results of calcula-
tions of the secondary electron path lengths proved to
be much shorter than experimental values of the track
radius [98].
The dimensions of a damaged region along the ion
trajectory in crystals and polymers were studied by
methods of small-angle X-ray (SAXS) and neutron
(SANS) scattering. The data were interpreted consider-
ing the system of tracks as a set of randomly arranged
extended cylinders possessing a reduced material den-
sity, which are aligned parallel to the primary ion beam.
For example, Svergun et al. [99] showed for the tracks
of Xe ions with an energy of about 1 MeV/amu in PET
668
NANOTECHNOLOGIES IN RUSSIA Vol. 3 Nos. 11–12 2008
VOLKOV et al.
that the size of a damaged zone with a homogeneous
density amounts to 26 nm.
4.3. Etching Latent Tracks
Conductometry is among methods that can be used
for judging on the track structure from data on the
kinetics of track etching [100]. The conductometric
data can be supplemented by studying the laws of track
etching with allowance for their swelling in the etchant.
The initial stage of alkaline etching is accompanied by
local swelling of the entire cross-linked polymer vol-
ume, in which the local swelling regions (LSRs) filled
with a polymer gel are formed. These regions, the
dimensions of which increase with the degree of swell-
ing (for Xe ions in PET, up to 150 nm), are not equilib-
rium since the hydrolysis process is continued inside
the gel. However, in the equilibrium state (that is
attained upon keeping a sample for several days in air
at 20°C). the LSR size becomes constant and equal
approximately to the diameter of the modified polymer
region around the track axis. In PET, this size amounts to
50 nm for indicated Xe ions, while for Ar (1 MeV/amu),
Kr (1–2 MeV/amu), and Bi (3.5 MeV/amu) this value
is 25–30, 35–40, and ~150 nm, respectively.
Figure 7 presents experimental results illustrating
the character of pore diameter variation with increasing
time of etching for PET irradiated by Xe ions. In this
curve, about 20-min-long region 1 corresponds to the
swelling of latent track in a KOH solution. This swell-
ing stage terminates with the formation of the track
membrane with a minimum pore size (D = 8–10 nm).
The following region 2, which is characterized by a
reduced etching rate, was previously revealed by the
conductometry. The slow etching (D = 8–12 nm) in this
region is related to cross-linking of the polymer via
covalent bonds [93]. The next special region 3 appears
in the vicinity of D = 20–25 nm and is not as pro-
nounced. This region was not observed previously,
since it is only revealed in the course of etching in an
alkaline solution of small concentration at a relatively
low temperature.
The results of investigations presented above can be
generalized in the form of a track scheme, which
includes a core with a diameter of 4–6 nm, a region with
D = 8–12 nm that is cross-linked by covalent bonds, a
region with D from 8–12 to 25–30 nm that is cross-
linked due to migration of the radiolysis products out of
the core, and a region with D from 30 to 50–60 nm that
cross-linked by hydrogen bonds.
4.4. Formation of Symmetric and Asymmetric Pores
Pores with diameters >100 nm in the track mem-
branes possess almost perfect cylindrical shapes (Figs. 8
and 9). This fact allows such membranes to be classi-
fied as isotropic, in which the pore shape is constant
along the filtration channel. Track membranes of this
kind are now frequently referred to as the traditional.
Figure 8a, showing micrographs of the planar surface
of a traditional membrane and a network type mem-
brane, demonstrates a regular pore structure of track
membranes. As can be seen, the micrograph of the track
membrane displays both separate pores and overlap-
80
0
70
60
50
40
30
20
10
100 200 300 400 500 600
t, min
1
2
3
Dy, nm l
Fig. 7. A plot of the pore diameter D versus time t of etching in KOH solution (0.25 mol/l; 61°C) for a PET film irradiated by Xe
ions to a fluence of N = 2 × 109 cm–2: (1) track swelling time and (2, 3) region of reduced etching rate for a pore diameter of ~10
and 25 nm, respectively [2].
NANOTECHNOLOGIES IN RUSSIA Vol. 3 Nos. 11–12 2008
MEMBRANES AND NANOTECHNOLOGIES 669
ping ones, which can be expected to reduce the selec-
tivity. However, the porous structure of track mem-
branes of the last generation is formed, with allowance
for the features of irradiation of the initial polymer film,
so as to obtain several pore arrays oriented at certain
angles relative to each other. For this reason, pores
inside the membrane are mutually divergent and,
hence, their coincidence over the entire membrane
thickness is excluded. This is clearly illustrated in Fig. 9a
showing a micrograph of the cross section of such a
membrane.
Using various methods of modification of the near-
surface layers of track membranes, it is also possible to
obtain asymmetric (cone- or bottle-shaped) pores. By
setting asymmetric conditions during sensitization and
etching of the irradiated polymer films (e.g., by using
different etchant concentrations on the opposite sides of
the film, introducing surfactants, etc.), it is possible to
obtain asymmetric membranes [101], for example, with
pores having the shape of a truncated cone. The possi-
bility of such regulation of the anisotropy of track
membranes is a significant advantage. Indeed, an
increase in the pore taper results in a growth in the
membrane productivity due to an increase in the effec-
tive porosity volume at a retained selectivity that is
determined by the minimum cross section of transport
pores.
The essence of one method of the formation of
asymmetric track membranes, which has been devel-
oped by the Flerov Laboratory of Nuclear Reactions
(JINR, Dubna), consists in introducing surfactants into
the etching solution and exposing one side of the film
(irradiated by high-energy ions) to UV radiation. A thin
layer of the surfactant formed on the film surface hin-
ders penetration of an etchant into the membrane at the
stage of track etching, so that a selective pore fraction
is formed in this region. At the same time, the free
etchant diffusion over the other part of the track leads
to the formation of a nonselective (bottleneck or cigar-
like) pore fraction [102]. The exposure of one mem-
brane surface to the UV light leads to an increase in the
etching rate on this side as compared to that on the
opposite side. As a result, pores formed in the mem-
brane in this case resemble asymmetric cigars (Fig. 10).
Investigations of the track membranes of this kind
(a)
(b)
Fig. 8. Micrographs of the surface of (a) track membrane
and (b) network type membrane.
1 μm
(b) (a)
Fig. 9. Micrographs of (a) the surface of a track membrane and (b) its transverse cleavage showing a porous structure comprising
several pore arrays sloped relative to each other.
1 μm
(a)
670
NANOTECHNOLOGIES IN RUSSIA Vol. 3 Nos. 11–12 2008
VOLKOV et al.
showed that, for the same selectivity, the asymmetric
track membranes exhibit 3–7 times higher productivity
as compared to the traditional track membranes. The
unique properties of such membranes, including the
transport asymmetry, are considered in Section 5.
The spread of pore parameters in the microfiltration
track membranes is mostly determined by overlap of
the neighboring pores. In the case of pores with diame-
ters below 100 nm, their spread is determined primarily
by the morphology of the polymer film (e.g., by the
microcrystalline structure of a PET film). In this caser,
the scale of roughnesses on the surface of a track mem-
brane (10 nm) is close to the pore size, which is
clearly illustrated by the atomic force microscopy
(AFM) image presented in Fig. 11. With allowance for
this circumstance, the scatter of pore diameters in track
membranes in the range of 10–100 nm amounts to ±10–
20% of the average pore size.
