Mineral liquid crystals

www.elsevier.com/locate/cocis

Current Opinion in Colloid & Interface

Mineral liquid crystals

Patrick Davidsona,*, Jean-Christophe P. Gabrielb,1

aLaboratoire de Physique des Solides, UMR 8502 CNRS, Bat. 510, Universite Paris-Sud, 91405 Orsay cedex, FrancebNanomix Inc., 5980 Horton Street, Suite 600, Emeryville, CA 94608, USA

Available online 22 January 2005

Abstract

This review describes recent developments in the field of liquid–crystalline suspensions of mineral nanoparticles. New families of

chemical compounds have been investigated in the last few years. The most common mesophases (nematic, lamellar and columnar) have now

been discovered in dispersions of disc-like and rod-like nanoparticles. New research thrusts presently focus on more subtle thermodynamic

effects such as those of polydispersity and gravity. The specific physical properties brought by the mineral building blocks to the liquid–

crystalline phases are now being examined. Mesomorphic ordering of the nanoparticles is increasingly used in materials science for

templating and for preparing composites.

D 2004 Elsevier Ltd. All rights reserved.

Keywords: Liquid crystals; Colloids; Mineral; Inorganic; Self-assembly; Nanoparticles

1. Introduction

The liquid–crystalline properties of colloidal suspen-

sions of mineral nanoparticles were first observed and

understood by Zocher in 1925 [1!!]. However, they were

almost forgotten and renewed interest in such systems

only appeared about 12 or 15 years ago [2!,3!]. This

renewed interest has increased much lately (our review

considers mostly articles published in 2001–2004) thanks

to two general trends. Firstly, there is nowadays a lot of

excitement around what may be called the dnanoTfashion due to new fabrication and characterisation

techniques that provide nanometric control of materials

and devices. Secondly, a new kind of solid-state

chemistry (dchimie douceT) has recently emerged, often

based on inorganic polycondensation methods and dsol–gelT processes. This chemistry involves reactions in

1359-0294/$ - see front matter D 2004 Elsevier Ltd. All rights reserved.

doi:10.1016/j.cocis.2004.12.001

* Corresponding author. Tel.: +33 1 69 15 53 93; fax: +33 1 69 15 60

86.

E-mail addresses: [email protected] (P. Davidson)8

[email protected] (J.-C.P. Gabriel).1 Tel.: +1 510 428 53 13; fax: +1 510 658 04 25.

solvents and at much lower temperatures, in contrast

with traditional solid-state chemistry. Consequently, it

provides a wealth of new colloidal suspensions of

anisotropic nanoparticles, often as a by-product of

research on low-dimensional compounds.

From a purely physical point of view, inspired by the

early work of Onsager [4!!], theorists have devised statistical

physics models [5!!� and numerical simulations [6!!,7!!] of

liquid–crystalline ordering that provide a fairly comprehen-

sive framework to understand the phase behaviour of such

suspensions. The most common mesophases, namely the

nematic, lamellar and columnar phases have now been found

in colloidal mineral systems comprised of rod-like and disc-

like entities. Very recent work instead focuses on subtle

effects such as those of particle polydispersity, gelation and

gravity. Another research direction consists in trying to

impart physical properties (e.g. magnetism), typical of

mineral compounds, to liquid–crystalline phases.

Chemists are currently exploring whole families of

materials such as layered double hydroxides, layered

niobate oxides, semi-conducting and metallic nanorods.

They also try to make use of the liquid–crystalline

properties to tailor new types of materials, for instance by

templating.

Science 9 (2005) 377–383

Fig. 1. Aqueous suspensions of electrostatically stabilised gibbsite platelets

viewed in polarised light. The concentrations increase from left to right

from 95 to 270 g l�1. The suspensions spontaneously demix into a dark

isotropic phase and the denser birefringent nematic phase. The proportion

of nematic phase increases with concentration. (Courtesy of D. van der

Beek and H. Lekkerkerker.)

