MONTE-CARLO AND MOLECULAR-DYNAMICS SIMULATIONS OF INTERFACIAL STRUCTURE IN LITHIUM-MONTMORILLONITE HYDRATES

Molecular structure in the interlayers of lithium-Wyoming montmorillon ite with one, two, or three adsorbed water layers was investigated for the first time by concurrent Monte Carlo and molecular dynamics simul ation, based on the MCY model of water-water interactions. Calculated layer spacings, as well as interlayer-species self-diffusion coefficie nts, were in good agreement with available experimental data. Inner-sp here surface complexes of Li+ with tetrahedral charge sites were obser ved in all hydrates, whereas outer-sphere surface complexes of Li+ wit h octahedral charge sites, found in the one-layer hydrate, dissociated from the clay mineral basal planes into a diffuse layer in the two- a nd 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 montmorillon ite surface. All of the adsorbed Li+ and interlayer water species exch anged 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 indic ated that layer clay spacings and interlayer species mobilities tend t o be underpredicted by the TIP4P model.