Structure, Energetics, and Dynamics of Cs+ and H2O in Hectorite: Molecular Dynamics Simulations with an Unconstrained Substrate Surface

Classical molecular dynamics simulations were performed for the smectite clay hectorite with charge-balancing Cs+ cations using a newly developed structural model with a disordered distribution of Li/Mg substitutions in the octahedral sheet and the fully flexible CLAYFF force field. Calculations for systems with interlayer galleries containing 0-19 H2O/Cs+ suggest that the monolayer hydrate is the only stable state at all relative humidities at ambient pressure and temperature, in agreement with experimental results and previous molecular calculations. The basal spacing of this structure is also in good agreement with experimental values. In contrast to previous molecular modeling results, however, the new simulations show that interlayer Cs+ occurs on 2 different inner sphere adsorption sites: above the center of ditrigonal cavities and above Si tetrahedra. Unlike previous simulations, which employed a rigid clay model and fixed orientations of the structural -OH groups, the present results are obtained for an unconstrained clay substrate structure, where the structural -OH groups are able to assume various orientations, including being nearly parallel to the clay layers. This flexibility allows the Cs+ ions to approach the surface more closely above the centers of the hexagonal rings. In this structural arrangement, Cs+ ions are not hydrated by the H2O molecules which share the same interlayer plane, but rather by the H2O molecules coordinated to the opposite surface. In contrast, on the external basal surface, a significant fraction of H2O molecules are adsorbed above the centers of ditrigonal cavities adjacent to adsorbed Cs+ ions. For these H2O molecules, both HHZO atoms coordinate and H-bond to O-b surface oxygen atoms. The mean residence times for the Cs+-H2O, Cs+-O-b, and H2O-O-b coordination pairs show that Cs+ ions are more strongly coordinated with O-b atoms than H2O molecules. This result is the opposite of the behavior in Ca-hectorite, due to the much smaller hydration energy of Cs+ compared to that of Ca2+.