OBSERVATION OF NEMATIC LIQUID-CRYSTAL TEXTURES IN AQUEOUS GELS OF SMECTITE CLAYS

Aqueous suspensions of two smectite clays, natural bentonite and synth etic laponite B, were prepared at concentrations ranging from 0.01 to 0.1 g/cm(3). Viewed at rest between crossed polarizers, concentrated g el suspensions (> 0.024 g/cm(3) for bentonite and > 0.020 g/cm(3) for laponite B) show optical birefringence which demonstrates the existenc e of a liquid-crystalline order of the platelike particles extending o ver macroscopic distances. The textures of these birefringent gels obs erved with polarized light microscopy are typical of a nematic phase. At concentrations slightly below that of the phase transition, the sus pensions show strong pretransitional effects such as shock or flow bir efringence. For both systems, large (approximate to 1 cm(3)) macroscop ically oriented domains of the nematic phase were grown by slowly dryi ng isotropic suspensions. The phase diagrams of these two systems vers us clay and NaCl concentrations have been established. Unexpectedly, t he nematic phase of bentonite is stabilized with increasing NaCl conce ntration; that of laponite B is not sensitive to it. Finally, the orig in of the liquid-crystalline character of these suspensions and its co nsequences upon their physical properties are briefly discussed in the light of previous experimental studies published in the literature.