Molecular simulations of swelling clay minerals

The authors carried out mol. simulations in the grand-canonical ensemble of water and cations in Wyoming and Arizona montmorillonite clay minerals, with varying relative humidity. Several water models and cations are used to investigate the swelling of these clays. The authors show how the water content depends on the type of clay, type of cation, the basal spacing, and the relative humidity. Related to the layering of water mols. in the interlayer space, the pressure normal to the clay sheets oscillates as a function of the basal spacing. Min. in corresponding free energy curves indicate the presence of dehydrated states and layered hydrates. The development of these stable states and the corresponding basal spacings are in agreement with exptl. data. D. profiles show significantly different interlayer structures depending on the type of clay and models used. A relation is shown between the formation of two-layer hydrates and the position of the cations. The simulations with the MCY water model underestimate the spacings of two- and three-layer hydrates, whereas simulations with the TIP4P model produce a better agreement. Therefore, the TIP4P model is recommended for simulating clay minerals. In addn., remarkable ordering of cations and water mols. in a one-layer Arizona montmorillonite hydrate is reported. [on SciFinder (R)]