Microscopic simulation of structure and dynamics of water and counterions in a monohydrated montmorillonite

Models of swelling clays are studied by computer simulations (Monte Carlo and molecular dynamics). We focus on the comparison of structural and dynamic properties of two montmorillonites with different kinds of counterions Na+ and Cs+. The calculated values are compared with available experimental quantities such as interlayer spacing as a function of water content and diffusion coefficients of both water molecules and counterions in the monohydrated state. The results are consistent with experimental values and previous simulations. For the dynamics, the short time behavior of water as observed with quasielastic neutron scattering is in agreement with simulated one. For the ions, the experimental values are related to macroscopic long time motions and are much smaller than the short time values calculated from MD. Thus, the present study provides a detailed insight in the microscopic dynamics of ions related to the structure of the clay: it is shown that Cs+ diffuse faster than Na+ and that the arrangement of clay surfaces plays a significant role in the choice of the sites occupied by the cations as well as in their mobility.