Molecular-scale model for the mass density of electrolyte solutions bound by clay surfaces: Application to bentonites

A model to simulate the density of solutions adsorbed onto clay mineral surfaces is proposed. In this model, the alteration of the ionic distribution caused by the electric field associated with the surface charge of clay platelets is accounted for using an electrical triple-layer model with an overlapping diffuse layer. The combined effects of ion hydration and the electric field on the structure of water are introduced through their influence on the partial molar volume of water. This model, applied to Na-montmorillonite, simulates the distribution of the interplatelet solution density as a function of the distance to the mineral surface. High densities in the direct vicinity of the surface and slightly lower density (a few percent) than the normal density in the diffuse layer are obtained. These results show good consistency with the available data on bentonite and with the densities that can be inferred from molecular dynamics simulations. This model shows that the interplatelet distance plays an important role in the distribution of the mass density of the solution in the pore space of clay rocks.

Calculated density for a Na-montmorillonite with an equilibrium solution concentration Cf=10−3 molL−1 and an interplatelet half-distance r of 7 nm. MD stands for molecular dynamics simulation and data* for reconstructed profile using the data of Fig. 1 (see text).Figure optionsDownload as PowerPoint slide