Attractive double-layer interactions between calcium clay particles

The mechanism responsible for restricted swelling of calcium clays in water and in aqueous salt solutions is examined. Particular attention is given to the montmorillonite, vermiculite, and illite systems. The diffuse double-layer interactions are calculated using an advanced statistical mechanical method, the Anisotropic Hypernetted Chain approximation. In contrast to the predictions of the simple Poisson-Boltzmann theory, which is based on inadequate approximations, for divalent ions the double-layer interaction is strongly attractive at relatively small surface separations, provided the density of surface charge is reasonably large. The attraction is a consequence of the correlations between the ions, giving rise to an electrostatic fluctuation force. When the van der Waals forces between the dielectric media are included, the attraction becomes even larger. The attractive force between the Ca-clay particles gives rise to a stable state, a potential minimum. No specific binding or hydration effects involving the Ca²⁺ ions are needed to explain the existence of the minimum. However, the position of the potential minimum as derived using the model, based on diffuse double-layer and van der Waals forces only, is influenced by hydration interactions. Some implications of the potential minimum are discussed.