Liquid–crystalline aqueous clay suspensionsLaurent J. Michot*†, Isabelle Bihannic*, Solange Maddi*, Sergio S. Funari‡, Christophe Baravian§, Pierre Levitz¶,and Patrick Davidson�

*Laboratoire Environnement et Mineralurgie, Nancy Universite Unite Mixte de Recherche 7569, Centre National de la Recherche Scientifique–InstitutNational Polytechnique de Lorraine, BP40, 54501 Vandœuvre Cedex, France; ‡HASYLAB, NotkeStrasse 85, D-22603, Hamburg, Germany; §Laboratoired’Energetique et de Mecanique Theorique et Applique, Nancy Universite Unite Mixte de Recherche 7563, Centre National de la RechercheScientifique–Institut National Polytechnique de Lorraine–Universite Henri Poincare, 2 Avenue de la Foret de Haye, BP160, 54504 Vandœuvre Cedex,France; ¶Laboratoire de Physique de la Matiere Condensee, Unite Mixte de Recherche 7643, Centre National de la Recherche Scientifique–EcolePolytechnique, 91128 Palaiseau Cedex, France; and �Laboratoire de Physique des Solides, Unite Mixte de Recherche 8502, Centre National de la RechercheScientifique–Universite Paris Sud Bat 510, 91405 Orsay Cedex, France

Edited by Herman Z. Cummins, City College of the City University of New York, New York, NY, and approved August 31, 2006 (received for reviewJune 21, 2006)

This article demonstrates the occurrence of a true isotropic�nematictransition in colloidal Brownian aqueous suspensions of naturalnontronite clay. The liquid–crystalline character is further evidencedby polarized light microscopy and small-angle x-ray scattering exper-iments in the presence and absence of modest external magneticfields. The complete phase diagram ionic strength�volume fractionthen exhibits a clear biphasic domain in the sol region just before thegel transition in contrast with the situation observed for otherswelling clays in which the sol�gel transition hinders the isotropic�nematic transition. Small-angle x-ray scattering measurements of gelsamples reveal strong positional and orientational orders of theparticles, proving unambiguously the nematic character of the geland, thus, clearly refuting the still prevalent ‘‘house of cards’’ model,which explains the gel structure by means of attractive interactionsbetween clay platelets. Such order also is observed in various otherswelling clay minerals; therefore, this very general behavior must betaken into account to reach a better understanding of the rheologicalproperties and phase behavior of these systems.

colloids � liquid crystal � phase transitions

Swelling clay minerals are layered compounds that bear anegative layer charge compensated by interlayer exchange-

able cations whose valence and hydration properties control bothswelling and colloidal behavior. One of the most importantproperties of swelling clay minerals is their ability to form yieldstress materials when dispersed in water. This feature, exten-sively used in various industrial applications (drilling fluids, foodindustry, cosmetic industry, etc.), also plays a major role in manyfundamental processes occurring at the Earth’s surface, such asslipping processes in plate-boundary faults (1–4) or landslidetriggering (5–9). For these reasons, numerous studies havefocused on the rheology of aqueous clay suspensions withparticular emphasis on yield stress, thixotropy, and aging (10–15). However, most studies neglect a key feature of clay minerals,i.e., their anisotropic shape. Actually, due to their high aspectratio typically ranging between 25 and 1,000, these materialsshould very likely form liquid–crystalline phases (16), such asthose observed for rod-like clay particles, such as imogolite inaqueous media (17), or organophilic sepiolite clay particles innonaqueous solvents (18). A phase transition was indeed ob-served by Langmuir (19, **) as early as 1938 in suspensions ofnatural hectorite swelling clay. However, all subsequent studiesfailed to reproduce this crucial observation and give evidence ofa clear thermodynamic liquid–crystalline order but insteadrevealed a dominant gel formation (20). Such behavior isobserved for both highly polydisperse natural samples (21) andsynthetic monodisperse ones (22). The structure and formationmechanisms of the gel are still under debate. Indeed, althoughsome of the gel features indicate nematic ordering (23, 24), notrue thermodynamic nematic order was ever clearly evidenced.††

In this manuscript, we show that aqueous suspensions of naturalclay minerals can really exhibit a true isotropic�nematic transi-

tion, that the nematic phase displays strong orientational order,and that it can be aligned in modest magnetic fields, a distinctivefeature of liquid–crystalline phases.

