Structural Arrangements of Isomorphic Substitutions in Smectites: Molecular Simulation of the Swelling Properties, Interlayer Structure, and Dynamics of Hydrated Cs–Montmorillonite Revisited with New Clay Models

Three
new structural models of montmorillonite with differently
distributed Al/Si and Mg/Al substitutions in the tetrahedral and octahedral
clay layers are systematically developed and studied by means of MD
simulations to quantify the possible effects of such substitutional
disorder on the swelling behavior, the interlayer structure, and mobility
of aqueous species. A very wide range of water content, from 0 to
700 mgwater/gclay is explored to derive the
swelling properties of Cs–montmorillonite. The determined layer
spacing does not differ much depending on the clay model. However,
at low water contents up to 1-layer hydrate (∼100 mgwater/gclay) the variation of specific locations of the tetrahedral
and octahedral substitutions in the two TOT clay layers slightly but
noticeably affects the total hydration energy of the system. Using
atom–atom radial distribution functions and the respective
atomic coordination numbers we have identified for the three clay
models not only the previously observed binding sites for Cs+ on the clay surface but also new ones that are correlated with the
position of tetrahedral substitution in the structure. The mobility
of Cs+ ions and H2O diffusion coefficients,
as expected, gradually increase both with increasing water content
and with increasing distance from the clay surface, but they still
remain 2 to 4 times lower than the corresponding bulk values. Only
small differences were observed between the three Cs–montmorillonite
models, but these differences are predicted to increase in the case
of higher charge density of the clay layers and/or interlayer cations.