Evidence of glassy behaviour of water molecules in confined states

A molecular dynamics simulation of SPC/E water confined in a realistic sili ca pore is presented. The simulation has been performed at different hydrat ion levels at ambient temperature to study the single-particle dynamics. Ow ing to confinement and to the presence of a strong hydrophilic surface, the dynamic behaviour of the liquid appears to be strongly dependent on the hy dration level. At higher hydration two quite distinct subsets of water mole cules are detectable. Those belonging to the first layer close to the subst rate suffer a severe slowing down. While the behaviour of the remaining one s is more similar to bulk water. At lower hydrations we observe the onset o f a slow dynamics due to the cage effect. The intermediate scattering funct ion displays a double-step relaxation behaviour whose long time tail is str ongly non-exponential. Moreover, for low hydrations, the intermediate scatt ering function clearly displays an overshooting, which can be assigned to t he so called 'boson peak'. The conventional picture of the stochastic singl e-particle diffusion therefore already loses its validity at room temperatu re for confined water.