4.5. Applications of Polymer Track Membranes
In recent years, track membranes find increasing
application in conditioning natural waters, analyzing
environmental pollutants [103], and purifying gases
and liquids for microelectronics. It is also necessary to
specially mention the use of track membranes for cre-
ating clean (dust- and microbe-free) laboratories and
technological rooms. Owing to specific features of their
porous structure, track membranes possess the mini-
mum value of gasdynamic pressure under conditions of
a diffusion gas transfer in comparison to the mem-
branes of other types. The main element of the clean
room technology is a filtering modulus based on a track
membrane, which provides a diffusion gas exchange
between the clean zone and environment and maintains
the necessary oxygen to carbon dioxide concentration
ratio [104]. The main manufacturer of clean rooms
based on track membranes in Russia is the Research
Center for Applied Nuclear Physics (Dubna). Such
rooms have been successfully employed for several
years at the Shubnikov Institute of Crystallography
(Moscow) for investigations in the field of nanotechnol-
ogy. Track membranes are also widely used in Russian
hospitals for safely conducting the procedures of pro-
phylactic and donor plasmapheresis. Specialists of the
Flerov Laboratory of Nuclear Reactions (JINR, Dubna)
developed an original method for the jointing of PET-
based track membranes, which was used for the devel-
opment of cassettes for plasma filters. Such filters avail-
able from the Plazmofiltr Company (St. Petersburg) and
Trackpore Technology Company [82] are characterized
by compact design, simplicity, and relatively low cost
in comparison to foreign analogs. Track membranes are
also widely used for isolating cells from biological sus-
pensions, studying their shapes and dimensions, purify-
ing proteins, viral preparations, and drugs, and prepar-
ing antiviral vaccines.
As was mentioned above, specialists of the Keldysh
Research Center (Moscow) developed and imple-
mented a special technology of membrane desalination
of Caspian Sea water, constituting a basis for the Man-
gistauss desalination plant, which is the biggest of this
kind in the CIS countries. An important element of this
technology is microfiltration via track membranes.
Original technologies have been developed and pat-
ented for the track membrane etching and quality mon-
itoring [105–107], manufacture of separating devices
and spiral wound filtering element based on track mem-
1 um
Fig. 10. Micrograph of a transverse cleavage of an exisym-
metric track membrane. Fig. 11. AFM image of the surface of a track membrane
with a nominal pore size of 50 nm.
NANOTECHNOLOGIES IN RUSSIA Vol. 3 Nos. 11–12 2008
MEMBRANES AND NANOTECHNOLOGIES 671
branes [108], and the microfiltration technology and
system design [109–111]. Active participants in the cre-
ation and development of microfiltration elements
based on track membranes were specialists of the Fle-
rov Laboratory of Nuclear Reactions (JINR, Dubna),
Vladipor Company [52], Akvapor Company, and Ekofil
Company.
A technological line for the etching of polymer films
and the manufacture of spiral wound filtering element
based on microfiltration track membranes [112] was
created at the Keldysh Research Center (Moscow) [5].
Strict pore geometry and insignificant deviations of the
pore size from the nominal ensure reproducibly high
selectivity of these membranes, while small thickness
of the initial film makes possible a high density of
membrane packaging in the spiral wound filtering ele-
ment. Such design and performance characteristics
cannot be achieved using the existing polymer microfil-
tration membranes of obtained by other methods.
Recently, the technology of microfiltration based on
track membranes has been implemented at the Moscow
Oil-Processing Plant and some other enterprises.
5. ION-EXCHANGE MEMBRANES
Ion-exchange membranes constitute a very impor-
tant class of membrane materials. Among these, we
should specially mention high-molecular-weight mem-
branes based on polymers containing functional ion-
exchange groups, inorganic membrane materials, and
hybrid materials of the organic/inorganic type [113].
Despite the fact that inorganic membrane materials and
ion exchangers were discovered much earlier, the high-
molecular-weight membranes are now most widely
employed. These media consist of flexible polymer
chains containing functional groups (–COOH, –SO3H,
–NH3OH, etc.), which are capable of replacing protons
and OH groups by cations or anions from solution.
In the middle of the 20th century, several scientific
schools have been founded in the USSR, which were
concentrated on the research in the field of ion-
exchange membranes and related processes. These
schools, residing at the Kuban State University (Krasn-
odar) and Voronezh State University, as well as in Kiev
and Almaty, are still actively working in the indicated
field. Somewhat later, research in this direction was ini-
tiated at the Kurnakov Institute of General and Inor-
ganic Chemistry (Moscow), Moscow State University,
North Caucasus Technical University (Stavropol), and
Vyatka State University. At present, this field is also
actively developed at the Institute of Problems of
Chemical Physics (Chernogolovka), Frumkin Institute
of Physical Chemistry and Electrochemistry (Mos-
cow), Engels Institute of Technology at the Saratov
State University, and in some other scientific centers.
Recently, a special Institute of Membranes has been
established at the Kuban State University (Krasnodar).
Historically, electromembrane technologies always
received much attention in Russia, beginning with elec-
trodialysis that originated from the well known and
widely used ion exchange [114–116]. Water desalina-
tion setups based on the electrodialysis process were
employed since the middle of the 20th century [117].
The first industrial desalination plants with a produc-
tivity of up to 25 m3/day were developed by
K.M. Saldadze with coworkers and manufactured at the
Tambovmash enterprise. In 1965, a laboratory proto-
type setup for deep desalination was developed [118]
based on a three-stage electrodialysis process, which
allowed water with a resistivity of up to 22.4 MΩ cm to
be obtained. The commercial production of these
devices was organized at the Kuban State University
(Krasnodar) and at the Membrane Technology Center
established later at this university. Complex setups for
the production of deionized water with a yield of 1.2
and 10 m3/h [119] have been also developed for the
microelectronic industry. Later, a new setup was devel-
oped that allowed deeply desalinated water to be
obtained by means of multistage direct flow electrodi-
alysis with a yield of up to 250 l/h [120]. Another setup
for the deep desalination of water by electrodialysis
with a yield of 2 m3/h was developed at the Voronezh
State University. New setups for the production of
deionized water were described in [119, 121]. Many
researchers still believe that electrodialysis with an ion-
exchange resin column will replace the traditional ion
exchange in the production of high-purity high-ohmic
water [119, 121, 122]. Desalination and the production
of drinking water are still among the most popular
applications of electrodialysis [113, 123–125].
Ion-exchange membranes are also widely used in
fuel cells (FCs). The principles of FC operation,
together with the theoretical foundations and principles
of direct electricity production via chemical processes,
are described in the monograph by Davtyan [126]. At
present, significant success has been achieved with FCs
based on polymeric electrolytes (polyelectrolytes) in
the form of thin membranes. An important impact stim-
ulating investigations in the field of membrane FCs was
the agreement concerning research in hydrogen power
engineering between the Russian Academy of Sciences
and the Norilsk Nickel Company [127].
In selecting the structure of membrane materials, it
is necessary to consider the conditions of their func-
tioning and the related requirements to their properties.
The candidate materials have to possess high ion (pro-
ton) conductivity, stability to redox media (at ambient
and elevated temperatures), and good mechanical
strength. The latter is important since, for example, a
polyelectrolyte in FCs has the form of a thin membrane
that must bear water in the course of operation. The
material must also possess a long working life, that is,
retain characteristics during long-term FC operation
(dozen thousand hours). At present, there are no mem-
branes meeting all of the aforementioned requirements.
For this reason, the R&D of membranes for FCs con-
672
NANOTECHNOLOGIES IN RUSSIA Vol. 3 Nos. 11–12 2008
VOLKOV et al.
sists to a considerable extent in the search for optimum
variants for particular applications.