Results and DiscussionThe clay mineral used in this study is a nontronite from SouthernAustralia (25). Nontronite is a naturally occurring swellingdioctahedral clay mineral related to the montmorillonite-beidellite series in which most aluminum atoms are replaced byiron (III) ions. The structural formula of the nontronite used inthe present study was recently refined (26) as (Si7.55Al0.16Fe0.29)(Al0.34Fe3.54Mg0.05) O20(OH)4 Na0.72. Based on unit-cell param-eters, its density can be estimated at �3.0 g�cm3. After purifi-cation and size fractionation, four different fractions wereobtained. The results presented in this report deal with the thirdsize fraction in which the elementary lath-shaped nontroniteparticles have an average length and width of 147 and 52 nm withstandard deviations of 40% and 38%, respectively. Suspensionsprepared with this size fraction are stable over years, whereas inhigher size fractions sedimentation starts occurring after a fewweeks, which affects the phase diagram (27). Naked-eye obser-vations in polarized light of vials filled with suspensions ofincreasing volume fractions (at a fixed ionic strength of 10�4 M)reveal the following features. (i) At volume fractions � � 0.6%,the suspensions are isotropic liquids (Fig. 1a) and exhibit f lowbirefringence for � � 0.2%. (ii) At 0.6% � � � 0.8%, thesuspensions are biphasic with a clear phase separation betweena denser birefringent phase at the bottom and an isotropic oneat the top (Fig. 1 b–d). As expected, the proportion of birefrin-gent phase gradually increases with the overall clay volumefraction. It must be emphasized that this observation representsa clear-cut and reproducible instance of isotropic�nematic phaseseparation in aqueous suspensions of natural clay minerals. In

Author contributions: L.J.M., I.B., P.L., and P.D. designed research; L.J.M., I.B., S.M., C.B., P.L.,and P.D. performed research; S.M., S.S.F., and C.B. contributed new reagents�analytic tools;L.J.M., I.B., C.B., P.L., and P.D. analyzed data; and L.J.M., I.B., P.L., and P.D. wrote the paper.

The authors declare no conflict of interest.

This article is a PNAS direct submission.

Abbreviation: SAXS, small-angle x-ray scattering.

†To whom correspondence should be addressed. E-mail: laurent.michot@ensg.inpl-nancy.fr.

**In a footnote (p. 877) in his original paper (19), Langmuir clearly mentions that the phaseseparation was nonreproducible. Furthermore, some of the features of the phasetransition observed by Langmuir are a bit awkward. Indeed, the transition appearswithin the gel phase, with the isotropic phase separating out of the birefringentmaterial. Furthermore, the relative proportion of isotropic phase increases with the totalconcentration, which is the contrary to what should be observed in the case of anisotropic�nematic phase transition.

††One of us (P.D.) did observe some phase separation in a couple of laponite samples andmentioned it in ref. 23. However, as in the case of Langmuir’s experiments (19), theobservation was not reproducible, and none of us working with laponite samples haveobserved such a phase separation since.

© 2006 by The National Academy of Sciences of the USA

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contrast with all previous studies, these clay suspensions thenreach the isotropic�nematic transition line at thermodynamicequilibrium before gelling. (iii) At � � 0.83%, the suspensionsare birefringent gels (Fig. 1e), which means that the sol�geltransition actually occurs at a volume fraction only slightly largerthan the nematic edge (0.8%) of the biphasic region. The phaseseparation is also readily observed by polarized light microscopy(Fig. 1f ). A few weeks after sample preparation, birefringentdroplets are clearly visible in the top isotropic phase; they slowlysediment and coalesce to form the nematic phase. The phaseseparation is complete after a few months. Similar phenomenaare observed with nontronites of size 4. This nematic phasedisplays a typical threaded texture, and the detection of flick-ering reveals that the suspensions are Brownian.

