COMPUTER-SIMULATION OF INTERLAYER MOLECULAR-STRUCTURE IN SODIUM MONTMORILLONITE HYDRATES

Molecular structure in the interlayers of sodium Wyoming montmorilloni te with one, two, or three water monolayers was investigated by Monte Carlo and molecular dynamics simulation, based on tested water-water, water-counterion, clay-water, and clay-counterion potential unctions. Calculated layer spacings and thermodynamic properties, as well as int erlayer water configurations and interlayer-species self-diffusion coe fficients, were in good agreement with available experimental data. In ner-sphere surface complexes of Na+ with tetrahedral charge sites pers isted in all hydrates, whereas the outer-sphere surface complexes with octahedral charge sites found in the one-layer hydrate dissociated pa rtially into an incipient diffuse layer in the two- and three-layer hy drates. Even in the three-layer hydrate, interlayer water molecules di d not replicate their structure or mobility in bulk aqueous solution. Improved diffraction and spectroscopic data on interlayer water are ne eded to guide future simulation studies.