The first successful solution for an FC with sul-
fonated polystyrene membranes was realized in 1962 in
the development of an energy supply system for the
Gemini satellite [128], where the FC had a working life
of about 800 h. The next important step in the develop-
ment of polyelectrolytes and related FCs was made in
1966 by the DuPont Company [129], which patented a
polyelectrolyte based on a copolymer of tetrafluoro-
ethylene and a perfluorinated sulfonated monomer of gen-
eral structure. At present, the above criteria are most satis-
factorily obeyed by perfluorinated sulfonated membranes
(Nafion, MF-4SK), poly(ether–etherketones) and aromatic
systems of the polybenzimidazole type.
The main manufacturer of ion-exchange membranes
in Russia is Shchekinazot Company [131], which pro-
duces high-quality membranes from domestic resins
according to the classical technology. The wide assort-
ment of membranes includes cation exchangers (MK-40,
MK-40L, MK-41IL), anion exchangers (MA-40,
MA-41I), and bipolar membranes (MB-1E, MB-2I,
MB-3I) [132]. Heterogeneous ion-exchange membranes
MK-40, MA-40, MK-40L, MA-41I, MK-41IL are
intended primarily for use in electrodialysis and electrol-
ysis setups. Membranes of the MF-4SK type are avail-
able from Plastpolimer Company. These membranes are
in good demand and, despite, active competition, have
entered the world market. The transport properties of
domestic membranes are somewhat inferior as compared
to foreign analogs, but proper conditioning allows these
properties to be significantly increased [133]. A compar-
ative analysis of the properties of Russian membranes
and their foreign analogs can be based on [134–137].
Glüsen and Stolten [138] analyzed the level of tech-
nology and potential possibilities of the polymer mem-
branes of various structures for FCs operating on hydro-
gen and methanol, including those working at high-tem-
peratures. It was concluded that Nafion still occupies
leading positions for hydrogen FCs with respect to both
achieved results and potential possibilities.
In recent years, one of the main directions of R&D
is the modification of Nafion and MF-4SK membranes.
Some of the possible approaches are considered in
more detail below. In some cases, the modification of
membranes by inorganic additives makes possible a
significant increase in the productivity (including that
at elevated temperatures and increased humidity)
[139–141].
5.1. Polyelectrolytes
In principle, the properties of a polyelectrolyte can
be predicted to the first approximation proceeding from
the chemical structure of the elementary unit of the base
polymer. For example, the proton conductivity of a
polyelectrolyte is determined by the presence of func-
tional groups (in particular, acid) capable of dissocia-
tion. These can be sulfate, phosphate, and carboxy
groups. Taking into account the ability of these groups
to dissociate, sulfate groups are preferred to phosphate
and the more so to carboxy groups. The organic radical
also influences the proton conductivity of the bound
functional groups. In this respect, a sulfate group
bound to an aromatic moiety will be more effective
that that bound to an aliphatic fragment. The properties
of acid groups bound to fluorinated radicals are close
to those of strong inorganic acids, and these groups are
preferred to the structures based on aromatic com-
pounds. Based on data on the activity of analogs with
close structures containing carboxy or ether groups
[143], it is possible to assume that this difference in
activity reaches two orders of magnitude. Recently, it
was demonstrated [142, 144] that the structure of a
peroxide initiator significantly influences the thermal
stability and working characteristics of the polyelec-
trolytes. It was experimentally confirmed that the use
of perfluorinated peroxide initiators during copoly-
merization provides for the formation of membranes
characterized by improved thermal stability. Recently,
Zhou et al. [145] thoroughly studied the influence of
the nature of terminal groups in the polymer on the
working life of perfluorinated polyelectrolytes of the
Nafion type.
In description of the properties of polyelectrolytes,
the copolymer composition is quantitatively character-
ized by the so-called equivalent mass, that is, by the
molecular mass of a polymer chain fragment per sul-
fonic acid group. Experience showed that the equiva-
lent mass of a copolymer must fall in an interval
between two limiting values. The upper limit is related
to the percolation threshold, that is, to the minimum
content of ionogenic groups at which the membrane
exhibits new (proton) conductivity as a result of the ion
cluster formation. The lower limit is related to deterio-
ration of the mechanical properties of the polymer as a
result of significant swelling and even dissolution of
this polyelectrolyte in water. According to patent [146],
the equivalent mass must fall within 800–1500;
Rusanov et al. [147] established that the optimum prop-
erties of Nafion-type polyelectrolytes are realized for
an equivalent mass of within 1000–1100 [130].
Inhomogeneous chemical composition of the ion-
exchange membranes based on high-molecular-weight
compounds and the flexibility of their polymer chains
determine features of the structure, in particular, its
inhomogeneity [15, 148]. This is manifested by the for-
mation of regions containing predominantly hydropho-
bic fragments of polymer chains (hydrocarbon chains,
aromatic groups, perfluorinated chains). The hydro-
philic functional groups form clusters inside these
regions, the sizes of which depend on the flexibility of
chains. In the literature devoted to membrane systems,
these small formations are frequently called “phases”
[113]. The hydration of membranes leads to some
increase in their dimensions both due to the inclusion of
a certain number of water molecules into the cluster and
NANOTECHNOLOGIES IN RUSSIA Vol. 3 Nos. 11–12 2008
MEMBRANES AND NANOTECHNOLOGIES 673
due to an increase in the number of functional groups
within each cluster [15, 149]. The inhomogeneity of the
membrane structure was postulated by Gierke et al
[149, 150] based on the presence of small-angle reflec-
tions in the X-ray diffraction patterns. It was suggested
that clusters have nearly spherical shapes and their
dimensions even in hydrated membranes amount to
several nanometers. At the same time, the existence of
a through transport via the membrane makes it neces-
sary to refine the model by admitting the presence of
thinner channels that connect clusters into a network
(Fig. 12).
The structural inhomogeneity of membranes was
confirmed by various methods, including SAXS [148,
150–152], Mössbauer spectroscopy [153], nuclear
magnetic resonance [154], porosimetry [155], SANS
[152], electron microscopy and others [156–157].
The model described above is used in most investi-
gations devoted to polyelectrolyte membranes, but
other models have been also formulated. In particular,
according to an alternative theory, membranes are com-
posed of hydrophobic blocks with comb-like walls
[148], the combination of which also forms a pattern
close to that outlined above. This model was theoreti-
cally justified in publications [158–161].
In order to elucidate differences in the internal struc-
ture of membranes of the Nafion (Du Pont) and MF-
4SK (Plastpolimer) types, the samples were studied in
three states [162]: (i) initial, (ii) upon drying at 100°C,
and (iii) after saturation of the dry samples with D2O.
The results showed that Nafion contains predominantly
small scatterers (pores), while the behavior of MF-4SK
exhibits a contribution of greater pores. The number of
small pores in Nafion was evaluated as about seven
times as large as that in MF-4SK. A difference in the
number of large pores is also significant. For a compa-
rable size of 20 nm, the number of such pores in MF-4SK
is much greater than in Nafion. However, the polymer
matrix of MF-4SK was saturated with D2O at a signifi-
cantly slower rate: for 2.5 h, Nafion and MF-4SK
absorbed 17.2 and 12.3% D2O, respectively. This result
indicates that not all large pores in MF-4SK are con-
nected by channels, so that not all of such pores are
filled and involved in the conduction process. Thus,
MF-4SK membranes is characterized by lower “con-
duction,” which is related to features of their fabrication
or storage.