Because of the presence of ferric iron in the octahedral layer,nontronite suspensions are fairly sensitive to magnetic fields, asshown by Fig. 1 g and h, which illustrates the strong alignment of thenematic phase submitted to a 1-T magnetic field. This interestingfeature, typical of liquid–crystalline phases, proves that this nematicsample is a fluid rather than a gel of appreciable yield stress. Themagnetic field can even be used to micropattern the orientation ofthe clay platelets by exploiting a classical transient hydrodynamicinstability observed upon a sudden change of field direction (Fig.1i) (28). The period of the modulation can easily be tuned byadjusting the magnetic field intensity.

A similar evolution with volume fraction also is obtained foran ionic strength of 10�3 M. In contrast, at 5.10�3 M, no phaseseparation was observed, and the system evolves directly fromisotropic liquid to birefringent gel. The complete phase diagramof this nontronite size fraction is presented in Fig. 2. At low ionicstrength, the biphasic domain is tilted toward larger volumefractions (29), revealing that the system is dominated by repul-sions. In contrast, at higher salt concentrations, the sol–gel

transition line displays a negative slope and crosses the biphasicregion. Such a shape is similar to what was observed in the caseof laponite (22). The complete phase diagram where the sol–gelline meets the flocculation line at high ionic strength suggeststhat this evolution is likely related to microflocculation pro-cesses. The shape of rheological f low-curve measurements con-firms this interpretation because the viscosity of liquid samples,at high ionic strength, increases at low shear stress. Furthermore,in this region of the phase diagram, significant aging effects are

Fig. 1. Visual observations of the isotropic�nematic transition in nontronite aqueous solutions. (a–e) Naked-eye observation of the samples. Vials (2 ml) werefilled with aqueous suspensions of sodium nontronite (ionic strength � 10�4 M) and observed between crossed polarizers (the isotropic phase in a–d appearsdark). (a) Isotropic liquid sample at a volume fraction � � 0.5%. (The small bright line observed at the bottom of the vial is due to a reflection on the curvedbottom.) (b) Onset of the phase separation at � � 0.61%. (c) Biphasic sample at � � 0.67%. (d) Biphasic sample at � � 0.72%. (e) Birefringent gel at � � 1%.( f) Polarized-light optical microscopy observations of a nontronite sample (� � 0.7%, ionic strength � 10�3 M) at the onset of phase separation. (g and h)Polarized-light optical microscopy observations of a biphasic sample of a nontronite suspension (� � 0.7%, ionic strength � 10�3 M) held in a flat capillarysubmitted to a horizontal 1-T magnetic field. White arrows indicate the directions of the polarizers. I, isotropic phase; N, nematic phase. (g) Extinction conditions.The capillary is barely visible because its axis is parallel to that of the polarizer. (h) Maximum transmission conditions. The nematic phase is almost uniformly brightbecause there are only very few defects left. The isotropic phase is not dark because of its large magnetic-field-induced anisotropy. (i) Transient hydrodynamicinstability observed upon a sudden change of magnetic field direction for a nontronite suspension (� � 0.7%, ionic strength � 10�3 M) held in a flat capillary.

Fig. 2. Phase diagram of sodium nontronite suspensions. Upon increasingvolume fraction, the suspensions first form an isotropic liquid (IL), then entera biphasic regime (B) followed by a small region of nematic sol (NS), and finallyform birefringent gels. The line between gel and liquid was determined byoscillatory shear measurements. At high salt concentration, the presence offlocs (F) was checked out by visual observation.

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observed, which once again can be linked to microflocculationevents that clearly deserve further investigation.

On the basis of published statistical physics models andnumerical simulations, neglecting polydispersity and electro-static effects, and considering an average particle diameter of�100 nm, a rough estimated value of �2% can be obtained forthe volume fraction corresponding to the isotropic�nematictransition (30). Despite such crude simplifications, this predictedvalue compares rather well with the experimental one (0.6–0.8%). Because models and simulations are based on excluded-volume particle interactions only, the experiments reported herestrongly suggest that attractive forces, often mentioned to de-scribe clay suspensions, are irrelevant for the onset of nematicordering at low ionic strength.

