Liquid crystal phase behavior of sterically-stabilized goethite

Abstract

The liquid crystalline phase behavior of sterically-stabilized goethite particles in toluene was studied using small-angle X-ray scattering. The results were compared with those from charged particles in water, with and without magnetic field: similarly rich phase behavior was found. Furthermore, the special magnetic properties were retained after coating the particles with amino-functionalized polyisobutylene chains. A remarkable difference between the aqueous and toluene samples is the latter's tendency to form gels. Smaller domains of the different liquid crystalline phases were observed and the columnar phase does not fully develop, furthermore a higher field is needed to align the full sample.

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Liquid crystal phase behavior of sterically-stabilized goethite
Esther van den Pol, Andrei V. Petukhov, Dominique M.E. Thies-Weesie, Dmytro V. Byelov, Gert J. Vroege
⇑
Van ’t Hoff Laboratory for Physical and Colloid Chemistry, Debye Institute for Nanomaterials Science, Utrecht University, Padualaan 8, 3584 CH Utrecht, The Netherlands
article info
Article history:
Received 11 June 2010
Accepted 2 September 2010
Available online 9 September 2010
Keywords:
Liquid crystals
Steric stabilization
Magnetic field
Colloids
Small-angle X-ray scattering
Gelation
abstract
The liquid crystalline phase behavior of sterically-stabilized goethite particles in toluene was studied
using small-angle X-ray scattering. The results were compared with those from charged particles in
water, with and without magnetic field: similarly rich phase behavior was found. Furthermore, the spe-
cial magnetic properties were retained after coating the particles with amino-functionalized polyisobu-
tylene chains. A remarkable difference between the aqueous and toluene samples is the latter’s tendency
to form gels. Smaller domains of the different liquid crystalline phases were observed and the columnar
phase does not fully develop, furthermore a higher field is needed to align the full sample.
Ó2010 Elsevier Inc. All rights reserved.
1. Introduction
Aqueous dispersions of charged goethite particles show rich li-
quid crystalline phase behavior [1–8]. Nematic, biaxial nematic,
smectic Aand columnar phases were found in these dispersions.
Furthermore, goethite particles have extraordinary magnetic prop-
erties [9]. They possess a permanent magnetic moment along their
long axis, but also an induced magnetic moment which is predom-
inantly along the shortest particle axis. Therefore, the particles
align parallel to a low magnetic field and perpendicular to a high
magnetic field. Those properties make goethite an interesting
model system.
In theoretical work, one usually considers purely entropic sys-
tems of hard particles. Charged particles have a double layer, that
depends on the ionic strength, which influences the effective par-
ticle dimensions and gives a softer interaction compared to the
hard particle interaction. The effect of the electrostatic repulsion
was clearly shown by changing the ionic strength of, for example,
TMV and gibbsite dispersions [10,11]. To get a better comparison
with theory, particles with a nearly hard particle interaction can
be obtained by sterically stabilizing them with a polymer layer
and disperse them in an organic solvent.
Sterically stabilized dispersions were obtained by grafting the
particles with amino-functionalized polyisobutylene (PIB) chains.
This has already been successful for several inorganic liquid crys-
talline particles, including goethite [12–15]. Before, this was usu-
ally done by an elaborate method which involves a gradual
solvent substitution upon addition of the stabilizing polymer
[12,16,17]. Here, we use an alternative, recently described method
in which the particles are freeze-dried in the presence of the stabi-
lizer [18]. For most systems used before the index of refraction (1.6
for gibbsite and boehmite and 1.5 for sepiolite) is close to that of
toluene (1.5), the solvent used to disperse the particles in. For goe-
thite the index of refraction is much larger, about 2.3, so the van
der Waals interaction will be stronger in this system.
In this article it is determined if the polymer coated particles form
an alternative model system compared to the charged particles, on
which much work has already been done, and if they have some
advantages. Therefore, the phase behavior of different charged and
polymer coated systems is compared. Also, the magnetic properties
of these systems were tested.
2. Materials and methods
2.1. Synthesis
The synthesis of the g35 dispersion was described before in Ref.
[15]. Iron nitrate was hydrolized at high pH according to Lemaire
et al. [19]. 1 M NaOH (Acros, reagent ACS, pellets, 97+%) was added
dropwise, under stirring, to a 0.1 M iron nitrate (Fisher Scientific,
p.a.) solution until a pH of 11–12 was reached. The precipitate was
aged for 9 days at room temperature. g38 was made using the
same method as g35, but with a slightly different iron nitrate con-
centration (0.08 M for g38 and 0.11 M for g35). The g13 and g14
systems were made in a similar way as the g17 system in the same
reference [15]. They were obtained by a slightly adjusted forced
hydrolysis method described by Krehula et al. [20]. Hundred and
0021-9797/$ - see front matter Ó2010 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcis.2010.09.002
⇑
Corresponding author.