There have been attempts to modify the existing
membranes, which are related to the fact that they do
not meet the growing requirements of science and tech-
nology. This is primarily valid for the thermal stability
and transport characteristics, including the transport
numbers for various ions and water, as well as the
clearly pronounced dependence of conductivity on the
humidity [113, 139, 140, 162]. In addition, the trans-
port of water in membranes is always accompanying
that of anions [15] both because the charge-carrying
ions are subject to hydration and due to the hydrody-
namic effect. This process is important, for example,
when membranes are used in FCs, since the transport of
protons accompanied by the transport of water can lead
to the loss of conductivity in the cathode part of the
membrane and thus deteriorate operation of the whole
element. In this context, investigations devoted to the
modification of membrane materials and the synthesis
of hybrid membranes based on inorganic and high-
molecular-weight components have been actively
developed in recent years [139, 140, 161, 162].
5.2. Modification of Membranes
by Polymer Systems
As was noted above, the hydrophobic character of
the fluorocarbon part of macromolecules and the
hydrophilic character of sulfonic groups lead to the
appearance of nanostructural inhomogeneities repre-
senting clusters with a radius of up to 3 nm, which are
connected by channels with a width of 1–2 nm. A
change (e.g., decrease) in the equivalent mass of a
membrane material may not only lead to an increase in
the proton conductivity of the related FC, but it can also
be accompanied by a decrease in the membrane
strength as a result of increased hydration (water
absorption) and a change in the volume of a swelled
material. For this reason, structural changes capable of
eliminating such drawbacks have been extensively
studied in recent decades. It was suggested, in particu-
lar, to modify the surface of Nafion type membranes by
Hydrophobic
Functional
matrix
group
H2O
H3O+
Fig. 12. Schematic diagram of the structure of a polymer
cation exchanger membrane in the H-form.
674
NANOTECHNOLOGIES IN RUSSIA Vol. 3 Nos. 11–12 2008
VOLKOV et al.
means of radiation-stimulated grafting of polystyrene
[163] or by grafting styrene in supercritical CO2 [164]
followed by sulfonation in concentrated sulfuric acid.
This modification leads to an increase in the glass tran-
sition temperature of the membrane material and to
some decrease in the degree of crystallinity (due to
incorporation of the amorphous fragments of polysty-
rene). Simultaneously, the ion conductivity of the mod-
ified system increases as compared to that in the initial
Nafion type material [164]. According to a patented
solution [165], Nafion type membranes are modified by
a mixture of polystyrene with divinylbenzene or water-
impermeable copolymers such as polyethylene,
poly(hexafluoropropylene), copolymers of hexafluoro-
propylene with propylene or ethylene, copolymers of
vinylidene fluoride with tetrafluoroethylene, etc.
It was also suggested to modify the surface of
Nafion type membranes with polypyrrole by means of
impregnation [166] or electrochemical introduction
[167]. The conducting polypyrrole additive improves
the effective transfer of electrons from the platinum
layer. According to another patent [168], the Nafion
surface is modified by polymerization of the applied
cationic monomers, which leads to the formation of a
protective layer. Various methods were proposed for the
modification of Nafion by polymers or polymerized
resins [169–174].
Interesting results were obtained for perfluorinated
MF-4SK membranes modified with polyaniline. Mem-
brane composites with polyaniline incorporated into a
perfluorinated membrane matrix are very interesting as
coatings for metal electrodes and as polymer composi-
tions possessing both electron and ion conductivity
[175]. In the latter case, the dimensions of polyaniline
particles are not limited by the pore size and can vary
from several nanometers to several dozen nanometers
(depending on the concentration of solutions used in
the synthesis and on the method of membrane prepara-
tion) [176].
The electric conductivity of polyaniline-modified
samples seems to depend on the method of membrane
synthesis. In particular, it was demonstrated [175] that
the contribution of electron component to the total con-
ductivity of composites can reach 60–70%. At the same
time, the proton conductivity of membranes signifi-
cantly decreases as a result of modification. It was
pointed out that the transport number of water in the
composite also decreases, which was explained by a
decrease in the hydrophilicity related to the presence of
polyaniline and by the association of water molecules
due to the formation of hydrogen bonds at the contact
of water clusters in the side segments of matrix tem-
plate and the nitrogen-containing aromatic chains of
polyaniline [177, 178].
In the case of membranes modified with polyaniline
in the bulk, the potential of the transition to a superlim-
iting state increases by more than 2 V as compared to
the initial membrane, which was explained [179] by a
change in the energy state of water in the composite as
a result of interpolyelectrolyte complex formation.
New attempts have been undertaken to create poly-
electrolytes, in which the conventional perfluorinated
sulfonated polymer systems are replaced by some other
commercially available polymers with an aryl frame-
work. These are polystyrenes, polycarbonates, polysul-
fones, poly(ethylene oxides) and some other, which
arte subject to sulfation, sulfonation, or modification
with proper reactants or polymers [180–187].
A very interesting idea was patented [188–191],
according to which hybrid polyelectrolyte membranes
and related electrode ensembles are obtained by form-
ing a proton-conducting polymer system via copoly-
merization of commercially available, cheap hydro-
philic and hydrophobic monomers containing iono-
genic groups (e.g., SO3H). These copolymers are
capable of containing water and transporting ions via
the structure, while exhibiting proton conductivity at
relatively low temperatures. The accessibility and low
cost of such polyelectrolytes allows the fabrication of a
complex of cells and makes possible extraction of the
expensive platinum catalyst by merely burning the cells
after use of this complex. A close approach [192] is
based on the preparation of polymer systems with
improved characteristics, where a polymer matrix is
synthesized using interpenetrating networks.
5.3. Modification of Membranes
by Inorganic Components
The approach to improvement of the working char-
acteristics of membranes by modifying them with inor-
ganic components is related to the need for increasing
the water-retention capacity of membranes and pre-
venting their dehydration. This is achieved by introduc-
ing a hydrophilic inorganic component that is capable
of chemical and/or coordination binding of water.
Using the introduction of inorganic components, it is
possible to increase the activity of cation- and anion-
exchange groups [139, 184, 193, 194]. Among various
types of inorganic components that can be used for this
purpose, frequently chosen are the oxides of polyvalent
elements (silicon, aluminum, zirconium).
The modification of membrane materials with inor-
ganic nanoparticles can be, in principle, performed in
two ways: first, by casting membranes from solutions
containing finely dispersed additives (e.g., SO2) [195]
and second, by synthesizing nanoparticles immediately
in the membrane matrix [140, 141]. In Russia,
researchers mostly consider the latter approach as more
promising and use it most frequently.
Membranes offer unique matrices for the synthesis
of nanoparticles, since nanopores can effectively col-
lect one of the reactants, for example, cations in the
case of cation-exchange membranes. Then, these nan-
opores can be the favorable sites of nanoparticle syn-
thesis, where the amount of reactants is controlled by
NANOTECHNOLOGIES IN RUSSIA Vol. 3 Nos. 11–12 2008
MEMBRANES AND NANOTECHNOLOGIES 675
these nanodimensional reactors, while the membrane
walls can effectively isolate the synthesized particles
from each other and reduce the surface tension, thus
ensuring thermodynamic stability of these particles. A
similar approach was used for the synthesis of materials
containing nanoparticles of silicon and zirconium
oxides, and acid zirconium phosphate [141, 196–198].
During this synthesis, particles with dimensions of sev-
eral nanometers were formed, which corresponded to
the pore size in the initial matrix (Fig. 13). This was
confirmed by the data of electron microscopy, X-ray
diffraction, electron microprobe, and high-resolution
NMR techniques.
In many cases, the introduction of nanoparticles led
to an increase in the proton conductivity of the modified
membrane as compared to the initial material. This was
achieved in the case of poly(arylene ether ketones) with
various degrees of sulfation, which were modified by
nanoparticles of acid zirconium phosphate [199].
