Microscopic simulations of interlayer structure and dynamics in bihydrated heteroionic montmorillonites

To simulate the diffusion of a tracer Cs+ in very compacted bentonites, the interlayer spaces of two bihydrated Na-montmorillonites containing a small quantity of cesium are studied by computer simulations (Monte Carlo and molecular dynamics). The calculated structural properties show that water and sodium cations behave in the same way as in pure homoionic Na-montmorillonites. The study of the dynamical properties shows that simulated self-diffusion coefficients of water are in agreement with short time coefficients measured by neutron scattering. For the ions, the experimental values are related to macroscopic long-time motion and are not directly comparable to simulations. However, the introduction of simulated values in macroscopic models gives results in agreement with tracer experiments.