Observations of orientational ordering in aqueous suspensions of a nano-layered silicate

We have studied colloidal suspensions of clay particles in aqueous salt solutions. These suspensions make excellent model systems for the study of interactions between plate-shaped particles, due to the inherent possibility of tuning their electrostatic repulsion with the concentration of the salt. Various gel and sol structures are possible, including nematic liquid crystalline order, although only qualitative identification of the latter in clay colloids has been available so far. Here, we briefly review our earlier synchrotron X-ray diffraction from gravity dispersed solutions of Na fluorohectorite, a synthetic swelling clay, over a large NaCl concentration range. Our use of liquid X-ray scattering techniques allowed us to identify regions in which particles reorient from horizontal to vertical alignments in strata coexisting at different heights within the sample. We thus identified two distinct gel regions characterized by differences in orientational anisotropy and domain size. In addition, we for the first time, present visual observations of birefringence in these systems; these new observations support the interpretation of our X-ray experiments, and thus our new results provide further evidence for nematic order.