On Viscoelastic, Birefringent, and Swelling Properties of Laponite Clay

be defined, an isotropic liquid, an isotropic gel, and a birefringence gel, where ... the isotropic and the birefringent gel coincides with the line w...
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Langmuir 1998, 14, 4718-4723

On Viscoelastic, Birefringent, and Swelling Properties of Laponite Clay Suspensions: Revisited Phase Diagram A. Mourchid,* E. Le´colier, H. Van Damme, and P. Levitz* Centre de Recherche sur la Matie` re Divise´ e, Centre National de la Recherche Scientifique, 45071 Orle´ ans Cedex 2, France Received January 29, 1998. In Final Form: June 4, 1998 Relations between thermodynamics, structural, and mechanical properties of Laponite suspensions were recently discussed in the literature. One important issue concerning the liquid/gel transition of the Laponite suspensions is to understand why a mechanical gel appears concomitantly with what appears as an incomplete nematic transition. To get some insight, we first give a more extended characterization of the viscoelastic properties of these suspensions near the liquid/gel transition. For this purpose, stress relaxation experiments are compared to direct determinations of the viscoelastic modulus in the frequency domain. This permits the following of viscoelastic properties, in the linear regime, on a very extended scale, from 10-5 to 102 rad/s. The data show that the relaxation mechanisms are very slow and are compatible with the presence of a large scale structural organization compared to the elementary particle size. The elastic modulus follows the power law: G′ ) A(C - C0)R. Only the concentration threshold varies with the ionic strength. In a second part, we compare, on the same system, how the osmotic pressure and the birefringent properties are correlated. As already shown by Gabriel et al., three optical domains can be defined, an isotropic liquid, an isotropic gel, and a birefringence gel, where numerous threadlike defects highly reminiscent of nematic texture are observed. An interesting new result is seen, a line that separates the isotropic and the birefringent gel coincides with the line where the plateau of the osmotic pressure ends up. Recalling that the osmotic plateau starts just at the liquid/solid transition, we propose a more complete phase diagram exhibiting a pseudobiphasic region with no macroscopic phase separation.

Introduction Clay colloid suspensions are involved in a large number of applications such as gelling agents, oil drilling fluids, and other important industrial processes. This interest is driven by the diversity of the phase diagram of these colloidal suspensions. We can get a Newtonian liquid, a viscoelastic gel, or a flocculated dispersion.1-3 These properties can be tuned by varying certain parameters such as the clay concentration or the ionic strength. The platelike shape and the size of the colloidal clay particles play a crucial role in the transition occurrence from one state to another.4-7 Relating microstructure to macroscopic flow behavior is an important issue. Recently, suspensions of Laponite emerged as one of the best material for fundamental studies of clay colloidal dispersions.8-13 This synthetic clay has a high chemical purity. Its elementary particles have small anisotropy * To whom correspondence should be addressed. E-mail: [email protected] and [email protected]. (1) van Olphen, H. An Introduction to Clay Colloid Chemistry; Wiley: New York, 1997. (2) Rand, B.; Pekenc, E.; Goodwin, J. W.; Smith, R. W. J. Chem. Soc., Faraday Trans. 1 1980, 76, 225. (3) Sohm, R.; Tadros, Th. F. J. Colloid Interface Sci. 1989, 132, 62. (4) Langmuir, I. J. Chem. Phys. 1938, 6, 873. (5) Forsyth, P. A.; Marcelja, S.; Mitchell, D. J.; Ninham, B. W. Adv. Colloid Interface Sci. 1978, 9, 37. (6) Mourchid, A.; Delville, A.; Lambard, J.; Le´colier, E.; Levitz, P. Langmuir 1995, 11, 1942. (7) Mourchid, A.; Delville, A.; Levitz, P. J. Chem. Soc., Faraday Discuss. 1995, 101, 275. (8) Dijkstra, M.; Hansen, J. P.; Madden, P. A. Phys. Rev. Lett. 1995, 75, 2236. (9) Ramsay, J. D. F. J. Colloid Interface Sci. 1986, 109, 441. (10) Gabriel, J.-C. P.; Sanchez, C.; Davidson, P. J. Phys. Chem. 1996, 100, 11139. (11) Pignon, F.; Piau, J.-M.; Magnin, A. Phys. Rev. Lett. 1996, 76, 4857. (12) Willenbacher, N. J. Colloid Interface Sci. 1996, 182, 501. (13) Kroon, M.; Wegdam, G. H.; Sprik, R. Phys. Rev. E 1996, 54, 6541.

