Single particle slow dynamics of confined water

Molecular dynamics simulations of SPC/E water confined in a silica pore are presented. The simulations have been performed at different hydration levels and temperatures to study the single particle dynamics. Due to the confinement and to the presence of a hydrophilic surface, the dynamic behaviour of the liquid appears to be strongly dependent on the hydration level. On lowering temperature and/or hydration level the intermediate scattering function displays a two-step relaxation behavior whose long-time tail is strongly non-exponential. At higher hydrations two quite distinct subsets of water molecules are detectable. Those belonging to the first two layers close to the substrate suffer a severe slowing down already at ambient temperature. The behaviour of the remaining ones is more resemblant of that of supercooled bulk SPC/E water. At lower hydrations and/or temperatures the onset of a slow dynamics due to the cage effect and a scenario typical of supercooled liquids approaching the kinetic glass transition is observed. Moreover, for low hydrations and/or temperatures, the intermediate scattering function clearly displays an overshoot, which can be assigned to the so-called ‘‘boson peak’’.