A similar method was used for the synthesis of
metal nanoparticles in ion-exchange membrane matri-
ces [200–203]. The metal particles formed in MF-4SK
membranes had dimensions within 1–5 nm and the
overall metal concentration reached 1022 metal atoms
per gram of dry membrane [202, 203]. Owing to the
small dimensions of these metal particles, the modified
membranes acquire special properties. In particular,
membranes containing transition metal particles exhib-
ited superparamagnetic properties [202, 203]. Compos-
ite membranes with inclusions of low-active metals
readily sorbed oxygen [204, 205], thus preventing its
transfer via the membrane material. The modification
of membranes with carbon nanoparticles [206] and car-
bon nanotubes [207] allowed the mechanism of con-
ductivity to be changed and their permeability for vari-
ous components to be controlled.
The possibility of improving the characteristics of
Nafion type membranes by introducing silicon dioxide
was studied in [208–210]. Almost simultaneously with
these investigations of the effect of SiO2 on the proper-
ties of Nafion type membranes, the intrinsic proton
conductivity of a series of heteropolyacids, including
phosphotungstic acid (PTA) and silicotungstic acid
(STA), was studied in [211–214]. It was established
that heteropolyacids exhibit high proton conductivity at
room temperature [211] and can be immobilized in Nafion
[212], the resulting compositions also possessing a good
room-temperature proton conductivity (>10–2 S/cm).
However, since heteropolyacids are highly soluble in
water, they are usually first immobilized in silica gel
[175]. Using PTA immobilized in silica gel in combina-
tion with poly(ethylene oxide), it is possible to obtain a
hybrid proton-conducting system with a conductivity
of 10–3 – 10–2 S/cm in the range from room temperature
up to ~140°C [214]. Arico et al [215] replaced in this
system poly(ethylene oxide) by Nafion and obtained a
Nafion–SiO2–PTA composition, which was success-
fully used in an FC operating on methanol at 145°C.
Analogous systems were obtained [195] by means of
repeated casting of Nafion solution upon preliminarily
ultrasonic mixing with SiO2 and PTA.
In concluding this analysis, it should be noted that
the properties of ion-exchange membranes can also be
controlled by other methods. In particular, it is possible
to employ external fields [216] or plasma-initiated graft
polymerization [217] for controlling the structure of
membrane channels. In addition, it was demonstrated
[218–220] that another promising approach is based on
mechanical modification (profiling) of the membrane
surface, which leads to an increase in the hydration and
conductivity.
Investigations directed to the creation of new and
the improvement of existing membrane systems are
continued. For example, Su et al. [221] proposed using
sulfated poly(phthalazanone ether ketones) filled with
sulfonated SiO2–SO3H particles. Ishikawa et al. [222]
obtained polyblockcopolymers of sulfonated poly(ether
ketone) containing benzophenone and methyl frag-
ments capable of cross-linking under the action of UV
radiation. Ding et al. [223] reported on cross-linkable
sulfonated poly(arylene ethers) soluble in organic sol-
vents, which are capable of forming flexible, transpar-
ent membranes on casting from solutions.
Another new but rapidly developing direction of
research is related to the synthesis of hybrid polymer
membranes. Honma et al. [224] proposed a new class of
amphiphilic organic–inorganic hybrid membranes,
which are obtained by a sol–gel method proceeding
from bridged polysiloxanes with acid (PTA) fragments.
It was also suggested [187] to use high-efficiency
proton-conducting membranes representing hybrid
polymer systems based on poly(vinyl alcohol) (PVA)
etherified by phenolsulfonic acids. Kato et al. [225]
reported on hybrid polymer membranes, which were
also prepared by the sol–gel method at various ratios of
3-glicydyloxypropylmethoxysilane and phosphonace-
10 nm
Fig. 13. Electron micrograph of an MF-4SK membrane
with hydrated silicon oxide synthesized inside.
676
NANOTECHNOLOGIES IN RUSSIA Vol. 3 Nos. 11–12 2008
VOLKOV et al.
tic acid, and proved to possess high thermal stability,
mechanical strength, optical transparency, and electric
conductivity. The method of synthesis and the proper-
ties of organic–inorganic hybrid polyelectrolyte mem-
branes, obtained by radical copolymerization of tri-
methoxysilylmethylstyrene and phenylvinylphospho-
nic acid, were described in [226].
A very interesting method was proposed for the
preparation of hybrid polyelectrolyte membranes and
related electrode ensembles [188–191]. This approach
is based on formation of a proton-conducting polymer
system by copolymerization of readily available and
cheap hydrophilic and hydrophobic monomers contain-
ing ionogenic groups (e.g., SO3H)., An analogous prob-
lem was solved in [144, 192], where accessible mono-
mers were used to obtain proton-conducting membrane
systems with satisfactory strength, water content, and
conductivity [162].
The above results give an incomplete list of achieve-
ments in the field of creation and characterization of
ion-exchange membranes. Anyhow, these data indicate
that Nafion type polyelectrolyte membrane still remain
competitive in the entire series of existing proton-con-
ducting materials.
However, as far as the methanol fuel cells are con-
sidered, the potential possibilities of Nafion type mem-
branes (despite still high R&D level) are inferior to
those of some polycondensation membranes. Polycon-
densation membranes, especially those based on poly-
benzimidazoles are more promising materials for high-
temperature FCs [147, 227–229]. Some other investiga-
tions related to the aforementioned membranes pos-
sessing high-conductivity with respect to lithium and
hydrogen were reported in [141, 185–187, 196–199].
Highly promising materials for the FCs operating at
elevated temperatures (100–500°C) are offered by inor-
ganic membranes based on acid sulfates and phos-
phates [184, 194, 230, 231].
6. TRANSPORT ASYMMETRY
6.1. Electrolyte Solutions
Despite the long history of investigations into trans-
port processes in artificial membranes, even laboratory
experiments are far from approaching the characteris-
tics inherent in cell membranes. The transport of ions
and molecules through cell membranes is characterized
by high efficiency, selectivity, and the absence of
energy consumption for a change in the ambient
medium composition [232]. A mechanism ensuring the
high efficiency of real transport processes in natural
membranes is still unclear. An even more mysterious is
the phenomenon of transport asymmetry, according to
which the permeability of a membrane significantly
changes in response to alteration of the flow direction
or the component concentration gradient.
In the nature, there are many systems where the
transfer of components in one direction proceeds faster
than in opposite directions. A well-known example is
the transport of alkali metal ions through cell mem-
branes, where the motion of ions can even proceed
against their concentration gradient [233]. The asym-
metry of transfer through membranes of mammal cells
was repeatedly demonstrated (see, e.g., [234–236]).
These membranes have a complicated structure, with
coarse pores emerging on one side and fine pores, on
the opposite side. However, the observed asymmetry
cannot be explained proceeding from this difference in
the structure. Tekle et al. [237] demonstrated that the
predominant direction of ion motion can be determined
both by the orientation of a membrane and by the nature
of transported ions. For example, at a low salt concen-
tration, Ca2+ and ethidium bromide ions are predomi-
nantly transferred in one direction, while the positive
ions of propidium and ethiduum homodimer, in the
other. It should be also noted that this distinction is not
observed under conditions of a high salt concentration,
where all indicated ions are transported in the same pre-
ferred direction.
