Molecular simulation of water confined in nanoporous Ca-silica

This paper reports on a molecular simulation study of the thermodynamics, structure and dynamics of water confined at ambient temperature in charged silica nanopores of a width H = 10 and 20 Å. The adsorption isotherms for water resemble those observed for experimental samples; the adsorbed amount increases continuously in the multilayer adsorption regime until a jump occurs due to capillary condensation of the fluid within the pore. Strong layering of water in the vicinity of the silica surfaces is observed as marked density oscillations are seen up to 8 Å from the surface in the density profiles for confined water. Our results also indicate that the Ca²⁺ counterions remain in a space close to the silica surface whatever the pore width and the adsorbed amount of water. For all pore sizes and adsorbed amounts, the self-diffusivity of confined water is lower than the bulk due to the strong hydrophilic nature of the pore surface. Our results also suggest that the self-diffusivity of confined water is sensitive to the adsorbed amount of water molecules.