MONTE-CARLO AND MOLECULAR-DYNAMICS STUDIES OF INTERLAYER STRUCTURE INLI(H2O)(3)-SMECTITES

Monte Carlo and molecular dynamics simulations were performed to eluci date interlayer structure in hydrated Li-smectites (hectorite, beidell ite, or montmorillonite with interlayer Li+) at low water content (H2O /Li = 3). Previous spectroscopic studies of these stable clay mineral hydrates have led to interlayer structural models based on a postulate d inner-sphere surface complex comprising Lit bound directly to the sm ectite surface while surmounted by exactly three solvating water molec ules that execute hindered rotational motions. Our simulation results, based on tested water-water, Li+-water, water-clay mineral, and Li+-c lay mineral potential functions, showed that the nature of the interla yer Li+ solvation complexes in fact depends critically on the location of negative charge sites within the smectite layers. Inner-sphere sur face complexes were observed to form exclusively on Li-beidellite (tet rahedral charge sites), outer-sphere surface complexes formed exclusiv ely on Li-hectorite (octahedral charge sites), and both types of surfa ce complex formed on Li-montmorillonite, which also contains bath type s of charge site. The Li+ solvation number in these clay hydrates can vary from two to four. Rotational motions of the water molecules solva ting Li+ occurred (on a picosecond time scale) only if inner-sphere su rface complexes had formed, again strongly contradicting the spectrosc opic models. Improvement of these models and the spectroscopic data is needed to resolve the major differences between our simulation predic tions and the current experimental interpretations of interlayer struc ture on Li(H2O)(3)-smectites.