Molecular Dynamics and Statistical Mechanics of Water Near an Uncharged Silicate Surface

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Abstract
Properties such as the swelling, mass transfer, and porosity in aqueous colloidal systems and natural soils are thought to vary in magnitude depending upon the extent of surface‐induced perturbation in the water near silicate surfaces (vicinal water). Any satisfactory theoretical understanding of the interrelation between the perturbations in vicinal water and the properties of colloidal systems must involve the statistical mechanical analysis of an appropriate molecular model for the vicinal water. We have conducted a preliminary molecular dynamics simulation in order to elucidate the effects of an uncharged silicate surface on the structure and dynamics of vicinal water. Statistical mechanics were used to interpret position and velocity trajectories computed in the molecular dynamics simulation. The results of our simulation indicate that the vicinal water differs substantially from bulk water over appreciable distances for the static orientation of molecular dipole moments and the rate of relaxation of these moments and substantially over smaller distances for hydrogen bonding patterns and the rate of self‐diffusion. No significant differences between the radial distribution functions of vicinal and bulk water are evident. Because vicinal water may be perturbed to large distances from a clay surface, one must take extreme care in deriving transport equations in saturated and unsaturated media which are high in clay. We may conclude that a satisfactory derivation of the solute transport equations in systems high in colloids and having low water content does not exist.