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Enhanced small-angle scattering connected to the Widom line in simulations of supercooled water
Stockholm University, Faculty of Science, Department of Physics. (Kvantkemi)
Stockholm University, Faculty of Science, Department of Physics. (Kvantkemi)
Stockholm University, Faculty of Science, Department of Physics. (Kvantkemi)
(English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690Article in journal (Refereed) Submitted
Abstract [en]

We present extensive simulation results on the TIP4P/2005 water model and show that it displays significantly enhanced small-angle scattering in the supercooled regime. The simulations exhibit a Widom line (TW), emanating from a liquid-liquid critical point (LLCP) in the supercooled region; TW is characterized by a maximal Ornstein-Zernike correlation length and strong small-angle scattering. The good agreement between the simulated small-angle scattering and recent experimental SAXS data [Huang et al., J. Chem. Phys. 133, 134504 (2010)] thus provides indirect evidence for the existence of a Widom line in supercooled water; both the LLCP and singularity-free (SF) scenarios are however consistent with the presence of TW. Simulations performed at 1, 1,000 and 1,500 bar show an increasing abruptness of a high-density (HDL) to low-density (LDL) liquid crossover associated with crossing TW, while simulations at 2,000 bar show a very gradual transition at lower temperatures indicating that the critical pressure (whether at T=0, as in the SF scenario, or above as in the LLCP scenario) is below 2,000 bar in this simulation model. Maxima in the isothermal compressibility and negative thermal expansion coefficient nearly coincide with TW at 1, 1,000 and 1,500 bar. Analysis of the tetrahedrality parameter Q reveals that the HDL-LDL structural transition is very sharp at 1,000 and 1,500 bar, and that structural fluctuations become strongly coupled to density fluctuations upon approaching TW. Furthermore, the tetrahedrality distribution becomes bimodal at ambient temperatures, an observation that possibly provides a link between the HDL-LDL transition and the structural bimodality in liquid water indicated by x-ray spectroscopic techniques.

National Category
Physical Sciences
Identifiers
URN: urn:nbn:se:su:diva-47122OAI: oai:DiVA.org:su-47122DiVA: diva2:373044
Available from: 2010-11-29 Created: 2010-11-29 Last updated: 2017-12-12Bibliographically approved
In thesis
1. Structure, Dynamics and Thermodynamics of Liquid Water: Insights from Molecular Simulations
Open this publication in new window or tab >>Structure, Dynamics and Thermodynamics of Liquid Water: Insights from Molecular Simulations
2011 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Water is a complex liquid with many unusual properties. Our understanding of its physical, chemical and biological properties is greatly advanced after a century of dedicated research but there are still many unresolved questions. If answered, they could have important long-term consequences for practical applications ranging from drug design to water purification. This thesis presents results on the structure, dynamics and thermodynamics of liquid water. The focus is on theoretical simulations applied to interpret experimental data from mainly x-ray and neutron scattering and spectroscopy techniques. The structural sensitivity of x-ray and neutron diffraction is investigated using reverse Monte Carlo simulations and information on the pair-correlation functions of water is derived. A new method for structure modeling of computationally demanding data sets is presented and used to resolve an inconsistency between experimental extended x-ray absorption fine-structure and diffraction data regarding oxygen-oxygen pair-correlations. Small-angle x-ray scattering data are modeled using large-scale classical molecular dynamics simulations, and the observed enhanced scattering at supercooled temperatures is connected to the presence of a Widom line emanating from a liquid-liquid critical point in the deeply supercooled high pressure regime. An investigation of inherent structures reveals an underlying structural bimodality in the simulations connected to disordered high-density and ordered low-density molecules, providing a clearer interpretation of experimental small-angle scattering data. Dynamical anomalies in supercooled water observed in inelastic neutron scattering experiments, manifested by low-frequency collective excitations resembling a boson peak, are investigated and found to be connected to the thermodynamically defined Widom line. Finally, x-ray absorption spectra are calculated for simulated water structures using density functional theory. An approximation of intra-molecular zero-point vibrational effects is found to significantly improve the relative spectral intensities but a structural investigation indicates that the classical simulations underestimate the amount of broken hydrogen bonds.

Abstract [sv]

Vatten är en komplex vätska med flera ovanliga egenskaper. Vår förståelse av dess fysiska, kemiska och biologiska egenskaper har utvecklats mycket sedan systematiska vetenskapliga studier började genomföras för mer än ett sekel sedan, men många viktiga frågor är fortfarande obesvarade. En ökad förståelse skulle på sikt kunna leda till framsteg inom viktiga områden så som medicinutveckling och vattenrening. Denna avhandling presenterar resultat kring vattnets struktur, dynamik och termodynamik. Fokusen ligger på teoretiska simuleringar som använts för att tolka experimentella data från huvudsakligen röntgen- och neutronspridning samt spektroskopier. Den strukturella känsligheten i röntgen- och neutrondiffraktionsdata undersöks via reverse Monte Carlo metoden och information om de partiella parkorrelationsfunktionerna erhålls. En ny metod för strukturmodellering av beräkningsintensiva data presenteras och används för att lösa en motsägelse mellan experimentell diffraktion och EXAFS angående syre- syre parkorrelationsfunktionen. Data från röntgensmåvinkelspridning modelleras med storskaliga klassiska molekyldynamiksimuleringar, och den observerade förhöjda småvinkelspridningen vid underkylda temperaturer kopplas till existensen av en Widomlinje härrörande från en vätske- vätske kritisk punkt i det djupt underkylda området vid höga tryck. En undersökning av inherenta strukturer i simuleringarna påvisar en underliggande strukturell bimodalitet mellan molekyler i oordnade högdensitetsregioner respektive ordnade lågdensitetsregioner, vilket ger en tydligare tolkning av den experimentella småvinkelspridningen. Dynamiska anomalier i underkylt vatten som har observerats i inelastisk neutronspridning, speciellt förekomsten av lågfrekventa excitationer som liknar en bosontopp, undersöks och kopplas till den termodynamiskt definierade Widomlinjen. Slutligen presenteras densitetsfunktionalberäkningar av röntgenabsorptionsspektra för simulerade vattenstrukturer. En approximation av intramolekylära nollpunktsvibrationseffekter förbättrar relativa intensiteteri spektrumen avsevärt, men en strukturanalys visar att klassiska simuleringar av vatten underskattar andelen brutna vätebindningar.

Place, publisher, year, edition, pages
Stockholm: Department of Physics, Stockholm University, 2011. 102 p.
Keyword
Liquid water, supercooled water, diffraction, structure modeling, molecular dynamics, x-ray spectroscopy, EXAFS, SAXS
National Category
Physical Sciences
Research subject
Theoretical Physics
Identifiers
urn:nbn:se:su:diva-56711 (URN)978-91-7447-287-5 (ISBN)
Public defence
2011-05-27, lecture room FA31, AlbaNova universitetscentrum, Roslagstullsbacken 21, Stockholm, 13:00 (English)
Opponent
Supervisors
Note
At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 6: Submitted. Paper 7: Submitted. Paper 8: Manuscript. Paper 9: Submitted. Available from: 2011-05-05 Created: 2011-04-22 Last updated: 2011-04-26Bibliographically approved

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