Asymmetry of the transport characteristics of artifi-
cial membranes was not seriously treated by scientists,
which can be related to two factors. First, all theoretical
models developed until recently predicted the symme-
try of transfer processes. Second, the presence of an
asymmetry requires thorough analysis on the condi-
tions of its manifestation, otherwise it would be possi-
ble to build a perpetuum mobile based on asymmetric
membranes. Apparently, these very circumstances
explain the fact that the original experiments [238, 239]
demonstrating asymmetric diffusion permeability did
not attract much attention. In these experiments, the
asymmetric effects were observed for ion-exchange
membranes of the MK-40 and MK-41 types modified
by tetrabutylammonium (TBA) and sodium dodecyl-
sulfate (SDS), respectively. This modification led in
fact to two-layer membranes exhibiting sharply asym-
metric transport properties. The coefficient of asymme-
try Q (defined as the ratio of permeabilities in the oppo-
site directions) exhibited extremal dependence on the salt
concentration difference across the membrane (Fig. 14).
Note that this large asymmetry can by no means be
explained by experimental errors.
From the present-day notions, a qualitative explana-
tion of the mechanism of observed asymmetry that was
given in [238] seems to be quite reasonable. The only
“disadvantage” of that experiment was the dynamic
two-layer character of the membrane, since a redistri-
bution and “washing out” of the modifying agent could
take place in the course of ion transfer. That experiment
did not allow a clear and simple model system possess-
ing the transport asymmetry to be constructed. For this
reason, the original communications were not properly
assessed.
The effects of asymmetry in artificial membranes
with clearly defined structures admitting theoretical
description were rediscovered only quite recently, and
NANOTECHNOLOGIES IN RUSSIA Vol. 3 Nos. 11–12 2008
MEMBRANES AND NANOTECHNOLOGIES 677
this gave rise to an outburst of new experiments aimed
at the search and explanation of the observed asymme-
try effects. the main feature of these membranes is the
nanometer size of pores. It should be noted that the arti-
ficial membranes exhibiting asymmetric transport were
created using a technology proposed by Russian scien-
tists [240, 241], representing track membranes obtained
upon the high-energy ion bombardment of a film, fol-
lowed by controlled etching until obtaining a preset
pore size (see Section 4).
Using a special method [241] developed for the
track membrane formation, Siwy and Fulinski [242]
obtained a PET membrane with a single nanometer-
sized pore of a cone shape. The pore had the shape of a
truncated cone with a base diameter of 500 nm, the
cross-sectional diameter of 2 nm, and a length of 12 μm.
The membrane was cation-selective, since it is known
that PET membranes acquire a negative charge as a result
of the dissociation of -COOH groups. This implies that
the transport of positively charged ions (K+) toward their
lower concentration proceeds faster than the transport of
negatively charge ions (Cl–). As a result, a current passes
in the system even in the absence of an external potential
difference applied to the membrane (naturally, in the
presence of a concentration gradient of the salt). The
asymmetry of ion transport in such membranes was
demonstrated in [241, 243]. The proposed model of ion
motion in the electric field with allowance for the ion–
wall interaction showed evidence for the asymmetry of
the average ion flow in the periodic field and a strong
dependence of the asymmetry coefficient Q on the ratio
of diameters of the cone base and cross section.
Detailed investigation of the ion transport process in
this system was performed in [244] using PET mem-
branes with a thickness of L = 12 μm and cone-shaped
pores with the narrow neck radius of 1.5-18 nm and a
base radius of 300–370 nm. The ion transport was stud-
ied in a system with KCl solutions of different concen-
tration (0.1 and 1.0 M) on the opposite sides of the
membrane. The current was measured in the absence of
a potential drop across the membrane and in the pres-
ence of a bias voltage. The ion transfer asymmetry was
monitored by measuring currents in the absence of an
external potential difference for the opposite directions
of the ion concentration gradient. The asymmetry was
calculated as the ratio of currents passing from the cone
neck to base and in the reverse direction (in fact, the
permeability ratio Q). The corresponding states of the
system are illustrated in Fig 15. The membrane
becomes selective when (as was noted above) the elec-
tric double layers (EDLs) formed at the channel surface
begin to play a significant role in the ion transport, that
is, when the Debye radius becomes comparable with
the pore size.
The effect of asymmetry is due to a change in the
ratio of the EDL and the pore sizes in various configu-
rations of the system. When the solution concentration
is higher at the pore base, a decrease in the EDL thick-
ness proceeds over a much longer distance than in the
opposite case (see Fig. 15).
The asymmetry effect was also observed [198] on
MF-4SK membranes inhomogeneously modified with
hydrated zirconium oxide nanoparticles, where a differ-
ence in the measured permeability coefficients was
about 40% and depended on the solution concentration.
Apparently, this system features a different mechanism
of transport asymmetry, which is related to the mutual
influence of nanoparticles (mediated by the polymer
matrix) on the ion transport. Detailed elucidation of the
mechanism of asymmetry ion this case requires addi-
tional investigations.
For theoretical explanation of the transport asym-
metry in the aforementioned system of cone-shaped
pores, it is not necessary to attract any new theoretical
concepts in addition to those developed in the classical
works of Derjaguin [17, 245–248] on the membrane
transport with allowance for the contribution of surface
forces. Only the application of this theory to cone-
shaped channels of nanometer dimensions brought the
novelty. Detailed calculations of the ion current in such
pores were performed in [249]. In the absence of an
external potential difference, the ion flux through the
channel [mol/s] can be expressed as follows:
(6.1)
where subscript i refers to ion species, Zi is the ion
charge, Di is the diffusion coefficient, z is the coordinate
along the channel axis, h(z) id the pore radius at the
point with coordinate z, and Veff(z) is the effective
potential of the ion–pore wall interaction.
As was noted above, the effective potential distribu-
tion depend on the concentration field in the pore.
Ji
cit cib
–
dz Zie
kBT
--------- Veff z()exp / Dih2z()()
0
L
∫
----------------------------------------------------------------------------,=
1.0 0.2 0.4
1.5
2.0
Q
Δc, g-eq./l
Fig. 14. Plot of the asymmetry coefficient Q versus electro-
lyte concentration difference ΔC.
678
NANOTECHNOLOGIES IN RUSSIA Vol. 3 Nos. 11–12 2008
VOLKOV et al.
Therefore, the ion fluxes also depend on this field,
which implies that the asymmetry effect takes place.
The nature of asymmetry in this system is quite clear:
the concentration fields in the pore and the EDL config-
urations are significantly different for ion transfer in the
opposite directions. As a result, ions differently interact
with the pore walls and the effective transfer coeffi-
cients are also different. There is another factor that can
also influence the ion transfer: the diffusion coefficient
can also depend on the ion concentration. In this case,
the difference in the concentration fields for the ion
transfer in the opposite directions will also leads to a
difference in the diffusion coefficients.
Later, the effect of transport asymmetry was theoret-
ically also explained in [250], where a discrete model
of the ion transport was considered. According to this
model, ions migrate via active centers characterized by
various binding energies. There is a quite clear analogy
between this model and that developed in [249]: in both
cases, the ion–wall interaction is inhomogeneous along
the channel axis.
Another important property of the models predict-
ing the asymmetric ion transport is their substantially
nonlinear character. It should be noted that, as the elec-
trolyte concentrations on the opposite sides are leveled
and/or the system weakly deviated from equilibrium,
no asymmetry in the ion transfer coefficients is
observed in the system. This is a very important cir-
cumstance, which is sometimes not taken into account
in constructing the models of asymmetric transport.
Restoration of the symmetry upon weak deviation of
the system from equilibrium is a necessary prerequi-
site; otherwise, such asymmetric membranes could be
used to build a perpetuum mobile.
