Diffusion of Na and Cs in montmorillonite

The state and dynamics of water and cations in pure and mixed Na-Cs-montmorillonite as a function of the interlayer water content were investigated in the present study, using Monte Carlo and classical, molecular-dynamics methods. While highly idealized, the simulations showed that the swelling behavior of hetero-ionic Na-Cs-montmorillonite is comparable to the swelling of a homo-ionic Na- or Cs-montmorillonite. The mixed Na-Cs-montmorillonite is characterized by intermediate interlayer distances compared to homo-ionic Na- and Cs-montmorillonites. Dry, hetero-ionic Na-Cs-montmorillonite is characterized by a symmetric sheet configuration, as is homo-ionic Cs-montmorillonite. We found that at low degrees of hydration the absolute diffusion coefficient of Cs is less than for Na, whereas at greater hydration states the diffusion coefficient of Cs is greater than for Na. An analysis of the relative diffusion coefficients (the ratio between the diffusion coefficient of an ion in the interlayer and its diffusion coefficient in bulk water) revealed that water and Na are always less retarded than Cs. With large interlayer water contents, tetralayer or more, Na ions preferentially form outer-sphere complexes. The mobility perpendicular to the clay surface is limited and the diffusion is equivalent to two-dimensional diffusion in bulk water. In contrast, Cs ions preferentially form 'inner-sphere complexes' at all hydration states and their two-dimensional diffusion coefficient is less than in bulk water. The question remains unanswered as to why experimentally derived relative diffitsion coefficients of Cs in the interlayer of clays are about 20 times less than those we obtained by classical molecular dynamics studies