E-mail address: g.j.vroege@uu.nl (G.J. Vroege).
Journal of Colloid and Interface Science 352 (2010) 354–358
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twenty-five millilitre of 25% tetramethyl ammonium hydroxide
(TMAH, Aldrich, 25% w/w in water) was added, under vigorous stir-
ring, to 350 mL of 0.16 M iron nitrate. The solution was aged for 5
days at 85 °C. The obtained dispersions were centrifuged and stabi-
lized in water of pH 3 as described in Ref. [15].
Sterically stabilized dispersions were obtained using amino-
modified polyisobutylene stabilizer SAP 230 TP, which has two
tails of approximately 17 isobutylene subunits, according to the
method described in [18]. Goethite (20 mL, 3% v/v) was added to
SAP 230 TP (Infineum, UK, 5 g), after which 1-propanol (Acros,
99%, 30 mL) was added. The mixture was shaken vigorously until
the polymer and particles appeared well dispersed. The solvent
was then removed by using a rotational vacuum evaporator and
a highly viscous paste was obtained. This paste was spread out
as a film at the inside of a round-bottom flask, then frozen in liquid
nitrogen and connected to a vacuum setup. After freeze-drying, a
mixture of goethite particles and stabilizing polymer was obtained,
which was easily redispersable in toluene (J.T. Baker, 99.5%). Final-
ly, excess SAP 230 TP was removed by three cycles of centrifuga-
tion (2 h, 1425 g) and redispersion in toluene. Although we do
not know the exact amount of the polymer adsorbed at the particle
surfaces, the gained stability of the suspension confirms that this
procedure worked well.
2.2. Characterization
Particle size distributions were determined by transmission
electron microscopy (TEM) using a Technai 10 and 12 (FEI com-
pany) electron microscope. The particles have a more or less rect-
angular boardlike shape with three different dimensions: length L,
width W, and thickness T. The length and width of about 500 par-
ticles was measured with iTEM imaging software to determine the
average length hLiand width hWiand their standard deviation d
L
and d
W
. The length polydispersity is then defined as
r
L
=d
L
/hLi.
The thickness was difficult to determine because most particles
lay on their largest-area side on the TEM grid. For each sample
about 10–20 particle thicknesses were measured.
2.3. SAXS experiments
Samples with different volume fractions were prepared in flat
glass capillaries (VitroCom) with internal dimensions of 0.2 
4.0 100 mm
3
. Capillaries were closed by flame sealing and by
using glue (Araldite AW 2101). They were kept in a vertical position
to allow the establishment of the sedimentation equilibrium profile.
To study the liquid crystalline phase behavior as a function of poly-
dispersity and time, small-angle X-ray scattering (SAXS) measure-
ments were performed. These measurements were conducted at
the BM26 DUBBLE beamline of the European Synchrotron Radiation
Facility (ESRF, Grenoble, France) [21] during measurement sessions
in 2008 and 2009. The microradian resolution setup described in Ref.
[22] was used. A variable permanent magnet (from the ID02 beam-
line) was used, which could reach a magnetic field of up to 1.5 T. The
field was generated by NdFeB permanent magnets and the distance
between them could be adjusted to reach the desired field strength.
3. Results and discussion
The particle dimensions of the systems used are shown in
Table 1. A part of each of the aqueous dispersion (gxx) was used
to coat the particles with PIB to get sterically-stabilized dispersions
in toluene (gxxp). An example of TEM micrograph of the sterically-
stabilized particles is given in Fig. 1.
At first sight the process of particle coating already seemed to
be successful, since streaming birefringence was observed in the
new dispersions. Both systems with a low and high polydispersity
were studied to get a complete picture of the behavior of these ste-
rically stabilized dispersions.
3.1. Low polydispersity
It was found before that charge stabilized systems with a low
polydispersity form isotropic, nematic and smectic Aphases [5].
In sterically stabilized dispersions the same phases were found.
The SAXS patterns of the nematic and smectic Aphase of the
g13p system can be seen in Fig. 2a and f. For both phases scattering
is observed at small angles corresponding to the length correla-
tions; for the smectic phase the scattering peaks are much sharper
because of the layerlike ordering and even a second-order ring is
vaguely discernible. However, no scattering is observed at larger
angles, which is different from the scattering obtained from aque-
ous dispersions (Fig. 3).
The absence of wider-angle scattering can be partially caused
by the presence of many small domains with all different orienta-
tions. This is confirmed by the small-angle scattering which is a full
ring. If the broad scattering peak at a large angle is spread over all
orientations it is possible that it is not detected because the
remaining intensity is too low. In the toluene samples a faster sed-
imentation of goethite is observed and the particles have a strong
tendency to form gels. Liquid crystal phases form at a small scale
but it seems that gelation takes place before large domains can
form. The polymer layer of about 4 nm [24,17] makes that the par-
ticles will probably be able to come closer together if compared to
the charged particles with a Debye length of around 10 nm. There-
fore, the van der Waals interaction will be stronger, which might
cause the gelation.
