MONTE-CARLO SIMULATION OF INTERLAYER MOLECULAR-STRUCTURE IN SWELLING CLAY-MINERALS .1. METHODOLOGY

Monte Carlo (MC) simulations of molecular structure in the interlayers of 2:1 Na-saturated clay minerals were performed to address several i mportant simulation methodological issues. Investigation was focused o n monolayer hydrates of the clay minerals because these systems provid e a severe test of the quality and sensitivity of MC interlayer simula tions. Comparisons were made between two leading models of the water-w ater interaction in condensed phases, and the sensitivity of the simul ations to the size or shape of the periodically-repeated simulation ce ll was determined. The results indicated that model potential function s permitting significant deviations from the molecular environment in bulk liquid water are superior to those calibrated to mimic the bulk w ater structure closely. Increasing the simulation cell size or alterin g its shape from a rectangular 21.12 Angstrom x 18.28 Angstrom x 6.54 Angstrom cell (about eight clay mineral unit cells) had no significant effect on the calculated interlayer properties.