P. Davidson, J.-C.P. Gabriel / Current Opinion in Colloid & Interface Science 9 (2005) 377–383378

2. Nematic phases

2.1. Rod-like nanoparticles

The formation of nematic phases in suspensions of rod-

like nanoparticles is now a very well documented phenom-

enon well described by statistical physics models. Recent

research activity in this area has aimed to expand the variety

of mineral compounds that show nematic ordering. For

example, CdSe semi-conductor nanoparticles, of aspect

ratios tuneable up to 15, were grafted with long aliphatic

chains and dispersed in cyclohexane [8!]. These suspensions

show schlieren textures in polarised light microscopy and

display spontaneous isotropic/nematic phase separation in a

given range of concentration. Their phase diagram, deter-

mined by NMR, exhibits some temperature dependence (a

rather unusual feature in this field) and a large I/N biphasic

region, which was interpreted as due to the existence of

attractive interactions between the nanorods [9!]. Another

recent example is that of aqueous suspensions of surfactant-

coated gold nanorods that seem to display liquid–crystalline

textures [10]. Even though the electronic properties of such

suspensions have not been examined yet, these reports

should stimulate a lot of research activity.

2.2. Disc-like nanoparticles

Assemblies of disc-like nanoparticles have lately focused

a lot of attention because they are also expected to exhibit

nematic phases, as was first demonstrated by Langmuir in

1938 with clay particles [11!!]. The case of clay suspensions

is still plagued by the fact that, upon increasing concen-

tration, they gel before reaching a true isotropic/nematic

phase transition. This prevents the suspensions to reach true

thermodynamic equilibrium, which makes their understand-

ing much more difficult [12]. The gelation of clay

suspensions is still a matter of considerable debate but even

if these systems show nice nematic textures and large

nematic order parameters [13!,14], it is clear that, contrary to

an idea sometimes met in literature, the isotropic/nematic

transition in no way explains the gelation. A simple proof of

this is that isotropic clay gels can easily be produced. Then,

the gelation is merely a nuisance from the liquid–crystalline

point of view, which prevents the nematic transition to fully

take place at the macroscopic scale. Two recent studies

report the existence of small-scale heterogeneities that

suggest the possible coexistence of nematic and isotropic

microscopic domains [15!,16!].

Clear-cut evidence for the observation of nematic order-

ing, unhindered by gelation, of suspensions of disc-like

particles was only reported in the last few years. This was

achieved in several systems but the most detailed and

comprehensive study is probably that of gibbsite (Al(OH)3)

particles. These nanodiscs of about 150 nm diameter and

10–15 nm thickness were dispersed in two different ways:

they were grafted with polyisobutene chains to form

sterically stabilised suspensions in toluene [17!!] and they

were also electrostatically stabilised in water by addition of

aluminium polycations [18!,19!!] (Fig. 1). In both cases, the

complete isotropic/nematic coexistence region could be

observed as the gelation transition was either suppressed

or pushed to larger concentrations. Then, the phase-

transition volume fractions agree reasonably well with

statistical physics models and numerical simulations of the

nematic ordering of disc-like particles. Interestingly, sus-

pensions of polydisperse colloidal gibbsite platelets display

a remarkable isotropic/nematic phase separation where the

isotropic phase is denser than the nematic one [20!]. This

phenomenon was explained by a strong fractionation effect

of the particle thickness between the two phases [21].

Meanwhile chemists have been exploring new families of

materials such as the famous layered double hydroxides

(LDHs) and a group reported in 2003 the liquid–crystalline

behaviour of colloidal Mg/Al LDH aqueous dispersions

[22!!]. Judging from optical textures (Fig. 2), in the absence

of X-ray scattering experiments, the mesophase is most

probably nematic and no gelation phenomenon was

observed in this system. Another group has observed hints

of liquid–crystalline behaviour in this family of compounds

[23]. If confirmed, these promising results should inspire

large-scale investigations of the lyotropic mesomorphism of

these chemically important compounds.

Exfoliation of layered niobate and titanate oxides by

intercalation with organoammonium ions has been the focus

of much research work during the last 10 years [24,25].

