HYDRATION TRANSITIONS IN A WEAKLY-HYDRATED FLUOROHECTORITE - A HENDRICKS-TELLER ANALYSIS

摘要

We study samples obtained by dry-pressing colloidal suspensions of Na- fluorohectorite clay particles, under a uniaxial constraint. The resulting clay compounds are assemblies of particles consisting of 80 fluorohectorite platelets that stack by sharing an intercalated cation (Na~+). In addition to water films possibly remaining in the mesoporous space around the particles, there can be water molecules intercalated within the stacks: depending on the ambient temperature and humidity level, water molecules are incorporated in a stepwise manner, and the preferred configurations of molecular packing are denoted in terms of intercalated water layers". Such configurations have been studied in natural clays like vermiculite or montmorillonite, and have been shown to be true thermodynamic equilibrium states [1]. Each hydration state corresponds to a well-defined platelet separation, which is conveniently measured by monitoring the X-ray scattering peaks due to the Bragg diffraction by the clay nano-stacks. In particular, Na-fluorohectorite exhibits at least three hydration states (OWL, 1WL, 2 WL), whose characteristic platelet separations have been mapped out as a function of temperature at very high and very low relative humidity [2].