  A Molecular Dynamics simulation of the microscopic structure of water
confined in a silica pore is presented. A single cavity in the silica glass has
been modeled as to reproduce the main features of the pores of real Vycor
glass. A layer analysis of the site-site radial distribution functions evidence
the presence in the pore of two subsets of water molecules with different
microscopic structure. Molecules which reside in the inner layer, close to the
center of the pore, have the same structure as bulk water but at a temperature
of 30 K higher. On the contrary the structure of the water molecules in the
outer layer, close to the substrate, is strongly influenced by the
water-substrate hydrophilic interaction and sensible distortions of the H-bond
network and of the orientational correlations between neighboring molecules
show up. Lowering the hydration has little effect on the structure of water in
the outer layer. The consequences on experimental determinations of the
structural properties of water in confinement are discussed.
