Monte Carlo simulation of total radial distribution functions for interlayer water in Li-, Na-, and K-montmorillonite hydrates

Recent H-1/D-2 isotopic-difference neutron diffraction experiments on interlayer water in the two-layer hydrates of Wyoming montmorillonite with Li+ or Na+ counterions have shown that the organization of the water molecules differs from that in the bulk liquid. Monte Carlo simulations were performed to investigate molecular mechanisms underlying these structural differences in terms of radial distribution functions for H-O and H-H spatial correlations. Our simulations of the first-order difference total radial distribution function, G(H)(r), for interlayer water were in good agreement with published experimental data based on H-1/D-2 isotopic-difference diffraction patterns for the two-layer hydrates of Na- and Li-montmorillonite. Detailed examination of our results showed that the H-O and H-H spatial correlations found among interlayer water molecules differ from those obtained in the bulk liquid. Moreover, O-O spatial correlations are longer-ranged than in the bulk liquid because of cation solvation effects. A Monte Carlo simulation of G(H)(r) for the two-layer hydrate of K-montmorillonite predicted differences from those of the other two montmorillonite hydrates that should be observable by neutron diffraction.