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Ultrafast X-ray probing of water structure below the homogeneous ice nucleation temperature
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Ultrafast X-ray probing of water structure below the homogeneous ice nucleation temperature
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Journal Article
Ultrafast X-ray probing of water structure below the homogeneous ice nucleation temperature
2014
Overview
Femtosecond X-ray laser pulses are used to probe the structure of liquid water in micrometre-sized droplets that have been cooled below the homogeneous ice nucleation temperature, revealing the existence of metastable bulk liquid water down to temperatures of 227 kelvin.
Anomalous bevaviour of supercooled water
Water's anomalous physical properties become markedly enhanced upon supercooling below the freezing point and even seem to diverge towards infinity at around 228 K. Two papers in this issue use contrasting techniques to study this little-explored 'no-man's land' of water where extremely fast ice formation has prohibited measurements of the liquid state. Jonas Sellberg
et al
. use femtosecond X-ray laser pulses to measure bulk liquid water structure in droplets evaporatively cooled to 227 K. Even at this temperature some droplets remained liquid on a millisecond timescale. Pushing this technique further can shed light on controversial scenarios that aim to describe and explain the many anomalous properties of water. Jeremy Palmer
et al
. use six advanced computational methods to demonstrate the existence of two metastable liquid phases of ST2 water at the same deeply supercooled condition, undergoing a liquid–liquid transition that meets stringent thermodynamic criteria and could explain the behavior of water in this regime.
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
,
2
,
3
). This has prompted debate about conflicting theories
4
,
5
,
6
,
7
,
8
,
9
,
10
,
11
,
12
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 (
T
H
) at approximately 232 kelvin
13
and above about 160 kelvin
14
, 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
2
,
15
. Water crystallization has been inhibited by using nanoconfinement
16
, nanodroplets
17
and association with biomolecules
16
to give liquid samples at temperatures below
T
H
, but such measurements rely on nanoscopic volumes of water where the interaction with the confining surfaces makes the relevance to bulk water unclear
18
. 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
19
,
20
,
21
below
T
H
. We find experimental evidence for the existence of metastable bulk liquid water down to temperatures of
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.
