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Wide-angle X-ray diffraction and molecular dynamics study of medium-range order in ambient and hot water
Stockholm University, Faculty of Science, Department of Physics.
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2011 (English)In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 13, no 44, 19997-20007 p.Article in journal (Refereed) Published
Abstract [en]

We have developed wide-angle X-ray diffraction measurements with high energy-resolution and accuracy to study water structure at three different temperatures (7, 25 and 66 degrees C) under normal pressure. Using a spherically curved Ge crystal an energy resolution better than 15 eV has been achieved which eliminates influence from Compton scattering. The high quality of the data allows for a reliable Fourier transform of the experimental data resolving shell structure out to similar to 12 angstrom, i.e. 5 hydration shells. Large-scale molecular dynamics (MD) simulations using the TIP4P/2005 force-field reproduce excellently the experimental shell-structure in the range 4-12 angstrom although less agreement is seen for the first peak in the intermolecular pair-correlation function (PCF). The Shiratani-Sasai Local Structure Index [J. Chem. Phys. 104, 7671 (1996)] identifies a tetrahedral minority giving the intermediate-range oscillations in the O-O PCF and a disordered majority providing a more featureless background in this range. The current study supports the proposal that the structure of liquid water, even at high temperatures, can be described in terms of a two-state fluctuation model involving local structures related to the high-density and low-density forms of liquid water postulated in the liquid-liquid phase transition hypothesis.

Place, publisher, year, edition, pages
2011. Vol. 13, no 44, 19997-20007 p.
National Category
Physical Chemistry Atom and Molecular Physics and Optics
URN: urn:nbn:se:su:diva-71018DOI: 10.1039/c1cp22804hISI: 000296383100038OAI: diva2:483653
8Available from: 2012-01-25 Created: 2012-01-25 Last updated: 2014-04-28Bibliographically approved
In thesis
1. X-ray scattering and spectroscopy of supercooled water and ice
Open this publication in new window or tab >>X-ray scattering and spectroscopy of supercooled water and ice
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis presents experimental studies of water and ice at near-atmospheric pressures using intense x-rays only accessible at synchrotrons and free-electron lasers. In particular, it focuses on the deeply supercooled, metastable state and its implications on ice nucleation.

The local structure of the liquid phase was studied by x-ray scattering over a wide temperature range extending from 339 K down to 227 K. In order to be able to study the deeply supercooled liquid, micron-sized water droplets were evaporatively cooled in vacuum and probed by ultrashort x-ray pulses. This is to date the lowest temperature at which measurements of the structure have been performed on bulk liquid water cooled from room temperature. Upon deep supercooling, the structure evolved toward that of a low-density liquid with local tetrahedral coordination. At ~230 K, where the low-density liquid structure started to dominate, the number of droplets containing ice nuclei increased rapidly. The estimated nucleation rate suggests that there is a “fragile-to-strong” transition in the dynamics of the liquid below 230 K, and its implications on water structure are discussed.

Similarly, the electronic structure of deeply supercooled water was studied by x-ray emission spectroscopy down to 222 K, but the spectral changes expected from the structural transformation remained absent and explanations are discussed. At high fluence, the non-linear dependence of the x-ray emission yield indicated that there were high valence hole densities created during the x-ray pulse length due to Auger cascades, resulting in reabsorption of the x-ray emission.

Finally, the hydrogen-bonded network in water was studied by x-ray absorption spectroscopy and compared to various ices. It was found that the pre-edge absorption cross-section, which is associated with distorted hydrogen bonds, could be minimized for crystalline ice grown on a hydrophobic BaF2(111) surface with low concentration of nucleation centers.

Place, publisher, year, edition, pages
Stockholm: Department of Physics, Stockholm Univeristy, 2014. 148 p.
supercooled water, ice, x-ray scattering, x-ray spectroscopy, free-electron laser
National Category
Physical Chemistry
Research subject
Chemical Physics
urn:nbn:se:su:diva-103009 (URN)978–91–7447–902–7 (ISBN)
Public defence
2014-05-30, FA32, AlbaNova Universitetscentrum, Roslagstullsbacken 21, Stockholm, 10:15 (English)

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 2: Manuscript. Paper 3: Manuscript. Paper 4: Manuscript. Paper 5: Manuscript. Paper 6: Manuscript.

Available from: 2014-05-08 Created: 2014-04-26 Last updated: 2015-09-18Bibliographically approved

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Wikfeldt, Thor KjartanSellberg, JonasPettersson, Lars G.M.Nilsson, Anders
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