In a field of 120 mT, the nematic and smectic phase aligned par-
allel to the field, as can be seen in Fig. 2b and g. There was still no
clear scattering observed at larger angles, which suggests that the
alignment is not as good as in the charge stabilized dispersions,
Table 1
Goethite particle dimensions.
System hLi(nm)
r
L
(%) hWi(nm)
r
W
(%) hTi(nm) hLi/hWi
g38(p) 307 38 70 31 27 4.4
g35(p) 282 35 68 32 25 4.1
g17 220 17 62 29 23 3.5
g14(p) 189 14 57 25 17 3.5
g13(p) 215 13 69 20 18 3.2
Fig. 1. A typical TEM micrograph of sterically-stabilized goethite particles g35p.
E. van den Pol et al. / Journal of Colloid and Interface Science 352 (2010) 354–358 355

Author's personal copy
although now second order peaks are observed within the smectic
phase. However, a ring remains visible which implies that domains
with all orientations are present even in a magnetic field of
120 mT. Lower in the same sample the smectic phase does not re-
act to this field at all. Apparently, the structure is more gel like than
in aqueous dispersions. Applying a field for a longer time might in-
duce a better alignment.
At 440 mT, it can be seen that the particles were changing ori-
entation (Fig. 2c and h). Going to a field of 715 mT, the nematic
phase was mostly aligned perpendicular to the field (Fig. 2d). The
smectic phase still showed domains with different orientations
(Fig. 2i). This behavior is similar to that observed in charge stabi-
lized dispersions (Fig. 4)[25].
Using high fields (1.4 T), perpendicular alignment was observed
everywhere in the sample (Fig. 2e and j). In the smectic scattering
pattern it can be seen that the alignment of the layers was not per-
fect yet, more time in the field is needed to reach that state.
The magnetic-field-induced reorientation phenomenon was
further studied for charge and sterically stabilized dispersions of
g14. The critical field strength, where the particles change orienta-
N
SmA
0 mT 1.4 T715 mT440 mT120 mT
B
(a) (d)(c)(b)
(f) (i)(h)(g) (j)
(e)
Fig. 2. SAXS patterns of the nematic (a–e) and smectic (f–j) phase of the g13p system in a magnetic field. The scale bar is 0.05 nm
1
.
N SmA
Fig. 3. Typical SAXS patterns of the nematic phase (g35) and smectic phase (g17) of
aqueous goethite dispersions. Reprinted with permission from [5,23]. Copyright
2008, American Institute of Physics and 2008, IOP Publishing.
B
(a) (b) (c)
(d) (e)
3 mT
440 mT 1.4 T
280 mT120 mT
Fig. 4. SAXS patterns of the smectic phase of g17 in a magnetic field [25]. The scale
bar is 0.05 nm
1
. Reproduced by permission of The Royal Society of Chemistry.
3 mT
1.4 T280 mT
120 mT
B
(a)
(d)(c)
(b)
Fig. 5. SAXS patterns of the smectic phase of the g38p system in a magnetic field.
The scale bar is 0.05 nm
1
.
B
(a) (b)
Fig. 6. SAXS pattern of (a) the columnar phase of the g35p system and (b) of the
smectic/columnar phase of the g38p system in a magnetic field of 1.4 T. The scale
bar is 0.05 nm
1
.
356 E. van den Pol et al. / Journal of Colloid and Interface Science 352 (2010) 354–358

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tion from parallel to perpendicular to the field, was found to be the
same for both dispersions. So, the magnetic properties of the par-
ticles do not change if coated with PIB. It was shown before that
a surfactant layer on ferrofluids can cause spin-pinning, thereby
creating a non-magnetic surface layer on the particles [26–28].
Apparently, this is not the case for our goethite particles.
For this system the q-values were used to calculate distances
between the particles in charge and sterically stabilized samples.
For the small-angle peaks hardly any difference was found, so
the particles seem to be at the same distance from each other in
the length direction. Wider-angle peaks were only observed in high
fields for the PIB-samples, so only those could be compared. At
these high fields, the wider-angle peaks of the sterically stabilized
samples were at a larger angle, meaning that the particles were
closer together (45 nm compared to 60 nm). This confirms the
hypothesis made earlier in this section. It might be caused by the
shorter range of the interaction. A strong repulsion is present in
the toluene samples when the distance between the particle sur-
faces is smaller than 8 nm (twice the thickness of the polymer
layer of 4 nm [17,24]) leading to deformation of the polymer lay-
ers. In the aqueous samples the double layers start to overlap at
the surface-to-surface distance of about 20 nm (twice the Debye
length, which is around 10 nm) and the interaction is softer.
