Swelling Pressure in Systems with Na-Montmorillonite and Neutral Surfaces: A Molecular Dynamics Study

In the present work, we used molecular dynamics simulations to evaluate the distance between montmorillonite layers, and between montmorillonite and a neutral surface for a range of applied pressures as in an oedometer-like setup. The neutral surface was represented by a layer of pyrophyllite. Due to explicit water in the simulation, the resulting osmotic pressure and basal distance relationship differs from that of DLVO theory for both interlayer types. In agreement with experiment, basal distances change abruptly between hydration states at certain pressures. Within a given hydration state, the basal distances were found to vary with applied pressure also seen experimentally. For the pressure range in the study, we found that water adjacent to neutral surfaces has interlayer character. The hydration state of the interlayer between montmorillonite and the neutral surface was found to be either the same or one water layer less compared to the pure montmorillonite interlayer. Water densities in this region were determined, and the results show no significant difference from bulk density as some other studies reported. We also looked into reswelling and found that it occurred without hysteresis.