Water in porous glasses. A computer simulation Study

We report molecular dynamics simulations of water confined in a cylindrical silica pore. The pore geometry and size is similar to that of typical pore s in porous Vycor glass. In the present study we focus on the dependence of microscopical structural and dynamical properties on the degree of hydrati on of the pore. We have performed five simulations of systems between 19 an d 96 % hydration. In all cases, water adsorbs strongly on the pore surface, clearly demonstrating the hydrophilic nature of the Vycor surface. Two lay ers of water molecules are affected strongly by the interactions with the g lass surface. With decreasing degree of hydration an increasing volume in t he center of the pore is devoid of water molecules. At 96 % hydration the c enter is a continuous and homogeneous region that has, however, a lower den sity than bulk water at ambient conditions. A well-pronounced mobility prof ile exists, where molecules in the center of the pores have substantially h igher self diffusion coefficients than molecules on the pore surface. The s pectral densities of center of mass and hydrogen atom motion show the signa ture of confinement for the molecules close to the pore surface, while the spectral densities in the center of the pore are similar to those in bulk w ater. The molecular dynamics results are in good agreement with recent expe riments. Our data indicate that the dependence of experimental data on the level of hydration of the Vycor sample is due to the different relative con tribution of molecules adsorbed on the pore surface and bulk-like molecules in the interior of the pore to the experimental averages. (C) 1999 Elsevie r Science B.V. All rights reserved.