compared to other natural clay materials. Relations between thermodynamics, structural, and mechanical properties of Laponite suspensions were recently discussed in the literature. In our previous paper,6 we have shown that the suspensions of Laponite RD undergo a liquid/gel transition without macroscopic phase separation as the particle concentration increases. Shear rheology and osmotic pressure were used to locate the transition line between the liquid (sol) and the gel. Two main results were observed. First the liquid/gel transition is shifted to lower concentrations when ionic strength increases above 10-4 M. Second, looking at the osmotic pressure evolution with particle concentration for a defined ionic strength, we have observed a plateau beginning just above the liquid/gel transition. This plateau ends at a more higher particle concentration. More recently, Gabriel et al. have observed typical nematic liquid-crystal textures for Laponite B suspensions in the gel or solid phase.10 At low particle concentration, in the sol or liquid phase, the suspensions observed between cross-polarizers appear isotropic. In the gel phase two successive domains are observed: an optically isotropic gel at low particle concentration where pretransitional effects are observed and a permanent nematic one at a more higher concentration. This study was focused on the nematic liquidcrystal texture of Laponite. Such evolutions are not obvious for a classical isotropic-nematic phase transition where an isotropic liquid phase at low solid fractions is followed by a biphasic region, a nematic liquid, and finally a nematic gel at high solid fraction (case of V2O5 suspensions14,15). One important issue concerning the liquid/gel transition of the Laponite suspensions is to understand why a viscoelastic gel appears concomitantly with what (14) Davidson, P.; Garreau, A.; Livage, J. Liq. Crystallogr. 1994, 16, 905. (15) Davidson, P.; Bourgaux, C.; Schoutteten, L.; Sergot, P.; Williams, C.; Livage, J. J. Phys. II 1995, 5, 1577.

S0743-7463(98)00117-6 CCC: $15.00 © 1998 American Chemical Society Published on Web 07/30/1998

Properties of Laponite Clay Suspensions

Langmuir, Vol. 14, No. 17, 1998 4719

appears as an incomplete nematic transition. To get some insight, the aim of our work is 2-fold. First, we will give a more extended characterization of the viscoelastic properties of these suspensions near the liquid/gel transition. Second, we will compare, on the same system (Laponite RD), how the osmotic pressure and the birefringent properties are correlated. This second part will permit discussion of the status of the osmotic plateau and allow a more complete phase diagram to be proposed. II. Experimental Section The dispersions were prepared by adding Laponite powder to aqueous solutions at constant pH and variable ionic strength. Laponite RD (Si8Mg5.45Li0.4H4O24Na0.7) is dispersed in water as individual platelike sheets having a thickness of 10 Å and an average diameter close to 300 Å. These particles bear a structural negative charge that is balanced by Na+ counterions located all around the microcrystalline particles, in the so-called ionic double layer. Positive and negative charges can appear on the edge due to dissociation of amphoteric surface acids such as -MgOH, -LiOH, or -SiOH. To limit the appearance of positive lateral charges and to avoid congruent dissolution of the particles with time, the pH of the suspensions was fixed to 10 by addition of NaOH.16 The ionic strength was changed using successive addition of NaCl. The samples were then stirred with the aid of an homogenizer at high speed for 10 min, sealed, and stored for 1 week under nitrogen at room temperature prior to any measurements. This protocol of preparation allowed homogeneous transparent dispersions with clay concentrations (C) ranging from 0 to 3% (w/w) and ionic strengths (I) ranging from 10-4 to 10-2 M to be obtained. Above I ) 2 × 10-2 M, flocculation of the dispersions is observed. The osmotic stress method under N2 was used in order to get higher solid concentrations. Longterm storage in a glovebox under N2 atmosphere prevents contamination of the suspensions by atmospheric carbon dioxide. As shown elsewhere,17 this contamination promotes acidification of the samples and induces a slow but noticeable release of Mg2+ in water solution. The net result is a global increases of the ionic strength that is associated with a structural and a mechanical evolution of Laponite suspensions. The rheological experiments were divided into two sets. (i) A Carri-med CSL 50 (a controlled stress rheometer) was used to obtain the frequency variation of viscoelastic modulus of the suspensions in the range of 10-2 to 10+2 rad/s. The sample was placed between a cone and a plate, which were sealed within a thermostated housing with constant humidity, at 20 °C. Two types of experiments were performed. The first one permits following the reconstitution of the gel structure which was disturbed during the setting up of the sample between the cone and the plate (the geometry used). The material was subject to weak sinusoidal excitation and the complex viscoelastic modulus, G′ + iG′′, was recorded versus time. When the time curves of G′ and G′′ have reached a plateau (typically after 3 h), it was considered that the suspension had recovered to its equilibrium gel state. In the second experiment, the evolutions of G′ and G′′ versus frequency were measured in the linear elastic domain of the sample where the viscoelastic modulus is independent of the applied stress. This domain of linear viscoelasticity of the suspensions was determined by a stress sweep at constant frequency. In the liquid phase but near the liquid/gel transition, continuous shear experiments were performed in order to determine the zero shear rate viscosity. (ii) Measurements were also made using a controlled shear rate rheometry (Haake RV20), at room temperature and controlled humidity level. The method consisted of a vane immersed in the suspension and rotating at constant low speed. The resulting stress that appears within the material was recorded as a function of time. The measurement can be recorded during several hours. The geometry of the vane consisted of six thin (16) Thompson,D. W.; Butterworth, J. T. J. Colloid Interface Sci. 1992, 151, 236. (17) Mourchid, A.; Levitz, P. Phys. Rev. E 1998, 57, R4887.

