Distribution of Water and Counterions in Vermiculite Clays

Abstract

Williams, Soper, Skipper, and Smalley (J. Phys. Chem. B 1998, 102, 8945) reported neutron scattering results on the counterion−water distribution vermiculite clay surfaces, in which a bilayer of water was found to be preferred at the surface rather than the counterions. This observation is in contrast to the predictions of the Poisson−Boltzmann equation, as pointed out by these authors. We report calculations on a one-dimensional adsorption model that includes nearest-neighbor interactions for the competition between water and the counterion for the lattice site of the clay surface. The association for either species is assumed to be of electrostatic origin, where the molecular parameters are based on the known structures of water, the counterion, and the clay surface. With no adjustable parameters these calculations indicate that less than 15% of the sites are “bound” with the counterions. These results suggest that it is the much larger concentration of the water and the confined volume between the clay...