The structures of the suspensions were further analyzed by smallangle x-ray scattering (SAXS) experiments. Very dilute isotropicsuspensions showed a scattering intensity, I, monotonously decreas-ing with increasing scattering vector modulus, q, as I � q�2, provingthe bidimensional nature of the scattering objects. At larger volumefractions, SAXS patterns display correlation peaks due to theshort-range positional (i.e., ‘‘liquid-like’’) order of the clay platelets.Typical patterns obtained from the isotropic and birefringent partsof a biphasic sample, are presented in Fig. 3 a and b. The anisotropiccharacter of the birefringent phase is visible and, together with theabsence of sharp Bragg reflections, proves its nematic nature.However, the pattern of Fig. 3b indicates a very poorly aligned‘‘powder’’ sample, with a distribution of almost randomly oriented

nematic domains. In contrast, when submitted to a magnetic field(Fig. 3c), in agreement with optical observations (Fig. 1g), theanisotropy of the SAXS pattern is very pronounced, revealing a verystrong orientation of nontronite particles. The diffuse-peak posi-tions (d) are the same for both the powder and aligned nematicphases with distances, d, of �70 nm between clay platelets. In thegel phase (Fig. 3d), highly anisotropic SAXS patterns were recordedfrom samples aligned by shear-flow achieved when the suspensionswere gently centrifuged into the capillaries.

The evolution of d with inverse volume fraction (Fig. 4a)displays a first regime, for � � 1.1%, where d is proportional to1��. Such behavior (d � t��, where t is the layer thickness) istypical of the one-dimensional swelling of a pure lamellar phase(31) and therefore suggests a strong lamellar local order of theplatelets in the nematic phase. The slope obtained in this regimeis t � 0.70 � 0.05 nm, a value very close to the thickness of asingle clay sheet (0.8 nm), proving that the layers are perfectlyexfoliated in suspension. The nematic phase then appears asresulting from the orientation of individual charged clay plate-lets, which is different from the situation encountered forpositively charged layered double hydroxides (32), where a phasetransition was observed for much thicker stacks of platelets. Thelinear regime, observed in the nematic phase, ends close to thelimit of the biphasic region. There, a crossover occurs towardanother regime at lower volume fraction (33), where the distancescales as ��1/3, suggesting isotropic volume swelling.

Such an evolution of distance versus volume fraction is notspecific to nontronite but also is observed for all of the size-

Fig. 3. SAXS studies. (a) Two-dimensional SAXS pattern of the isotropic phase of a nontronite suspension (� � 0.7%, ionic strength � 10�3 M) (b) SAXS patternof the nematic phase of the same suspension. (c) SAXS pattern of the nematic phase of the same suspension aligned in a magnetic field of 1 T. (d) SAXS patternof a gel sample (� � 3%, ionic strength � 10�3 M). (e) Plots of I�q2 vs. q corresponding to patterns a, b, and d. For patterns a and b, the first diffuse peak is tooclose to the beamstop and is not observed. The intensity of pattern d was divided by 2 for enhanced readability.

Fig. 4. Plots of the average interparticledistances versus inverse volume fraction. Thestraight line corresponds to one-dimensionalswelling, d � t��, where t � 0.7 nm is thethicknessofasingleclay sheet. (a)Nontronitesuspensions. (b) Suspensions of montmorillo-nites from Arizona, Milos, and Wyoming.

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fractionated swelling clays of similar size that we have investi-gated [Milos (Greece), Arizona, and Wyoming montmorillo-nites], which points to a very general behavior of smectites interms of positional short-range order (Fig. 4b). The clear-cutevidence of a first-order isotropic�nematic transition in nontro-nite suspensions, reported here, therefore strongly suggests thatthe birefringence of smectite clay gels is indeed the sign oflong-range orientational order. In this context, at volume frac-tions larger than �1%, the shear-thinning, nonlinear rheologicalproperties of clay suspensions should be discussed in relation totheir nematic order and not according to the house of cardsmodels that assumes a connected network of interacting clayplatelets. Such a feature must then definitely be taken intoaccount for any future modeling of the rheological behavior ofclay minerals suspensions both for industrial applications andnatural processes.