Figure 1. Evolution of the relative zero shear rate viscosity (b) versus the volume fraction for Laponite suspensions at I ) 5 × 10-3 M. The broken line represents eq 2 (Krieger-Dougherty equation). blades located at equal angles around a cylindrical shaft. The dimensions of the blades were 22 mm diameter and 16 mm height. Before the experiment was started, the gel was allowed to relax during about 3 h, as the material was disturbed by the immersion of the vane. Stress evolution during deformation and stress relaxation well after cessation of the rotation are recorded. Measurements of the stress relaxation function have been achieved either for fixed C with I variable or for C variable with I constant. The purpose is to apply a finite deformation and to measure the resulting stress during the period of application of this deformation and after ceasing it. The first part of the curve is an almost linear increase of the stress with time. The slope of this curve, (dτ/dt)t)0, leads to the determination of the elastic modulus at high frequency, G′ (ω . 1 rad/s), according to the equation18

G′ )

Rc2 - R2 dτ 2ΩRc2 dt

(1)

where R and Rc are the inner and outer radius, respectively, of the geometry used and Ω is the angular velocity of the vane. The second part of the stress-time curve, obtained after cessation of the sample shearing, represents the relaxation function of the gel. The birefringence of the samples was studied between cross polarizers using test tubes of 2 cm diameter filled with 20 cm3 of suspension. A more detailed observation was performed with an optical microscope using flat glass capillaries of 0.4 mm thickness.

III. Viscoelactic Properties near the Liquid/Solid Transition 1. Viscoelastic Properties of the Liquid Phase near the Transition. In the liquid phase but near the liquid/gel transition, continuous shear experiments were performed. The effective viscosity decreases as the shear rate increases. Interpolation at zero shear rate permits estimation of the zero shear rate viscosity, η0. For a defined ionic strength of 5 × 10-3 M, the evolution of η0/ηwater as a function of particle volume fraction is shown in Figure 1. A strong divergence is observed near a volume fraction Φ0 ) 0.0023. This limit is in good agreement with the appearance of a yield stress. As shown in Figure 1, a Krieger-Dougherty type equation can be used to fit the (18) Alderman, N. J.; Meeten, G. H.; Sherwood, J. D. J. NonNewtonian Fluid Mech. 1991, 39, 291.

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Mourchid et al.

a

b Figure 3. Shear stress relaxation function for different Laponite suspensions with different solid concentrations and different ionic strengths. The curves are normalized by the G′ at high frequency.

time, it is possible to draw a master curve for these different relaxation curves. This implies that the relaxation time, t0, and the exponent, a, do not change strongly with C and I. Only the elastic modulus increases with either the concentration of Laponite or the ionic strength. In the approximation of linear response of a viscoelastic material, it is possible to get back the complex viscoelastic modulus, G(ω), from the temporal evolution of the stress written as

τ(t) ) Figure 2. (a) The applied shear rate and the resulting deformation as a function of time, used in the vane geometry. (b) Transient evolution of the shear stress for a viscoelastic suspension of Laponite of C ) 3% w/w and I ) 2 × 10-3 M.

experimental data. In such a case, we have

η0 ηwater

) (1 - φ/φ0)-τ

(2)

[ ( )] t t0

a

(3)

