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DIFFUSION OF Na AND Cs IN MONTMORILLONITE

Paul Scherrer Institut, 5232 Villigen PSI, Switzerland

^* E-mail address of corresponding author: georg.kosakowski{at}psi.ch

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 diffusion coefficients of Cs⁺ in the interlayer of clays are about 20 times less than those we obtained by classical molecular dynamics studies.

Key Words: Cs⁺ • Diffusion • Molecular Dynamics • Montmorillonite • Na⁺ • Swelling • Water