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Molecular structure and dynamics of liquid water: Simulations complementing experiments
Stockholm University, Faculty of Science, Department of Physics.
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 [en]
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: urn:nbn:se:su:diva-120808ISBN: 978-91-7649-264-2 (print)OAI: oai:DiVA.org:su-120808DiVA: diva2:854642
Public defence
2015-10-23, sal FB42, AlbaNova universitetscentrum, Roslagstullsbacken 21, Stockholm, 09:15 (English)
Opponent
Supervisors
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
List of papers
1. Evaporative cooling of microscopic water droplets in vacuo: Molecular dynamics simulations and kinetic gas theory
Open this publication in new window or tab >>Evaporative cooling of microscopic water droplets in vacuo: Molecular dynamics simulations and kinetic gas theory
(English)Manuscript (preprint) (Other academic)
Keyword
Evaporation, water, droplet, evaporative cooling, molecular dynamics simulations, kinetic gas theory, Knudsen evaporation rate, cooling rate, evaporation rate
National Category
Atom and Molecular Physics and Optics Condensed Matter Physics
Research subject
Theoretical Physics
Identifiers
urn:nbn:se:su:diva-120667 (URN)
Available from: 2015-09-17 Created: 2015-09-15 Last updated: 2016-01-29Bibliographically approved
2. Ultrafast X-ray probing of water structure below the homogeneous ice nucleation temperature
Open this publication in new window or tab >>Ultrafast X-ray probing of water structure below the homogeneous ice nucleation temperature
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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.

National Category
Physical Sciences
Research subject
Theoretical Physics
Identifiers
urn:nbn:se:su:diva-106283 (URN)10.1038/nature13266 (DOI)000337350200031 ()
Available from: 2014-07-31 Created: 2014-07-31 Last updated: 2017-12-05Bibliographically approved
3. Benchmark oxygen-oxygen pair-distribution function of ambient water from x-ray diffraction measurements with a wide Q-range
Open this publication in new window or tab >>Benchmark oxygen-oxygen pair-distribution function of ambient water from x-ray diffraction measurements with a wide Q-range
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2013 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 138, no 7, 074506Article in journal (Refereed) Published
Abstract [en]

Four recent x-ray diffraction measurements of ambient liquid water are reviewed here. Each of these measurements represents a significant development of the x-ray diffraction technique applied to the study of liquid water. Sources of uncertainty from statistical noise, Q-range, Compton scattering, and self-scattering are discussed. The oxygen-hydrogen contribution to the measured x-ray scattering pattern was subtracted using literature data to yield an experimental determination, with error bars, of the oxygen-oxygen pair-distribution function, g(OO)(r), which essentially describes the distribution of molecular centers. The extended Q-range and low statistical noise of these measurements has significantly reduced truncation effects and related errors in the g(OO)(r) functions obtained. From these measurements and error analysis, the position and height of the nearest neighbor maximum in g(OO)(r) were found to be 2.80(1) angstrom and 2.57(5) respectively. Numerical data for the coherent differential x-ray scattering cross-section I-X(Q), the oxygen-oxygen structure factor S-OO(Q), and the derived g(OO)(r) are provided as benchmarks for calibrating force-fields for water.

National Category
Physical Sciences
Research subject
Theoretical Physics
Identifiers
urn:nbn:se:su:diva-88695 (URN)10.1063/1.4790861 (DOI)000315263500038 ()
Funder
Swedish Research Council
Note

AuthorCount:6;

Available from: 2013-03-25 Created: 2013-03-25 Last updated: 2017-12-06Bibliographically approved
4. Intermediate range molecular correlations in liquid water
Open this publication in new window or tab >>Intermediate range molecular correlations in liquid water
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(English)Manuscript (preprint) (Other academic)
National Category
Physical Chemistry Atom and Molecular Physics and Optics
Research subject
Theoretical Physics
Identifiers
urn:nbn:se:su:diva-120661 (URN)
Available from: 2015-09-17 Created: 2015-09-15 Last updated: 2016-01-29Bibliographically approved
5. Highly Compressed Two-Dimensional Form of Water at Ambient Conditions
Open this publication in new window or tab >>Highly Compressed Two-Dimensional Form of Water at Ambient Conditions
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2013 (English)In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 3, 1074Article in journal (Refereed) Published
Abstract [en]

The structure of thin-film water on a BaF2(111) surface under ambient conditions was studied using x-ray absorption spectroscopy from ambient to supercooled temperatures at relative humidity up to 95%. No hexagonal ice-like structure was observed in spite of the expected templating effect of the lattice-matched (111) surface. The oxygen K-edge x-ray absorption spectrum of liquid thin-film water on BaF2 exhibits, at all temperatures, a strong resemblance to that of high-density phases for which the observed spectroscopic features correlate linearly with the density. Surprisingly, the highly compressed, high-density thin-film liquid water is found to be stable from ambient (300 K) to supercooled (259 K) temperatures, although a lower-density liquid would be expected at supercooled conditions. Molecular dynamics simulations indicate that the first layer water on BaF2(111) is indeed in a unique local structure that resembles high-density water, with a strongly collapsed second coordination shell.

National Category
Physical Sciences
Research subject
Theoretical Physics
Identifiers
urn:nbn:se:su:diva-88360 (URN)10.1038/srep01074 (DOI)000313551300002 ()
Note

AuthorCount:10;

Available from: 2013-03-25 Created: 2013-03-13 Last updated: 2017-12-06Bibliographically approved
6. The Boson peak in supercooled water
Open this publication in new window or tab >>The Boson peak in supercooled water
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2013 (English)In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 3, 1980Article in journal (Refereed) Published
Abstract [en]

We perform extensive molecular dynamics simulations of the TIP4P/2005 model of water to investigate the origin of the Boson peak reported in experiments on supercooled water in nanoconfined pores, and in hydration water around proteins. We find that the onset of the Boson peak in supercooled bulk water coincides with the crossover to a predominantly low-density-like liquid below the Widom line T-W. The frequency and onset temperature of the Boson peak in our simulations of bulk water agree well with the results from experiments on nanoconfined water. Our results suggest that the Boson peak in water is not an exclusive effect of confinement. We further find that, similar to other glass-forming liquids, the vibrational modes corresponding to the Boson peak are spatially extended and are related to transverse phonons found in the parent crystal, here ice Ih.

National Category
Engineering and Technology Physical Sciences
Research subject
Theoretical Physics
Identifiers
urn:nbn:se:su:diva-92266 (URN)10.1038/srep01980 (DOI)000320433800001 ()
Funder
Swedish Research Council
Note

AuthorCount:5;

Available from: 2013-07-25 Created: 2013-07-25 Last updated: 2017-12-06Bibliographically approved

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