Unraveling the Phase Behavior of Water Confined in Nanochannels through Positron Annihilation

Abstract

Water confined in cylindrical pores of silica with a diameter of 2 nm was studied by positron annihilation lifetime spectroscopy. Instead of freezing at 273 K, the glass-like transition through the viscous state to the solid phase was observed in the wide temperature range of 225–188 K. Reducing the vapor pressure over confined water allowed us to obtain the negative pressure pn = −164 MPa in liquid water, and the phase transition temperature range changed to 234–189 K. The temperature dependence of the ortho-positronium (o-Ps) lifetime in water under pn became consistent with the surface tension as in most liquids, as opposed to the anomalous temperature dependence in water under the saturated vapor pressure. This indicates a reduction in the number of hydrogen bonds in confined water under pn. On this basis, to explain the anomalous temperature dependence, we propose a hypothesis of a partial energy transfer to distant water molecules via the OH-stretching modes instead of repelling the molecules surrounding o-Ps. Our results indicate that thermally generated defects in the confined ice are accompanied by permanent defects, which predominate in the ice formed under pn. Both types of defects have the same volume of 70 Å3, which is much greater than in hexagonal ice Ih. We conjecture that the defects are larger due to the influence of the silanol groups on the structure of solid water. These observations imply that the structure of confined ice differs significantly from that of bulk ice and the vapor pressure influences its formation.