A puzzling issue is how clay particle properties (dimensions,polydispersity, electric charge, flexibility, etc.) control gelation orliquid–crystalline formation. In the case of the nontronite used inthis study, upon increasing volume fraction, the isotropic�nematictransition is observed before gelation, whereas suspensions of otherclay minerals exhibit gelation before any phase transition takesplace. The behavior of nontronite might be assigned to its lath-shape, and further model calculations and simulations are neededto explore in detail such an assumption. In any case, besides theirrelevance to the understanding of the rheological behavior ofswelling clay minerals, the features revealed in the present papershould strongly contribute to improve our knowledge of the phasebehavior of charged anisotropic colloids.

Materials and MethodsFor clay preparation, the clay sample was ground, exchangedthree times in 1 M NaCl, and washed by dialysis using ultrapurewater until a conductivity of �5 �S was obtained. The suspen-sions were then placed in Imhoff cones for 24 h to discard themajor mineralogical impurities (mainly iron oxide and feldspar).Size fractionation procedures were then applied by centrifugingthe stock suspension under different gravitational fields (7,000 �g, 17,000 � g, and 35,000 � g). The supernatant obtained aftercentrifugation at the highest speed was concentrated by roto-evaporation. Using such a procedure, four size fractions referredto as sizes 1–4 were obtained. Mineralogical purity was checkedby x-ray diffraction and infrared spectrometry, whereas sizes

were determined by transmission electron microscopy. Theaverage length and width of sizes 1–4 were 700 and 140 nm, 360and 90 nm, 150 and 50 nm, and 100 and 45 nm, with polydis-persities of 50%, 40%, 38%, and 35%, respectively. The sameprocedure was applied to montmorillonite samples from Ari-zona, Milos, and Wyoming. Their respective structural formulaecan be written as (Si7.95,Al0.05)(Al2.85, Mg1.07, Fe0.17)O20(OH)4Na1.11, (Si7.74,Al0.26)(Al3.0, Mg0.54, Fe0.46)O20(OH)4 Na0.79, and(Si7.76,Al0.24)(Al3.06, Mg0.48, Fe0.46)O20(OH)4 Na0.77. The respec-tive average sizes of the samples described in the present study(size 3) are 50, 55, and 75 nm, and the sheets exhibit veryirregular shapes.

Samples at different volume fractions were prepared byosmotic stress using either dextran or polyethyleneglycol solu-tions and dialysis membranes (Visking, London, U.K.) with acut-off value of 14,000 Da. Ionic strength was fixed in thereservoir to avoid problems related to the Donnan effect.

Rheological measurements were carried out on a TA 2000instrument. Oscillatory shear measurements were performed todetermine the elastic (G) and viscous (G) modulus of thesample, which were further used to locate the mechanicaltransition between sol and soft solid, the limit being taken whenG � G. Additional f low measurements were also carried outin controlled shear-rate mode.

Most SAXS experiments were carried out on beamline A2 atHASYLAB by using a fixed wavelength of 0.15 nm and a sampleto detector distance of 3 m. Bidimensional scattering patternswere collected on a CCD camera, and the curve intensities vs. q(q � 4�sin���, where 2� is the scattering angle and � thewavelength) were obtained by integrating the data in the direc-tion of the anisotropic pattern. Additional SAXS experimentswere performed with an in-house setup, using a wavelength of0.154 nm, a sample to detector distance of 1 m, and a 1-Tpermanent magnet. All samples were held in cylindrical Linde-man glass capillaries with diameters of 1 mm.

Polarized-light microscopy observations were performed withan Olympus (Tokyo, Japan) microscope, and the samples wereheld in flat glass capillaries (Vitrocom, Mountain Lakes, NJ).

Beam time at HASYLAB was supported by the European CommunityResearch Infrastructure Action under FP6 ‘‘Structuring the EuropeanResearch Area’’ Program Contract RII3-CT-2004-506008 (through theIntegrated Infrastructure Initiative ‘‘Integrating Activity on Synchrotronand Free Electron Laser Science’’).

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