Water plays a prominent role in many areas of physical and biological sciences. Water, though commonplace, is also highly unusual in comparison to most liquids. Promising models to explain water’s anomalous properties propose that it is a heterogeneous, temperature-dependent mixture of two structural motifs. Key phenomena related to this structural heterogeneity are predicted to occur for deeply supercooled water where fast crystallization has prevented experiments. We have developed a pulsed laser heating method for rapidly heating and cooling nanoscale water films adsorbed on a metal substrate at cryogenic temperatures. This approach allows us to investigate the properties of supercooled water such as its structure, self-diffusion, and relaxation and crystallization kinetics at temperatures ranging from ~180 K to ~260 K. Over this temperature range, water’s structure relaxes from its initial configuration to a steady-state configuration prior to appreciable crystallization. All the observed structural changes are reversible and reproducible by a linear combination of high- and low-temperature structural motifs. The fraction of the liquid with the high-temperature motif decreases rapidly as the temperature decreases from 245 to 190 K consistent with the predictions of two-state “mixture” models for supercooled water in the supercritical regime. A simple kinetic model based on a distribution of energetic barriers for rearrangement within a structurally heterogeneous environment accounts for the observed structural relaxation kinetics. For supercooled D2O, the structural transition and relaxation kinetics agree remarkably well with those for H2O once the temperatures are normalized by the temperature of maximum density, TMD, for the isotopologues.

Structure and dynamics of deeply supercooled water

ACS Spring 2022

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