The value of the exponent a is nearly 0.45. As shown in Figure 3 where the deformation was applied during a fixed

(4)

where µ(t) is the stress relaxation function and γ˘ (τ) is the time derivative of the deformation. Using Fourier transform of eq 4, we can write

G(ω) ) G′(ω) + iG′′(ω) )

with Φ0 ) 0.0023 and τ ) 0.9. Above an ionic strength of 10-4 M, Φ0 decreases as the ionic strength increases. 2. Linear Viscoelastic Properties of the Gel near the Transition. Figure 2a shows the applied shear rate and deformation used in the vane geometry. The resulting shear stress for a clay dispersion of C ) 3% (w/w) and I ) 2 × 10-3 M is shown in Figure 2b. The relaxation function decreases very slowly and shows no evidence of complete relaxation even after several hours. Moreover, it is relatively hard to see an asymptotic plateau at long time. This is indicative of a viscoelastic solid behavior having slow or very slow relaxation mechanisms. A similar behavior was observed for different clay concentrations and ionic strengths in the viscoelastic gel phase. The relaxation part of the curves exhibits a nonexponential variation. However, it was possible to model the evolution with a stretched exponential decay with a relaxation time, t0, of about 10000 s

exp -

∫0tµ(t - t′)γ˘ (t′) dt′

τˆ˘ (ω) γˆ˘ (ω)

(5)

where Fourier transforms of the time derivative of either the stress or the deformation are explicitly computed. A typical comparison between temporal relaxation and direct determination of the complex viscoelastic modulus is presented in Figure 4. Some oscillations can be observed on the computed G′ and G′′ curves. These artifacts are mainly due to a lack of accuracy concerning the rising and the decreasing parts of the shear rate that follows, as shown in Figure 2a, a step function. However, comparison between the two sets of data is good. The G′ and G′′ spectra obtained from the Fourier transform method overlap the direct experimental data over 3 orders of magnitude of the frequency scale, typically from 10-2 to 10 rad/s. The lower frequency accessible to the stress relaxation is on the order of 10-5 rad/s. One can see in Figure 4 that G′ varies slowly with the frequency. The G′′ shows an increase as the frequency decreases, but no maximum seems to be reached. One does not observe any intersection of G′ and G′′ in the accessible frequency range, and no defined correlation time can be estimated from such results. Such an agreement between the two sets of data permits checking that there is no experimental problem related to the specific geometry used for each experiment. Slipping at the interface sample/geometry, mainly when cone-plane geometry is used, is certainly negligible. Another interesting point is that one can extend the

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Langmuir, Vol. 14, No. 17, 1998 4721

Figure 4. Frequency response of the storage (G′) and loss (G′′) moduli for a Laponite suspension of C ) 3% w/w and I ) 2 × 10-3 M: (ss) experimental spectra measured with the Carrimed rheometer; (b and O) the computed spectra of G′ and G′′ obtained by using the Fourier transform of the transient shear stress.

Figure 5. Variation of the elastic modulus (G′) as a function of particle concentration at an ionic strength I ) 10-2 M. The curve corresponds to eq 5.

domain of frequency of the viscoelastic modulus spectra by using the Fourier transform of the transient stress recorded during a relatively long period of time (several hours). In conclusion, application of eq 5 confirms our previous results6,7 concerning the weak evolution of G′ and G′′ within the accessible frequency range. We have now two distinct ways to measure the elastic modulus at high frequency (see eq 1). A systematic determination of this parameter was conducted at fixed ionic strength and for different particle concentrations. As already mentioned elsewhere, the first important observation is the existence of a particle concentration below which the suspensions do not have a yield stress and where G′(ω) is lower than G′′(ω).6,7 Above this threshold, transient stress experiments exhibit a yield stress and G′ increases with the particle concentration as shown in Figure 5. This evolution is neither linear nor algebraic. Interesting enough, at fixed ionic strength, G′ can be written as

Figure 6. Variation of the elastic modulus (G′) as a function of (C - C0): (O) I ) 10-2 M and C0 ) 0.3% w/w; (2) I ) 6 × 10-3 M and C0 ) 0.45% w/w; (b) I ) 5 × 10-3 M and C0 ) 0.6% w/w; ([) I ) 2 × 10-3 M and C0 ) 1.35% w/w; (×) ) 10-4 M and C0 ) 1.85% w/w. The continuous curve follows eq 5.