Foreign investigations devoted to the problem of
asymmetric transport do not properly mention the pio-
neering experimental investigations of Russian research-
ers and even rediscover some theoretical notions
reported previously. For example, it was claimed as a
new finding of the model in [249] (as well as in [251])
that the attraction of ions (and/or other solution compo-
nents) to the channel walls accelerates the ion transport
through a membrane. However, this aspect of the mem-
brane transport is by no means new. The acceleration or
deceleration of the ion transport via pores in the presence
of such interactions was pointed out long ago [17] and
constitutes a basis of the theory of membrane processes
developed by the scientific school of Derjaguin.
Another possible variant of manifestation of the
asymmetry effect was described in [252], where it was
demonstrated that bilayer membranes can possess
asymmetric transport characteristics, but only in the
case of active membranes, in which the parameters (in
particular, potential difference on the membrane sur-
face exposed to the flow) depend on the electrolyte con-
centration. Such an asymmetry is, in a certain sense,
evident because it assumes a change in the membrane
characteristics depending on the orientation with
respect to the flow. Variation of the membrane charac-
teristics is also explicitly implied in the other models
considered above, where it can be quite clearly
explained by the physical influence of a solution on the
boundary layers in the membrane channels.
6.2. Gas Transport via Nanosized Channels
It is interesting to note that the track membranes
with cone-shaped pores exhibit the effect of asymmet-
ric transport not only for electrolytes, but for gases as
well. This was demonstrated [254, 255] for the track
membranes with nanosized cone-shaped pores, in
which the hydrogen and carbon dioxide permeabilities
varied by a factor of almost 2 upon reversal of the mem-
brane orientation relative to the flow. The transfer rate
was greater when the gas was supplied from the side of
a narrower pore neck. Apparently, the asymmetry
observed in this system is caused directly by nanosized
pores of the track membrane.
A significant anisotropy was also observed in multi-
layer metalloceramic membranes [81, 256, 257]. Such
a membrane comprised a support, representing a
K+Cl–
cb = 0.1 m
(a)
K+Cl–
cb = 1 m
(b)
ct = 1 m ct = 0.1 m
–
Fig. 15. Schematic diagram showing two states of the system corresponding to the transfer asymmetry (dashed curve indicate a
conditional boundary of the electric double layer).
NANOTECHNOLOGIES IN RUSSIA Vol. 3 Nos. 11–12 2008
MEMBRANES AND NANOTECHNOLOGIES 679
porous stainless steel (SS16L) plate, and a thin layer of
rutile deposited onto this support. The pore size in this
membrane was on the order of several microns. A cata-
lyst (Cu0.03Ti0.985O2 ± δ) deposited on the internal surface
of pores had a calculated thickness of 0.10–0.18 nm. The
membranes were modified by depositing (from the
ceramic side) a thin layer of P0.03Ti0.97O2, which had
2-nm pores. Investigation of the hydrogen permeability
of the modified membrane showed evidence of a strong
asymmetry, whereby permeability for the gas flow from
the phosphate–titanium coating side was more than
twice that for the gas supplied from the metal support
side (Fig. 16). The anisotropic permeability was also
observed for membranes deprived of the phosphate–
titanium coating, but in this case the difference was
about 10%. The membrane productivity also depended
on the gas flow direction.
The observed effect is of considerable significance,
especially with a view to the membrane catalysis that is
extensively developed in recent years [10, 11, 258, 259],
where membranes with nanosized pores are typically
employed. The passage to nanosized pores is related to
the fact that (i) this leads to a sharp increase in the sur-
face area and (ii) the typical catalysts also appear as
nanosized particles, which leads eventually to an
increase in the efficiency of catalytic processes.
It should be noted that an insignificant (within 10%)
anisotropy of the flow was also observed previously
[260] for multilayer membranes supported on α-Al2O3.
Several layers of this material were used to form mem-
brane layers with pore sizes on the order of dozens and
hundreds of nanometers, while the layer with nanome-
ter pores was made of γ-Al2O3 or SiO2 (or both).
The small anisotropy of gas transport in the direction
of flow can be explained as follows. A gas flow through
a porous body can be expressed as follows [261]:
(6.2)
where ke and be are constant coefficients characterizing
the porous body and η is the gas viscosity. This formula
shows that the flow depends not only on the parameters
of the porous body, but also on the average gas pressure p.
Since the gas transfer via the porous layers in various
directions proceeds at different average pressures, the
anisotropy of permeability can be manifested. It was
shown [260] that, within the framework of this model,
experimental data can be described for membranes with
a permeability asymmetry not exceeding 10–15%.
Apparently, the same factor can explain the anisot-
ropy of productivity and selectivity observed [262] in
the Fischer–Tropsch process on catalytic homogeneous
membranes of complicated geometry. At least, an anal-
ysis performed on the basis of Eq. (6.2) [263] for the
gas transport in various membranes showed that the
productivity of membranes significantly depends on the
geometry and flow direction. However, as was noted
above, the dependence of the gas flow on the average
j1
kBT
--------- 4
3
---ke
8kBT
πm
------------ be
η
---- p+
⎝⎠
⎛⎞
∇p,–=
pressure can explain only a weak asymmetry. A stron-
ger anisotropy of the gas transport cannot be described
within the framework of this model.
There were attempts [264] at explaining the asym-
metry of gas flow in membranes based on the purely
geometric factors, in particular, the existence of a sharp
transient zone between layers with pores of different
diameters and the anisotropy of the angular distribution
of the gas molecule velocities in the flow. As a result of
this anisotropy, the average velocity of molecules
reflected from the boundary between membrane layers
turns out to be greater than that of the molecules freely
crossing the boundary. This difference leads, on the
whole, to retardation of the gas transfer from coarse to
fine pores. However, this model contains a number of
free parameters, which were used by the authors to fit
the theory to experiment. Therefore, the correctness of
this theory can only be judged after independent deter-
mination of these parameters.
The results of numerical simulation [265] show that,
with allowance for the interaction between molecules
and their interaction with the pore walls, it is possible
to reveal the transfer asymmetry in the case of pores
with variable width (the asymmetric membranes in
[265] were modeled by the layers of balls of various
diameters, which implied the natural decrease in the
pore diameter). Thus, in constructing analytical mod-
els, it is necessary to take into account the relation
between the diameter of molecules and the pore size
and take into consideration the molecule–pore wall
interaction.
The authors believe that there are other factors that
must be also taken into account in explaining the asym-
metry of gas transport via nanoporous membranes.
These factors, which are related to the passage to chan-
nels on the nanometer scale, have been originally
pointed out for zeolites—typical representatives of
nanodimensional systems, in which pore diameters are
1.5
3.0 1
2
2
1
(a) (b)
J
Fig. 16. Hydrogen permeability of multilayer metallocer-
amic membranes (a) with and (b) without a phosphate–tita-
nium coating for the flow from the side of (1) the ceramic
layer and (2) the metal support.
680
NANOTECHNOLOGIES IN RUSSIA Vol. 3 Nos. 11–12 2008
VOLKOV et al.
comparable with the diameters of molecules. Zeolites
are used in various fields as catalyst carriers, adsor-
bents, and separation membranes [266]. Below we indi-
cate the main features of transfer in ultrathin channels,
which have been considered in the literature. We
believe that, without taking these factors into consider-
ation, it is hardly possible to explain the asymmetry of
transport in nanoporous membranes.
Surface forces. Surface forces, which are responsi-
ble for the adsorption of gases, are also operative in
capillary channels. Since the range of these forces
extends over dozens of nanometers, they have to be
taken into account in the analysis of transport in chan-
nels with the radii comparable with the range of surface
forces. Allowance for the affect of the long-range part
of the surface forces on the gas transport in nanosized
capillaries was originally made in [267, 268], where it
was shown that the surface forces significantly influ-
ence the gas flow velocity determined by the tempera-
ture and pressure gradients. For the flows caused by the
temperature gradients, the gas transfer rate can exhibit
a severalfold growth.
