Layer Charge Effects on Adsorption and Diffusion of Water and Ions in Interlayers and on External Surfaces of Montmorillonite

Molecular simulations using classical force fields were performed to study the adsorption and diffusion properties of water and ions in the high-charge (Arizona-type) montmorillonite clays with varying relative humidity (RH) at 298.15 K. The simulation results of basal distances derived from swelling free energy curves and of water uptake are in good agreement with experiments. Overall, the simulated self-diffusion coefficients of the interlayer species are in reasonable agreement with experiments and lower than those estimated for the external surfaces. Influence of the magnitude of the layer charge was studied by comparing these simulation results with those obtained for the low-charge (Wyoming-type) montmorillonite clays. Most importantly, these comparisons confirm the experimental finding that the high-charge clay generally shifts swelling transitions toward lower RH values. Therefore, the adsorption and dynamics of water and ions are significantly different in the low- and high-charge clays near the transition RH values. We find that the amount of water in an interlayer hydration state is mostly independent of the layer charge, probably due to steric reasons. In contrast, the externally adsorbed water content increases with increasing layer charge. Furthermore, the mobility of water and ions is generally lower in the high-charge montmorillonite than in the corresponding low-charge system. However, mobility of cations in the mesopores of high-charge montmorillonite is typically higher than that of the corresponding low-charge system which might be attributed to the presence of the tetrahedral substitutions in the latter case.