which a viscoelastic gel appears. Moreover, the elastic modulus is only sensitive to this transition point C0. All the G′ data collected on the Laponite suspensions, by using the Haake and the Carri-med rheometers, for different clay concentrations and ionic strengths can be plotted versus (C - C0). We can see in Figure 6 that all the experimental points are setting on a master curve given by eq 6. The results are that the amplitude A and the exponent R ()2.35) are independent of the clay concentration and of the ionic strength. This result suggests that the mechanical state of the suspensions of Laponite can be fixed by adapting the distance to the transition point C0. This parameter is only dependent on the ionic strength. As ionic strength increases, the clay concentration threshold C0 is shifted toward lower values. Such an evolution permits defining a transition line separating the viscous fluid from the viscoelastic gel as already discussed elsewhere.6,7 In this range of ionic strength, it is reasonable to assume the same mechanism of gelation, driven by the interplay of some interparticular interactions. The evolution of G′ above the sol/gel transition is relatively monotonic. However, it is relatively clear that the mechanical relaxation in the linear regime is dominated by long or very long time scales. This observation is supported by the form of G′ and G′′ spectra, which exhibit an evolution more pronounced at very low frequencies (10-5 to 10-4 rad/s, see Figure 4) and is in good agreement with several recent results.12 Such a mechanism of relaxation is compatible with the presence of large scale structural organization compared to the elementary particle size.6,11,19 IV. Revisited Phase Diagram Using Birefringent Properties and Osmotic Pressure

(6)

There is no permanent birefringence near the liquid/ gel transition. However an interesting experiment can be performed. For this, a gel prepared with particle concentration near the transition point was allowed to flow in a pipet of 1 cm diameter. The pipet was placed between cross-polarizers in the trajectory of a He-Ne laser beam. The axes of the polarizer and the analyzer were respectively parallel and perpendicular to the axis of the

where C0 is a good estimation for the solid fraction above

(19) Tassin, J.-F.; Robert, G.; Benyahia, L. Cah. Rhe´ o. 1996, 15, 238.

G′ ) A(C - C0)R

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pipet. At rest the suspensions did not show any permanent birefringence and the laser beam was not transmitted. When the suspension was starting to flow with a low average shear rate (∼10 s-1) two side intense bands, separated by a dark central band, were transmitted. When the flow was stopped, it took a few seconds for the sidebands to disappear. This experiment was similar to the one used by Langmuir for the study of the flow birefringence in some suspensions of colloidal particles such as bentonite clay.4 As noted by Langmuir, the anisotropic objects inside the suspension should mainly orient their long axis parallel to the pipet axis and perpendicular to the radius of the tube. This explains the observed flow birefringence. Two arguments suggest that the anisotropic objects are not constituted of single particles. (i) The ratio of the shear rate on the diffusion coefficient of a single particle leads to a very low Peclet number, ∼10-4, and seems to indicate that individual particles of Laponite cannot become oriented in these conditions. (ii) The time it takes for the sidebands to disappear when the flow stops is very long compared to the rotational time of a single particle of Laponite (∼10-4 s). In any case, such experiments and direct inspections at rest exhibit a region of the phase diagram, slightly above the liquid/gel transition line, where the gel is isotropic at rest and shows transient or flow birefringence under shear. At constant ionic strength and above some defined particle concentration, permanent birefringence is observed and numerous threadlike defects highly reminiscent of nematic texture are observed. These results obtained on Laponite RD are very similar to the work recently performed by Gabriel et al. on Laponite B.10 However, an interesting new result can be seen on Figure 7. The line which separates the isotropic and the birefringent gel coincides with the line where the plateau of the osmotic pressure ends up. Recalling that the osmotic plateau starts just at the liquid/gel transition, we can draw a more complete phase diagram exhibiting a pseudobiphasic region with no macroscopic phase separation. V. Discussion and Conclusion The classical suggestion for the gelation of the clay suspensions was given by van Olphen.1 The mechanism proposed was a microflocculation promoted by attraction between the oppositely charged edges and faces of the clay platelike particles and reinforced by van der Waals interactions, yielding to the “house of cards” structure. However, it is now admitted that this mechanism of gelation is favored only at high ionic strengths (>0.01 M NaCl for the Laponite suspensions), when the repulsive double layer potential is screened2,20 and at low pH where lateral amphoteric surface acid groups are positively dissociated, but where congruent dissolution is also acting.16 At low ionic strength, the potential barrier is sufficiently high to prevent the colloidal particles from coming into contact. Several recent studies using smallangle X-ray scattering measurements, transmission electron microscopy (TEM) observations of cryofractured gels, osmotic pressure determination and observation of nematic liquid-crystal textures were not able to demonstrate the existence of direct and sticky contact between particles.6,9,10,21-23 On the contrary these works have (20) Norrish, K. Discuss. Faraday Soc. 1954, 18, 120. (21) Avery, R. G.; Ramsay, J. D. F. J. Colloid Interface Sci. 1986, 109, 448. (22) Ramsay, J. D. F.; Lindner, P. J. J. Chem. Soc., Faraday Trans. 1993, 89, 4207.

