Molecular Simulation of Interlayer Structure and Dynamics in 12.4 Å Cs-Smectite Hydrates

Abstract

A detailed understanding of hydrated Cs-smectites is necessary to predict the permeability of clay liners to radiocesium cations at nuclear waste containment facilities. Monte Carlo (MC) and molecular dynamics (MD) modeling techniques were applied to three representative Cs-smectites to interpret a variety of experimental data on interlayer structure and dynamics. Spectroscopic and surface chemistry methods that attempt to differentiate interlayer water from water residing in micropores have provided data suggesting that, in stable 12.4 A Cs-smectite hydrates, the interlamellar water content is less than one-half monolayer. Convergence profiles in MC simulations predicted stable hydrates at interlayer water contents of 1/3 or possibly 2/3 water monolayer. Radial distribution functions and coordination number data illustrated the ability of Cs+ to organize water molecules into partial hydration shells and displayed the distortions of water structure induced by the clay surface. Molecular dynamics simulations of the MC-stable Cs-smectites revealed interlayer Cs+ to be strongly bound as innersphere surface complexes, in agreement with published bulk diffusion coefficients. The strongly adsorbed Cs+ can be associated with one of the species identified in 133Cs NMR spectroscopic studies of hydrated Cs-smectites. These cations typically exhibited jump diffusion, whereas continuous diffusion of H2O occurred.