This article demonstrates the occurrence of a true isotropic͞nematic transition in colloidal Brownian aqueous suspensions of natural nontronite clay. The liquid-crystalline character is further evidenced by polarized light microscopy and small-angle x-ray scattering experiments in the presence and absence of modest external magnetic fields. The complete phase diagram ionic strength͞volume fraction then exhibits a clear biphasic domain in the sol region just before the gel transition in contrast with the situation observed for other swelling clays in which the sol͞gel transition hinders the isotropic͞ nematic transition. Small-angle x-ray scattering measurements of gel samples reveal strong positional and orientational orders of the particles, proving unambiguously the nematic character of the gel and, thus, clearly refuting the still prevalent ''house of cards'' model, which explains the gel structure by means of attractive interactions between clay platelets. Such order also is observed in various other swelling clay minerals; therefore, this very general behavior must be taken into account to reach a better understanding of the rheological properties and phase behavior of these systems.colloids ͉ liquid crystal ͉ phase transitions S welling clay minerals are layered compounds that bear a negative layer charge compensated by interlayer exchangeable cations whose valence and hydration properties control both swelling and colloidal behavior. One of the most important properties of swelling clay minerals is their ability to form yield stress materials when dispersed in water. This feature, extensively used in various industrial applications (drilling fluids, food industry, cosmetic industry, etc.), also plays a major role in many fundamental processes occurring at the Earth's surface, such as slipping processes in plate-boundary faults (1-4) or landslide triggering (5-9). For these reasons, numerous studies have focused on the rheology of aqueous clay suspensions with particular 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 aspect ratio typically ranging between 25 and 1,000, these materials should very likely form liquid-crystalline phases (16), such as those observed for rod-like clay particles, such as imogolite in aqueous media (17), or organophilic sepiolite clay particles in nonaqueous solvents (18). A phase transition was indeed observed by Langmuir (19, ** ) as early as 1938 in suspensions of natural hectorite swelling clay. However, all subsequent studies failed to reproduce this crucial observation and give evidence of a clear thermodynamic liquid-crystalline order but instead revealed a dominant gel formation (20). Such behavior is observed for both highly polydisperse natural samples (21) and synthetic monodisperse ones (22). The structure and formation mechanisms of the gel are still under debate. Indeed, although some of the gel features indicate nematic ordering (23,24)...

Natural Na-Wyoming montmorillonite was size fractionated by successive centrifugation. Polydisperse particles with average sizes of 400, 290, and 75 nm were then obtained. As the structural charge of the particles belonging to three fractions (determined by cationic exchange capacity measurements) is the same, such a procedure allows studying the effect of particle anisotropy on the colloidal phase behavior of swelling clay particles. Osmotic stress experiments were carried out at different ionic strengths. The osmotic pressure curves display a plateau whose beginning systematically coincides with the sol/gel transition determined by oscillatory stress measurements. The concentration corresponding to the sol/gel transition increases linearly with particle anisotropy, which shows that the sol/gel transition is not directly related to an isotropic/nematic transition of individual clay particles. Indeed, a reverse evolution should be observed for an I/N transition involving the individual clay particles. Still, when observed between crossed polarizer and analyzer, the gel samples exhibit permanent birefringent textures, whereas in the "sol" region, transient birefringence is observed when the samples are sheared. This suggests that interacting clay particles are amenable to generate, at rest and/or under shear, large anisotropic particle associations.