Observation of nematic liquid-crystal textures in aqueous gels of smectite clays

Aqueous suspensions of two smectite clays, natural bentonite and synthetic laponite B, were prepared at concentrations ranging from 0.01 to 0.1 g/cm(3). Viewed at rest between crossed polarizers, concentrated gel suspensions (> 0.024 g/cm(3) for bentonite and > 0.020 g/cm(3) for laponite B) show optical birefringence which demonstrates the existence of a liquid-crystalline order of the platelike particles extending over macroscopic distances. The textures of these birefringent gels observed with polarized light microscopy are typical of a nematic phase. At concentrations slightly below that of the phase transition, the suspensions show strong pretransitional effects such as shock or flow birefringence. For both systems, large (approximate to 1 cm(3)) macroscopically oriented domains of the nematic phase were grown by slowly drying isotropic suspensions. The phase diagrams of these two systems versus clay and NaCl concentrations have been established. Unexpectedly, the nematic phase of bentonite is stabilized with increasing NaCl concentration; that of laponite B is not sensitive to it. Finally, the origin of the liquid-crystalline character of these suspensions and its consequences upon their physical properties are briefly discussed in the light of previous experimental studies published in the literature.