Detection of the liquid-liquid transition in the deeply cooled water confined in MCM-41 with elastic neutron scattering technique

TL;DR

This study uses elastic neutron scattering to investigate the phase behavior of deeply cooled water confined in MCM-41, revealing a likely first-order liquid-liquid transition that could explain water's anomalous low-temperature properties.

Contribution

First experimental evidence of a liquid-liquid transition in confined water using elastic neutron scattering, identifying phase separation and critical points.

Findings

Observation of LDL-HDL transition starting at ~1.12 kbar and 215 K

Identification of phase separation extending to higher pressures and lower temperatures

Estimation of the Widom line in confined water

Abstract

In this paper we present a review on our recent experimental investigations into the phase behavior of the deeply cooled water confined in a nanoporous silica material, MCM-41, with elastic neutron scattering technique. Under such strong confinement, the homogeneous nucleation process of water is avoided, which allows the confined water to keep its liquid state at temperatures and pressures that are inaccessible to the bulk water. By measuring the average density of the confined heavy water, we observe a likely first-order low-density liquid (LDL) to high-density liquid (HDL) transition in the deeply cooled region of the confined heavy water. The phase separation starts from 1.12 +- 0.17 kbar and 215 +- 1K and extends to higher pressures and lower temperatures in the phase diagram. This starting point could be the liquid-liquid critical point of the confined water. The locus of the Widom line is also estimated. The observation of the liquid-liquid transition in the confined water has potential to explain the mysterious behaviors of water at low temperatures. In addition, it may also have impacts on other disciplines, because the confined water system represents many biological and geological systems in which water resides in nanoscopic pores or in the vicinity of hydrophilic or hydrophobic surfaces.