Distribution of water and counterions in vermiculite clays

Williams, Soper, Skipper, and Smalley (J. Phys. Chem. B 1998, 102, 8945) re ported neutron scattering results on the counterion-water distribution verm iculite clay surfaces, in which a bilayer of water was found to be preferre d at the surface rather than the counterions. This observation is in contra st 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 wa ter and the counterion for the lattice site of the clay surface. The associ ation for either species is assumed to be of electrostatic origin, where th e molecular parameters are based on the known structures of water, the coun terion, and the clay surface. With no adjustable parameters these calculati ons indicate that less than 15% of the sires are "bound" with the counterio ns. These results suggest that it is the much larger concentration of the w ater and the confined volume between the clay surfaces that is responsible for the reported water-counterion distribution.