Evidence of Glassy Behaviour of Water Molecules in Confined States

A molecular dynamics simulation of SPC/E water confined in a realistic silica pore is presented. The simulation has been performed at different hydration levels at ambient temperature to study the single-particle dynamics. Owing to confinement and to the presence of a strong hydrophilic surface, the dynamic behaviour of the liquid appears to be strongly dependent on the hydration level. At higher hydration two quite distinct subsets of water molecules are detectable. Those belonging to the first layer close to the substrate suffer a severe slowing down. While the behaviour of the remaining ones is more similar to bulk water. At lower hydrations we observe the onset of a slow dynamics due to the cage effect. The intermediate scattering function displays a double-step relaxation behaviour whose long time tail is strongly non-exponential. Moreover, for low hydrations, the intermediate scattering function clearly displays an overshooting, which can be assigned to the so called 'boson peak'. The conventional picture of the stochastic single-particle diffusion therefore already loses its validity at room temperature for confined water.