Ultrafast X-ray probing of water structure below the homogeneous ice nucleation temperature

Abstract

Water has a number of anomalous physical properties, and some of these become drastically enhanced on supercooling below the freezing point. Particular interest has focused on thermodynamic response functions that can be described using a normal component and an anomalous component that seems to diverge at about 228 kelvin (refs 1-3). This has prompted debate about conflicting theories that aim to explain many of the anomalous thermodynamic properties of water. One popular theory attributes the divergence to a phase transition between two forms of liquid water occurring in the 'no man's land' that lies below the homogeneous ice nucleation temperature (TH) at approximately 232 kelvin and above about 160 kelvin, and where rapid ice crystallization has prevented any measurements of the bulk liquid phase. In fact, the reliable determination of the structure of liquid water typically requires temperatures above about 250 kelvin. Water crystallization has been inhibited by using nanoconfinement, nanodroplets and association with biomolecules to give liquid samples at temperatures below TH, but such measurements rely on nanoscopic volumes of water where the interaction with the confining surfaces makes the relevance to bulk water unclear. Here we demonstrate that femtosecond X-ray laser pulses can be used to probe the structure of liquid water in micrometre-sized droplets that have been evaporatively cooled below TH. We find experimental evidence for the existence of metastable bulk liquid water down to temperatures of 227(-1)(+2) kelvin in the previously largely unexplored no man's land. We observe a continuous and accelerating increase in structural ordering on supercooling to approximately 229 kelvin, where the number of droplets containing ice crystals increases rapidly. But a few droplets remain liquid for about a millisecond even at this temperature. The hope now is that these observations and our detailed structural data will help identify those theories that best describe and explain the behaviour of water.

Crystal nucleation as the ordering of multiple order parameters

“…Thus, the experimentally inaccessible supercooled liquid region is now widely known as "no-man's land" [216], although there have been efforts to overcome the difficulty by either strong spatial confinement [217,218], mixing an anti-freezing component [219,220], or rapid cooling [221].…”

A physically constrained classical description of the homogeneous nucleation of ice in water

“…Sellberg et al 21 used a fast free-electron X-ray laser technique to study the structure of water at the 100 fs time scale in micrometer-sized droplets down to about 227 K and found that the molecular nearest-neighbor coordination of water molecules increases sharply. These observations confirmed similar computational findings that there is a sharp increase in the fraction of four-coordinate water molecules in the homogeneous nucleation regime.…”

Fast Dynamics of Water Droplets Freezing from the Outside In

“…Deposition of a fresh layer of soot before each trial effectively isolated the droplet from any unintended ice nuclei on the substrate (the soot itself remained inactive as nucleus down to droplet temperatures of approximately −15 • C). When the chamber is evacuated, a water droplet here rapidly cools to sub-freezing temperatures by evaporative cooling and, if nothing is done to prevent it, would spontaneously freeze [26,27]. However, unrestricted evaporative cooling gives very little control over the temperature profile in the drop during the experiment.…”