In-situ SAXS study of aqueous clay suspensions submitted to alternating current electric fields

Aqueous colloidal suspensions of clay platelets display a sol/gel transition that is not yet understood. Depending on the nature of the clay, liquid-crystalline behavior may also be observed. For example, the suspensions of beidellite display a nematic phase whereas those of montmorillonite do not. Both beidellite and montmorillonite have a "TOT" structure but the structural electric charge is located in the tetrahedral layer for the former and in the octahedral layer for the latter. We built a setup to perform SAXS experiments on complex fluids submitted to an electric field in situ. We found that the fluid nematic phase of beidellite suspensions readily aligns in the field. However, the field had no influence on the gels, showing that the orientational degrees of freedom of the platelets are effectively frozen. Moreover, strong platelet alignment was induced by the field in the isotropic phase of both clays, in a similar way, regardless of their ability to form a nematic phase. This surprising result would suggest that the orientational degrees of freedom are not directly involved in the sol/gel transition. The ability to induce orientational order in the isotropic phase of clay suspensions can be exploited to prepare materials of controlled anisotropy.