Surface diffusion. The interaction of gas molecules
with the surface of capillary walls determines to a con-
siderable extent the surface diffusion transport of gas
molecules. In an analysis of gas flow in nanosized cap-
illaries, the surface diffusion transport becomes compa-
rable to that via the gas phase. The role of the surface dif-
fusion was considered in sufficient detail in [269–272],
where it was demonstrated that correct description of
the transfer of molecules requires using the kinetic
equation for the distribution function of adsorbed mol-
ecules, which was solved by Krylov et al. [270].
Calculations showed that surface diffusion coeffi-
cient Dc strongly depends on the degree of surface cov-
erage θ. To within a good approximation, this depen-
dence is described by the following relation:
(6.3)
It should be noted that the growth of the diffusion coef-
ficient predicted by this equation for high degrees of
surface coverage was experimentally confirmed for
zeolites.
Sieve effect. In channels with diameters close to the
size of molecules (which is the case in zeolites), the
motion of atoms is almost one-dimensional and quan-
tum effects can take place not only at low temperatures
(where under-barrier tunneling of atoms should be
taken into account), but also at room temperature (as a
result of quantization of the radial motion of atoms)
[274]. The energy of the quantized radial motion is
given by the following simple relation:
(6.4)
where is the Planck constant, σ is the diameter of the
hard molecular core, ands γn are coefficients on the
order of unity. Since the value of 2R–σ can be rather small,
DcD01
1θ–
------------.=
Ern 2γn
2"2m1– 2Rσ–()
2,n012…,,,==
the zero-point energy Er0 and the distance Er1–Er0
between levels can be very large. Indeed, for 2R–σ =
0.1 nm, the distance between levels is (E1–E0)/kB = 428 K.
At temperatures significantly lower than this, the
motion of atoms is actually one-dimensional, since the
radial motion is completely frozen.
The character of molecular transport in the channels
also exhibits a radical change when the value of Er0
exceeds the depth V of the potential well determined by
the surface forces. In this case, the adsorption of mole-
cules becomes impossible and, in addition, a kind of
potential barrier appears for the entrance of molecules
into the channel. The height of this barrier increases
with the size of molecules, that is, the channel acts as a
sieve, although formally (judging from the channel
geometry) the molecules can enter inside. Even a para-
doxical situation can take place where greater mole-
cules can enter the channel while smaller molecules can
not. For example, V/kB ≈ 200 K, σ ≈ 0.3 nm for helium
and V/kB ≈ 1000 K, σ ≈ 0.32 nm for neon, which
implies that 0.4-nm-diameter channels are inaccessible
for helium atoms, but accessible for greater neon
atoms.
Quasi-one-dimensional flow in nanosized channels.
As was noted above, the channel diameters in systems
employed in practice can be smaller than a doubled
diameter of molecules. In this case, one molecule can
block the motion of another and the transport becomes
one-dimensional. Channels of this kind are also present
in zeolites. For example, in a widely used zeolite ZSM-5,
the ratio of the channel diameter to that of argon atom
is 1.6, so that the motion of inert gas atoms in the chan-
nels of this zeolite can be considered one-dimensional.
The comparable diameters of atoms (molecules) and
channels also determine the high selectivity of mem-
branes based on zeolites, which increases the interest in
such systems.
The transport in quasi-one-dimensional channels
has certain specific features. It was shown [275] that,
for weakly-adsorbed gases, the flow grows in propor-
tion to the pressure difference, while in the case of
strong adsorption the flow quite rapidly attains a con-
stant level. Jobie et al. [276] observed deviations from
the Einstein law determining mean square displace-
ment as a function of the time. As was noted above, the
diffusion coefficient exhibited a sharp (more than ten-
fold) growth with increasing channel surface coverage
by adsorbed molecules [277]. A rather unexpected
behavior was observed for the selectivity of zeolite
membranes [275, 277–279] defined as
(6.5)
where subscripts i and f refer to the initial (input) and
final (output) concentrations, which exhibited strong
variations as a function of the ratio of component concen-
trations in the gas mixture. At a temperature of 300 K, the
selectivity reached a maximum value of ϕ = 380 for a
ϕc
1c–
-----------
⎝⎠
⎛⎞
i
/c
1c–
-----------
⎝⎠
⎛⎞
f
,=
NANOTECHNOLOGIES IN RUSSIA Vol. 3 Nos. 11–12 2008
MEMBRANES AND NANOTECHNOLOGIES 681
mixture of n-C4H10 and CH4 with a component ratio of
5 : 95. Upon an increase in the temperature to 623 K for
a 50 : 50 mixture, the flow of methane becomes 1.5 times
that of butane. Dramatic changes in the selectivity
depending on the gas mixture composition and temper-
ature were also observed for some other gas mixtures.
The gas diffusion “anomalies” observed in experi-
ments were described in [280, 281], where it was
pointed out that an important role in the case of single-
component systems in one-dimensional channels is
played by molecular clusters, since the excitation trans-
fer via such clusters has a barrier-less character. The
presence of clusters leads to an increase in the diffusion
coefficient with the gas density. For this reason, the
attraction between molecules significantly influences
the transport in quasi-one-dimensional channels. This
is illustrated in Fig. 17, which shows a comparison of
the experimental and theoretical dependences of the
relative flux on the relative temperature. As can be seen,
the relative flux exhibits a complicated behavior, but the
theory well agrees with the experimental data. The
observed behavior can be explained as follows. At low
temperatures, the thermal motion is “frozen” and the
flux vanishes because molecules cannot surmount
potential barriers in the zeolite crystal. As the tempera-
ture is increased, the thermal motion is activated, but
the degree of surface coverage is still high, the transfer
is mediated by collective effects, and the excitation is
transferred via clusters at a small activation energy. The
further growth in the temperature leads to the disap-
pearance of clusters and a decrease in the diffusion
coefficient as a result of decreasing coverage and
appearing;potential barriers for the diffusing species.
Finally, at still higher temperatures, the diffusion coef-
ficient increases again due to the Arrhenius factor.
7. CONCLUSIONS
The above analysis shows that the membrane tech-
nologies are presently among the highest in demand,
which is confirmed both by the variety of membranes
used in practice and the broad spectrum of applications
in the most important fields of human activities (power
engineering, ecology, medicine, chemistry, the oil and
gas industry, water desalination and conditioning, etc.).
This has led to a revival of interest in the investigation
of membranes and the development of new-generation
membrane systems. Obviously, these new membranes
can be created only within nanotechnologies that are
capable of increasing the efficiency of membrane oper-
ation by orders of magnitude. Switching to the nanos-
cale requires new ideas and implies the development of
a theoretical and experimental background of research.
As can be seen from our examples, nanoporous mem-
branes feature new effects that have not yet been
explained within the notions existing models. The
authors hope that the present review will provide addi-
tional stimulus toward expanding the scope of research
and the application of nanoscience in membrane sci-
ence and technology.
ACKNOWLEDGMENTS
The authors gratefully acknowledge the help in prepar-
ing this publication and express their thanks to V.V. Berez-
kin, V.M. Vorotyntsev, G.V. Grigor’ev, A.V. Desyatov,
V.G. Dzyubenko, V.P. Dubyaga, V.I. Zabolotskii,
N.N. Kazantseva, P.N. Martynov, V.V Nikonenko,
E.G. Novitskii, V.S. Khotimskii, and Yu.P. Yampol’skii.
This work was supported by the agency Rosnauka
(grants nos. 02.513.11.3358, 02.523.12.3022) and the
Russian Foundation for Basic Research.
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