Aqueous colloidal suspensions have been thus produced.

They are comprised of weakly interacting oxide nanosheets

and they can have the properties of physical hydrogels,

depending on pH [26,27]. The nanosheets are fairly flexible

and can adopt different morphologies [28]. The liquid–

crystalline nature of colloidal suspensions of K4Nb6O17 has

been demonstrated in 2002 by a Japanese group [29!].

Interestingly, nematic-like textures were observed at volume

fractions ranging from about 1% down to as low as 0.004%.

Liquid–crystalline samples of centimetric size spontane-

ously formed single domains under the influence of gravity.

These nanosheets have a quite large average diameter (1–2

Fig. 2. Optical textures in polarised light microscopy of colloidal Mg/Al

LDH dispersions. (a) At 18% w/w concentration, the isotropic/nematic

phase coexistence is revealed by the observation of dtactoidsT, i.e. littlenematic droplets floating in the isotropic phase. (b) At 22% w/w

concentration, the pure nematic phase displays a nice Schlieren texture.

(Adapted from Ref. [22!!], with permission.)

(e) (d)1

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

Vol

ume

frac

tion

of li

quid

cry

stal

line

dom

ain

(c) (b) (a)

P. Davidson, J.-C.P. Gabriel / Current Opinion in Colloid & Interface Science 9 (2005) 377–383 379

Am) and small thickness (1.8 nm), which may explain the

very low volume fractions at which birefringence occurs.

Organic dye molecules could be dissolved into this mineral

liquid crystal; they adsorbed onto the nanosheets and

showed some orientation when the samples were aligned

by gravity.

In a subsequent work, K4Nb6O17 single crystals were

exfoliated to form colloidal suspensions comprised of

particles of even larger (8 Am) diameter. The particle

diameter was then reduced from 8 to 0.15 Am by sonication

[30!!]. The isotropic/nematic transition occurred at volume

fractions that decrease with increasing particle diameter, in

agreement with statistical physics models of liquid–crystal-

line ordering (Fig. 3).

HNb3O8 and HTiNbO5 were also exfoliated with

tetrabutylammonium cations to form colloidal suspensions

of nanosheets although exfoliation was only incomplete for

the latter compound. These suspensions were reported to

display liquid–crystalline behaviour [31!].

00 0.01 0.02

(vol / vol)φ0.03

Fig. 3. Linear relationships between the ratio of nematic phase in biphasic

(I/N) samples and the overall volume fraction in colloidal suspensions of

layered niobates of different particle sizes. (a): 0.15 Am; (b): 0.38 Am; (c):

1.9 Am; (d): 6.2 Am; (e): 7.8 Am. (Reproduced from Ref. [30!!], with

permission.)

3. Lamellar phases

Lamellar phases have comparatively received much

less attention than nematic and columnar phases. The so-

called dSchiller SchichtenT (iridescent layers) have been

reported long ago by Zocher [32] and studied more

recently by AFM [33,34!,35]. They are comprised of

rather large rod-like akaganeite (h-FeOOH) or WO3

nanoparticles that sediment at the bottom of flasks. The

structure within these layers can be either liquid or

crystalline so that the sediments are formally analogous

to smectic A or smectic B phases, respectively. The

observation of a lamellar phase comprised of smaller

(brownian) rod-like mineral particles has not been

reported yet, to the best of our knowledge. This may

be due to polydispersity issues, as will be discussed later

in this article.