3.2. High polydispersity
Charge stabilized systems with a high polydispersity form iso-
tropic, nematic, smectic Aand columnar phases [5]. In sterically
stabilized systems (g35p,g38p) the isotropic, nematic and smectic
A(Fig. 5a) phases were clearly observed as well. Columnar features
were also recognized (Fig. 6a), but the scattering rings are not as
sharp as expected for a columnar phase. It seems that the system
prefers to form the columnar phase, but the particles get stuck be-
fore forming a well-defined phase. This supports the hypothesis
that gelation takes place in these PIB-systems.
The smectic phase aligned in a magnetic field of 120 mT
(Fig. 5b). Reorientation occurred at 280 mT and domains with dif-
ferent orientations can be observed in the SAXS pattern (Fig. 5c). In
a high magnetic field of 1.4 T, the smectic phase was mostly
aligned perpendicular to the field. Besides that, columnar features
started to appear (Fig. 5d). Slightly lower (0.5 mm) in the sample,
this can be seen more clearly (Fig. 6b). This behavior is similar
but clearly less pronounced compared to charge stabilized disper-
sions (Fig. 7)[25].
Like for the low polydispersity samples, high fields are needed
to orient the full sample. Further down in the capillary the struc-
ture seems to be more gel like. To illustrate this on a quantitative
level, we compare our SAXS results for the sterically stabilized
and aqueous suspensions without applied field since these two
systems respond somewhat differently to the external magnetic
field.
In Fig. 8a we present a 2D scattering pattern from the charge
stabilized g35 system, which can be now compared to a similar
pattern in Fig. 6a. Both systems clearly show polydomain colum-
nar phase in the presence of some coexisting smectic phase.
Fig. 8b compares the integrated scattering profiles of columnar
rings of the charge stabilized system g35 with that of the steri-
cally stabilized system g35p. The difference in the peak width is
very apparent. Note also the difference in the intercolumnar
spacing between these two systems, which is related with the
discussed above difference in the range of the steric and Cou-
lombic interactions. In order to estimate characteristic sizes of
domains Lin these two systems we applied Scherrer equation
L=2B
p
/dq, where dq is the full width at half maximum of the
peak profile and Bis a shape-dependent prefactor of order 1.
Taking B= 1 we have obtained L= 465 nm for g35p and
L= 4400 nm for g35. In the latter case the peak width is in fact
very close to the instrument resolution so that the value of
L= 4400 nm for the aqueous system can only be seen as the
low bound of the estimated domain size.
4. Conclusions
Sterically-stabilized goethite particles in toluene were obtained
and their phase behavior was studied. The phase behavior of sys-
tems with a low and high polydispersity is comparable to the
equivalent aqueous dispersions. The main difference is that the
PIB-systems show a stronger tendency to form gels. Therefore,
B
(a) (b)
(c)
60 mT
1 T
350 mT
Fig. 7. SAXS patterns of the smectic phase of g35 in a magnetic field [25]. The scale
bar is 0.05 nm
1
. Reproduced by permission of The Royal Society of Chemistry.
Fig. 8. SAXS pattern of coexisting columnar and smectic phases of the g35 system without applied field (a). Panel (b) presents integrated scattering profiles of columnar
phases in g35 (2D pattern in panel a) and g35p (2D pattern in Fig. 6a).
E. van den Pol et al. / Journal of Colloid and Interface Science 352 (2010) 354–358 357

Author's personal copy
smaller domains are formed and the columnar phase does not fully
develop.
In a magnetic field again similar behavior is observed as for the
charged systems, showing that the magnetic properties of the parti-
cles are notchanged by the adsorbedpolymer layer.The effect of gela-
tion is alsoobserved here. Thelower part of the samples does not react
to low fields; fields of around 1 T are needed to align the full sample.
For platelike particles PIB has proven to be a good stabilizer to
prevent gelation and observe isotropic-nematic phase separation
in systems that form gels in aqueous environments [14,17,18].In
marked contrast, for goethite it seems that the sterically stabilized
dispersions have a stronger tendency to form gels than the charge
stabilized ones. This might be caused by a stronger van der Waals
attraction between these particles of much higher index of refrac-
tion and the absence of Coulomb repulsion between the particles.
Acknowledgments
This work is part of the research program SFB TR6 of the ‘Stich-
ting voor Fundamenteel Onderzoek der Materie (FOM)’, which is
financially supported by the ‘Nederlandse Organisatie voor Wet-
enschappelijk Onderzoek (NWO)’ and ‘Deutsche Forschungsgeme-
inschaft (DFG)’. We thank the staff of the BM26 DUBBLE beamline
at the ESRF and A. Snigirev for their excellent support and Th.
Narayanan for sharing the magnet. Maurice Mourad is thanked
for his help with the PIB coating procedure.
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