Mourchid et al.

Figure 7. (a) Phase diagram for Laponite suspensions as obtained from (b) rheological, (O) osmometric, and (2) birefringence data: F, floc; IL, isotropic liquid; IG, isotropic gel; NG, nematic gel. (b) Evolution of the osmotic pressure as a function of particle concentration. C1 and C2 are the limits of the plateau where the suspensions are isotropic gels.

provided strong evidence for a more or less extended nematic order where face to face mutual orientation is favored. Our results and the work of Gabriel et al.10 reinforce theses conclusions. Structural organization of Laponite suspensions near the sol/gel transition reveals some hierarchy. Several length scales are involved. First of all, small-angle scattering reveals interparticular correlations. These structural correlations are compatible with the formation of orientational microdomains made of well-separated particles attempting to align their directors.6 Up to the micrometric size, these domains are distributed in space according a q-3 scattering law,6,22 i.e., in a more or less heterogeneous way for the translational order. At larger length scales, optical size nematic threaded textures are observed slightly above the liquid/gel transition. A similar mechanism of gelation had been already proposed by Bernal et al.24 to explain the formation of a gel phase in plant virus preparations (charged rodlike particle dispersions). According to these authors, the gel phase is due to the connection between finite nematic assemblies of particles named tactoids. (23) Morvan, M.; Espinat, D.; Lambard, J.; Zemb, Th. Colloid Surf., A 1994, 82, 193. (24) Bernal, J. D.; Fankuchen, I. J. Gen. Physiol. 1941, 25, 111.

Properties of Laponite Clay Suspensions

Mechanical relaxations near “equilibrium” are not exponential, very slow, and hardly connected with the individual dynamics of these nanometric particles. The hierarchy of the Laponite gel structure certainly plays a role. An interesting question concerns the connection between interparticular thermodynamics and large scale viscoelastic properties. This point is far from obvious when we look at the opposite evolution of the osmotic pressure and the elastic modulus as the ionic strength increases.6 Low-frequency mechanical properties can perhaps be connected with structural features appearing above the micrometer size, in good agreement which other recent works.11 However, one open question is to find a way to connect the different temporal and structural scales involved in the liquid/gel transition. In good agreement with ref 10, this work permits location of two transition lines. In the intermediate region, we show that the osmotic pressure transition is almost constant with the appearance of an isotropic gel at rest. Above the second line, at more higher particle concentration, permanent birefringent properties are observed reminiscent of a nematic texture. The intermediate region captures some properties of a biphasic regime. However two observations are not clearly understood: (i) there is no macroscopic phase separation; (ii) a mechanical transition from a liquid to a gel appears just at the beginning of the osmotic pressure plateau. It is tempting to consider that the first-order phase transition which appears to be

Langmuir, Vol. 14, No. 17, 1998 4723

missed is a nematic transition. The system is build from charged platelike particles having a small aspect ratio (less than 30). Two opposite interactions can induce a frustrated situation preventing the occurrence of a true first-order phase separation. First and most important are the effective excluded volume interactions, which tend to build a nematic order. However, electrostatic interactions between particles dressed with their ionic double layers favor perpendicular orientations,6 as is the case if one considers the total charge distribution inside and around a particle as a local electric quadrupole.8 Similar situations were recently reported for other slightly anisotropic particles.25 In both cases, suspensions of charged particles with a weak anisotropy form a gel phase well before the expected concentrations for a Onsager I-N transition.5,26 Moreover, competitions between electrostatic and effective excluded volume interactions raise a question concerning the status of this liquid/gel transition. Relation with a possible glass transition was recently discussed.13 Acknowledgment. It is a pleasure to thank Dr. P. Davidson for helpful and enlightening discussions on nematic textures. LA980117P (25) Buining, P. A.; Philipse, A. P.; Lekkerkerker, H. N. W. Langmuir 1994, 10, 2106. (26) Onsager, L. Ann. N. Y. Acad. Sci. 1949, 51, 627.