Neutron Spin Echo Studies on Dynamics of Confined Water

Abstract

Water confined in nanometer-scale pores exists under various conditions, such as water in rock, sandstones, proteins and membranes. The properties of confined water are very different from those of bulk water. Neutron spin echo (NSE) method covers suitable length and time scales to investigate the dynamics of confined water. An NSE study of confined water is exemplified on heavy water (D₂O) confined in porous silica glass MCM-41 C10 (pore diameter 2.04 nm). D₂O in C10 is stable as liquid over a temperature range down to 140 K. The intermediate scattering functions are analyzed by the Kohlrausch-Williams-Watts stretched exponential function. The relaxation times of confined D₂O in capillary condensed state follow remarkably well the Vogel-Tamman-Fulcher equation between 298 and 220 K, whereas below 220 K they show an Arrhenius type behavior. That is, the strong-to-fragile (STF) dynamic crossover occurs, which has never been seen in experiments on bulk water. On the other hand, for monolayer D₂O, the STF dynamic crossover was not observed in the temperate range measured. This could reflect the partial breaking of the hydrogen bonds of surface water.