Molecular Dynamics Simulations of Water Structure and Diffusion in Silica Nanopores

We present molecular dynamics (MD) simulations of water-filled silica nanopores such as those that occur in ordered oxide ceramics (MCM-41, SBA-15), controlled pore glasses (such as Vycor glass), mesoporous silica, bio-glasses, and hydrous silica gel coatings of weathered minerals and glasses. Our simulations overlap the range of pore diameters (1-4 nm) where confinement causes the disappearance of bulk-liquid-like water. In >= 2 nm diameter pores, the silica surface carries three statistical monolayers of density-layered water, interfacial water structure is independent of confinement or surface curvature, and bulk-liquid-like water exists at the center of the pore (this last finding contradicts assumptions used in most previous neutron diffraction studies and in several MD simulation studies of silica nanopores). In 1 nm diameter pores, bulk-liquid-like water does not exist and the structural properties of interfacial water are influenced by confinement. Predicted water diffusion coefficients in 1-4 nm diameter pores agree with quasi-elastic neutron scattering (QENS) data and are roughly consistent with a very simple core-shell conceptual model whereupon the first statistical water monolayer is immobile and the rest of the pore water diffuses as rapidly as bulk liquid water.