Phase transitions of interfacial water at 165 and 240 K. Connections to bulk water physics and protein dynamics

Abstract

We are considering water adsorbed as a monolayer on Vycor, a porous silica glass. The interfacial water molecules interact with the substrate through hydrogen bonding with the numerous silanol (Si-OH) groups present all over the surface. This special form of water exhibits peculiar dynamical properties. A combined calorimetric, diffraction, high resolution quasi-elastic and inelastic neutron scattering study shows that interfacial water experiences a glass transition at 165 K and a liquid-liquid transition at 240 K from a low-density to a high density-liquid. We show that this unusual behaviour, compared to the bulk, is due to a strong weakening of the hydrogen-bond strength, possibly due to the reduced number of hydrogen-bonds engaged by water molecules when they are in an interfacial two dimensional situation. The connections of these findings to the physics of bulk water and protein dynamics are discussed.