Liquid-crystalline aqueous clay suspensions.
This article demonstrates the occurrence of a true isotropic/nematic 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 strength/volume 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 sol/gel transition hinders the isotropic/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.
Michot
Laurent J
LJ
Laboratoire Environnement et Minéralurgie, Nancy Université Unité Mixte de Recherche 7569, Centre National de la Recherche Scientifique-Institut National Polytechnique de Lorraine, BP40, 54501 Vandoeuvre Cedex, France.
Bihannic
Isabelle
I
Maddi
Solange
S
Funari
Sérgio S
SS
Baravian
Christophe
C
Levitz
Pierre
P
Davidson
Patrick
P