Na/Cs montmorillonite: temperature activation of diffusion by simulation

The effect of temperature in the range of 0–150 °C was studied for homoionic montmorillonite clays with Na+ and Cs+ compensating ions in low hydration states. Both static and dynamic information concerning the interlayer ions and water molecules was obtained by Monte Carlo and molecular dynamics. Principal structural changes were limited to the water phase, marked difference was observed in the modes of diffusion of Cs+ and Na+ ions. Cs+ ion exhibited a clear site-to-site diffusion between sites allowing coordination to six oxygen atoms of the clay sheets, this behaviour persisted to high temperatures. Preferential sites for the Na+ counterion were significantly less well-defined, even at low temperatures. Water phase showed similar behaviour in the Na/Cs-monohydrated systems, a rapid approach to bulk dynamics was seen in the transition from monohydrated to bihydrated Na-montmorillonite.