We have carried out molecular 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. We 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 molecules in the interlayer space, the pressure normal to the clay sheets oscillates as a function of the basal spacing. Minima 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 experimental data. Density profiles show significantly different interlayer structures depending on the type of clay and models used. We show a relation between 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 our simulations with the TIP4P model produce a better agreement. Therefore, we recommend the TIP4P model for simulating clay minerals. In addition, we report remarkable ordering of cations and water molecules in a one-layer Arizona montmorillonite hydrate.