Smectic arrangement of bentonite platelets incorporated in gels of poly(acrylamide) induced by the interaction with cationic surfactants

Conformational and structural transitions in composite gels of poly(acrylamide) with incorporated suspensions of bentonite clay platelets induced by the interaction with cationic surfactants were investigated. X-ray diffraction measurements demonstrate that due to the interaction with cationic surfactants initially disordered clay platelets can arrange into highly ordered smectic structures incorporating about 5-6 layers. The corresponding conformational transition is accompanied by shrinking of the gels. Small-angle X-ray scattering data show that the self-ordering of the platelets increases with both the concentration of the platelets and the length of the hydrophobic tail of the surfactant. While moderately cross-linked gels do not inhibit the formation of smectic organo-clay aggregates, larger concentrations of highly cross-linked gels clearly limit the process. It was demonstrated that the structure and properties of the gels depend on their previous history. In gels dried on glass surfaces, the smectic ordering becomes anisotropic and the platelets orient parallel to the substrate. Models of the smectic polymer/clay/surfactant complex in the gel phase are formulated and discussed.