Our group reported in 2001 a very different kind of

mineral lamellar mesophase comprised of extended covalent

sheets obtained by exfoliation in water of the low-dimen-

sional H3Sb3P2O14 compound [36!!]. The lamellar meso-

phase is stable in a given range of volume fraction extending

down to 0.7%, which corresponds to the maximum swelling

of the phase. At this limit, the lamellar period is about 225

nm, which is comparable to the wavelengths of visible light

and gives the samples a bluish hue. Beyond the swelling

limit, excess water is expelled and the system becomes

biphasic. Small-angle X-ray (SAXS) experiments with

samples of the lamellar phase aligned in a Couette shear

cell (a device commonly used in the field of soft-condensed

matter) revealed a row of numerous lamellar reflections,

thus confirming the phase identification (Fig. 4). The

rheological properties of this phase were studied by

combined rheometry and SAXS in situ [37]. It is very

likely that the exfoliation of other two-dimensional com-

Fig. 4. Small-angle X-ray scattering pattern of the dswollenT lamellar phase

of H3Sb3P2O14. The sample was aligned in a Couette shear cell and, in a

suitable scattering geometry, a series of lamellar reflections can be

observed.


pounds will yield other lamellar liquid–crystalline suspen-

sions in the future.

4. Columnar phases

Aqueous suspensions of Ni(OH)2 disc-like nanoparticles

of 90 nm diameter and 10 nm thickness were reported to

form a hexagonal columnar phase [38] at fairly large weight

fractions (60–70%). Small-angle X-ray and neutron scatter-

ing techniques were used to show that this phase has a

structure quite similar to that of the hexagonal columnar

phase of disc-like thermotropic mesogens [39!!]. The disc-

like particles stack in columns that assemble on a two-

dimensional lattice, free to slide past each other. The

behaviour under shear flow of these suspensions of Ni(OH)2disc-like particles was examined in small-angle neutron

scattering experiments that revealed several regimes differ-

ing by the particles orientation with respect to the flow [40].

A similar organisation was also discovered in the same

suspensions of grafted gibbsite nanodiscs that display the

nematic phase already discussed above [41!!]. Paradoxically

enough, an interesting feature of this system is that it is quite

polydisperse (around 20%). The observation of a position-

ally ordered columnar phase in these suspensions proves

that the so-called dterminal polydispersityT is larger here

than those conjectured for the crystallisation of hard spheres

and for the lamellar ordering of hard rods. Moreover, as the

volume fraction increases, the system organises in a lamellar

phase rather than the columnar one. This illustrates a very

Fig. 5. Phase behaviour of suspensions of sterically stabilised gibbiste

nanodiscs of increasing volume fractions /. The first four tubes, viewed in

polarised light, respectively, show: an isotropic/nematic phase coexistence

at /=0.19, a pure nematic phase at /=0.28, a nematic/columnar phase

coexistence at /=0.41, and a pure columnar phase at /=0.47. The

columnar phase can be detected in natural white light (last tube on the right)

by the observation of small coloured bright spots due to the Bragg

reflections of light by the columnar lattice. (Reproduced from Ref. [41!!],

with permission.)

subtle interplay between diameter and thickness polydisper-

sities, depending on volume fraction (Fig. 5).

The columnar phase was also detected in the aqueous

suspensions of charged gibbsite platelets [19!!]. By varying

the ionic strength of this system, through salt addition, the

effective particle aspect ratio could be adjusted. This

provided the opportunity to verify the whole phase diagram

of disc-like particles predicted by numerical simulations

[7!!]: At low aspect ratio, a direct columnar/isotropic phase

transition was observed whereas the nematic/isotropic phase

transition was detected at large aspect ratio.

Interestingly enough, gravity has a very important

influence on this system, in agreement with theoretical

predictions [42]. A long enough vertical test tube (or

capillary) develops in a few months a vertical concentration

gradient, which results in the formation of several coexisting

phases (here, isotropic, nematic and columnar) with well-

defined heights and concentrations. A simple and elegant

model was devised to account for these observations [43].

Besides, very recently, we observed a magnetic-field-

induced nematic to columnar phase transition in suspensions

of goethite (a-FeOOH) nanorods [44].

5. Applications

This section describes studies that either specifically

make use of the liquid–crystalline order of mineral particles

to organise materials or exploit typical physical properties of

minerals to induce new phenomena in soft-condensed

matter.

Fig. 6. Small-angle X-ray scattering patterns of goethite suspensions in the

nematic (top: a, b) and isotropic (bottom: c, d) phases at low (left: a, c) and

high (right: b, d) magnetic field intensities. (Adapted from Ref. [53!!], with

permission.)


