The dynamics of water and cations in pure and mixed Na-Cs-montmorillonite were investigated as a function of the interlayer water content by using Monte Carlo and classical molecular-dynamics methods. It was found that, at low degrees of hydration, the absolute diffusion coefficient of Cs+ was less than of Na+. For greater hydration states, the diffusion coefficient of Cs+ was greater than that of Na+. An analysis of the ratio of the diffusion coefficient of an ion in the interlayer to its diffusion coefficient in bulk water revealed that water and Na+ were always less retarded than Cs+. For large interlayer water contents, tetralayer or more, the Na+ ions preferentially formed outer-sphere complexes. The mobility perpendicular to the clay surface was limited, and the diffusion was equivalent to two-dimensional diffusion in bulk water. In contrast, Cs+ ions preferentially formed 'inner-sphere complexes' at all hydration states and their two-dimensional diffusion coefficient was less than in bulk water. It was unclear why experimentally measured relative diffusion coefficients of Cs+ in the interlayers were some 20 times lower than those obtained using classical molecular dynamics studies.

Diffusion of Na and Cs in Montmorillonite. G.Kosakowski, S.V.Churakov, T.Thoenen: Clays and Clay Minerals, 2008, 56[2], 190-206