Molecular structure in the interlayers of lithium-Wyoming montmorillonite with one, two, or three adsorbed water layers was investigated for the first time by concurrent Monte Carlo and molecular dynamics simulation, based on the MCY model of water-water interactions. Calculated layer spacings, as well as interlayer-species self-diffusion coefficients, were in good agreement with available experimental data. Inner-sphere surface complexes of Li+ with tetrahedral charge sites were observed in all hydrates, whereas outer-sphere surface complexes of Li+ with octahedral charge sites, found in the one-layer hydrate, dissociated from the clay mineral basal planes into a diffuse layer in the two- and three-layer hydrates, a signature of the strong interaction between Li+ and water molecules. Interlayer water molecules tended to solvate Li+, although some were entrapped within cavities in the montmorillonite surface. All of the adsorbed Li+ and interlayer water species exchanged on the time scale (0.2 ns) of the molecular dynamics simulations. Comparisons with Monte Carlo results obtained using, instead of the MCY model, the TIP4P model for Li-water and Li-clay interactions indicated that layer clay spacings and interlayer species mobilities tend to be underpredicted by the TIP4P model.
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