The idea of preparing hybrid materials incorporating

mineral particles is not very new. Actually, clay+polymer

based composite materials represent a good example of such

an approach [45]. Early attempts have used imogolite

nanotubes that show a nematic phase in water [46] to

improve the mechanical properties of semi-rigid (Hydrox-

ypropylcellulose) or flexible (Polyvinylalcohol) polymers

[47,48]. This goal was only achieved with the semi-rigid

polymers. As templating has become lately a very popular

research trend, the idea naturally came of organising an

amorphous material like silica by using a mineral nematic

aqueous suspension. Two examples of this approach were

recently published: The first one involves the nematic phase

of vanadium pentoxide ribbons, a very well known system

[3!] that is still the subject of some detailed physical studies

[49]. Silica molecular precursors could be dissolved into this

nematic phase that was aligned in a magnetic field of 0.85 T,

i.e. much smaller than the 11.7 T used to align composites

obtained through organic templating [50]. Then, the pre-

cursors were polycondensed and the vanadium pentoxide

ribbons etched out. Mesoporous birefringent bulk materials

were thus produced where the pores are all aligned parallel

but lack long-range positional order [51!]. The second

example is very similar but is based on aqueous suspensions

of K4Nb6O17 nanotubes [52].

Colloidal aqueous suspensions of goethite nanorods

provide a very good illustration of the specific properties

that minerals can bring to the field of soft-condensed matter

[53!!,54,55]. These suspensions have nematic and isotropic

phases that show very peculiar magnetic properties. The

nematic phase can be aligned in very weak magnetic fields,

even compared to thermotropic nematics used in display

technology. Moreover, the nanorods align parallel to the field

at low field intensity (b350 mT) but they reorient perpendic-

ularly at high field intensities (N350 mT). The isotropic phase

also aligns in magnetic fields and its field-induced birefrin-

gence is much larger than usual lyotropic nematics. It also

shows the same reorientation phenomena around 350 mT as

the nematic phase (Fig. 6). These observations were

explained in the following way: Although bulk goethite is a

typical antiferromagnetic material, goethite nanorods still

bear a small remanent magnetic moment due to non-

compensated surface spins. Moreover, the anisotropy of

magnetic susceptibility of the nanorods is negative. The

magnetic moment dominates at low fields and favours

parallel alignment whereas the susceptibility anisotropy

dominates at high fields and favours perpendicular align-

ment. Suspensions of gibbsite nanodiscs also display very

strong and interesting orientation behaviour in magnetic

fields (D van der Beek, personal communication).

! of special interest.!! of outstanding interest.

6. Conclusion

Thanks to chemists and to the widespread interest in

nanoparticles, the field of mineral liquid crystals has been fast

expanding in recent years. Suspensions of new kinds of

particles have provided examples of the most common

mesophases (nematic, lamellar and columnar). More subtle

thermodynamic effects are being examined both from

experimental and theoretical points of view. The potential

interest of mineral particles for soft-condensed matter has

been illustrated in a few cases, a trend that should expand in

the future. Such mesophases have started to be used in

materials science and a new kind of chemistry within oriented

and/or confined environments may appear in the near future.

References and recommended reading

[1

!!

] H. Zocher, Uber freiwillige Strukturbildung in Solen. (Eine neue

Art anisotrop flqssiger Medien), Z. Anorg. Allg. Chem. 147 (1925)

91–110.

Possibly the first careful observations and analyses (in German) of the

liquid–crystallinity of suspensions of mineral particles. The subtitle (dAnew kind of anisotropic fluid materialT, i.e. our modern definition of a

liquid crystal) indicates very well the level of understanding reached by

Zocher, in contrast with previous reports by others. Anyone really

interested in this field should at least take a look at this article.

[2

!] J.C.P. Gabriel, P. Davidson, New trends in colloidal liquid crystals

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A review article that gives general principles and summarizes the literature

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!

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!!

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!!

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!!

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Mineral liquid crystals

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