Monte Carlo and molecular dynamics studies of interlayer structure in Li(H2O)3-smectites

Monte Carlo and molecular dynamics simulations were performed to elucidate interlayer structure in hydrated Li-smectites (hectorite, beidellite, or montmorillonite with interlayer Li(+)) at low water content (H(2)O/Li = 3). Previous spectroscopic studies of these stable clay mineral hydrates have led to interlayer structural models based on a postulated inner-sphere surface complex comprising Lit bound directly to the smectite surface while surmounted by exactly three solvating water molecules that execute hindered rotational motions. Our simulation results, based on tested water-water, Li(+)-water, water-clay mineral, and Li(+)-clay mineral potential functions, showed that the nature of the interlayer Li(+) solvation complexes in fact depends critically on the location of negative charge sites within the smectite layers. Inner-sphere surface complexes were observed to form exclusively on Li-beidellite (tetrahedral charge sites), outer-sphere surface complexes formed exclusively on Li-hectorite (octahedral charge sites), and both types of surface complex formed on Li-montmorillonite, which also contains bath types of charge site. The Li(+) solvation number in these clay hydrates can vary from two to four. Rotational motions of the water molecules solvating Li(+) occurred (on a picosecond time scale) only if inner-sphere surface complexes had formed, again strongly contradicting the spectroscopic models. Improvement of these models and the spectroscopic data is needed to resolve the major differences between our simulation predictions and the current experimental interpretations of interlayer structure on Li(H(2)O)(3)-smectites.