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Ultrafast X-ray probing of water structure below the homogeneous ice nucleation temperature
Stockholm University, Faculty of Science, Department of Physics. SLAC National Accelerator Laboratory, USA.
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2014 (English)In: Nature, ISSN 0028-0836, E-ISSN 1476-4687, Vol. 510, no 7505, 381-+ p.Article in journal (Refereed) Published
Abstract [en]

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(4-12) 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(19-21) 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.

Place, publisher, year, edition, pages
2014. Vol. 510, no 7505, 381-+ p.
National Category
Physical Sciences
Research subject
Theoretical Physics
Identifiers
URN: urn:nbn:se:su:diva-106283DOI: 10.1038/nature13266ISI: 000337350200031OAI: oai:DiVA.org:su-106283DiVA: diva2:735753
Available from: 2014-07-31 Created: 2014-07-31 Last updated: 2017-12-05Bibliographically approved
In thesis
1. Molecular structure and dynamics of liquid water: Simulations complementing experiments
Open this publication in new window or tab >>Molecular structure and dynamics of liquid water: Simulations complementing experiments
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Water is abundant on earth and in the atmosphere and the most crucial liquid for life as we know it. It has been subject to rather intense research since more than a century and still holds secrets about its molecular structure and dynamics, particularly in the supercooled state, i. e. the metastable liquid below its melting point. 

This thesis is concerned with different aspects of water and is written from a theoretical perspective. Simulation techniques are used to study structures and processes on the molecular level and to interpret experimental results. The evaporation kinetics of tiny water droplets is investigated in simulations with focus on the cooling process associated with evaporation. The temperature evolution of nanometer-sized droplets evaporating in vacuum is well described by the Knudsen theory of evaporation. The principle of evaporative cooling is used in experiments to rapidly cool water droplets to extremely low temperatures where water transforms into a highly structured low-density liquid in a continuous and accelerated fashion.

For water at ambient conditions, a structural standard is established in form of a high precision radial distribution function as a result of x-ray diffraction experiments and simulations. Recent data even reveal intermediate range molecular correlations to distances of up to 17 Å in the bulk liquid.

The barium fluoride (111) crystal surface has been suggested to be a template for ice formation because its surface lattice parameter almost coincides with that of the basal plane of hexagonal ice. Instead, water at the interface shows structural signatures of a high-density liquid at ambient and even at supercooled conditions.

Inelastic neutron scattering experiments have shown a feature in the vibrational spectra of supercooled confined and protein hydration water which is connected to the so-called Boson peak of amorphous materials. We find a similar feature in simulations of bulk supercooled water and its emergence is associated with the transformation into a low-density liquid upon cooling.

Place, publisher, year, edition, pages
Stockholm: Department of Physics, Stockholm University, 2015. 79 p.
Keyword
liquid water, supercooled water, molecular simulation, evaporative cooling
National Category
Atom and Molecular Physics and Optics Condensed Matter Physics
Research subject
Theoretical Physics
Identifiers
urn:nbn:se:su:diva-120808 (URN)978-91-7649-264-2 (ISBN)
Public defence
2015-10-23, sal FB42, AlbaNova universitetscentrum, Roslagstullsbacken 21, Stockholm, 09:15 (English)
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Note

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 1: Manuscript. Paper 4: Manuscript.

Available from: 2015-10-01 Created: 2015-09-17 Last updated: 2015-10-27Bibliographically approved

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