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X-Ray Investigations of the Liquid-Liquid Critical Point Hypothesis in Supercooled Water
Stockholm University, Faculty of Science, Department of Physics.
2020 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis presents experimental x-ray scattering studies on supercooled liquid water. A liquid-liquid transition between two structurally distinct configurations has been found in deeply supercooled water, indicating the existence of a liquid- liquid critical point. The experiments were performed at large-scale x-ray facilities, mostly using free electron x-ray lasers including PAL-XFEL in Korea, SACLA in Japan, LCLS in the USA, SwissFEL in Switzerland and European XFEL in Germany, as well as using synchrotrons including APS in the USA, PETRA III in Germany and ESRF in France.

Two conceptually different experimental approaches have been used to investigate the metastable phase of supercooled water. The first approach is based on rapid evaporative cooling of μm-sized water droplets that are injected into a vacuum chamber. Using this method, supercooled liquid water samples with temperatures down to approximately 227 K have been obtained, with the lowest temperature limited by homogeneous ice crystallization occurring after just a few milliseconds. In a second approach, structurally arrested high-pressure and therefore high-density amorphous ice samples are heated by an ultrafast infrared laser pulse. The fast heating melts the ice into a corresponding high-density liquid. At short time delays between the heating laser pulse and a subsequent x-ray probe pulse, the supercooled liquefied sample still experiences the high internal pressure of the initial state. At longer pump-probe delay times the supercooled water sample releases its internal pressure through structural relaxation. Hence, varying the pump-probe delay allows to probe the sample at different pressures.

Together, these two approaches have been used to access a region within the metastable phase diagram of supercooled water that has previously been inaccessible. Using elastic x-ray scattering measurements as a structural probe of the liquid, we identified the existence of a liquid-liquid phase transition in deeply supercooled water. The observed phase transition is interpreted as the transition between a high-density and a low-density liquid phase. At high pressure this phase transition is discontinuous or first-order like, featuring a characteristic double-peak feature in the observed x-ray scattering intensity of the first diffraction maxima. At ambient pressure, however, we observe a continuous shift of the first diffraction maxima that is consistent with a continuous or second-order phase transition between the two liquids. Further evidence of a continuous phase transition at ambient pressure is seen in the temperature dependent maxima of the measured correlation length, isothermal compressibility and heat capacity, which indicate the existence of a Widom line.

In summary, the experiments support the existence of a liquid-liquid critical point where the experimentally observed Widom line and phase coexistence line would both meet. The main result, however, is the first experimental observation of a liquid-liquid transition within a pure liquid.

Place, publisher, year, edition, pages
Stockholm: Department of Physics, Stockholm University , 2020. , p. 58
Keywords [en]
water, supercooled water, x-ray scattering, free electron x-ray laser, liquid-liquid phase transition, liquid-liquid critical point, x-ray speckle visibility spectroscopy
National Category
Atom and Molecular Physics and Optics
Research subject
Chemical Physics
Identifiers
URN: urn:nbn:se:su:diva-180847ISBN: 978-91-7911-092-5 (print)ISBN: 978-91-7911-093-2 (electronic)OAI: oai:DiVA.org:su-180847DiVA, id: diva2:1426158
Public defence
2020-06-12, sal FB52, AlbaNova universitetscentrum, Roslagstullsbacken 21, digitally via Zoom: https://stockholmuniversity.zoom.us/s/239996391, 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 7: Manuscript. Paper 8: Manuscript. Paper 9: Manuscript. Paper 10: Manuscript.

Available from: 2020-05-20 Created: 2020-04-23 Last updated: 2020-05-25Bibliographically approved
List of papers
1. Temperature-Independent Nuclear Quantum Effects on the Structure of Water
Open this publication in new window or tab >>Temperature-Independent Nuclear Quantum Effects on the Structure of Water
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2017 (English)In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 119, no 7, article id 075502Article in journal (Refereed) Published
Abstract [en]

Nuclear quantum effects (NQEs) have a significant influence on the hydrogen bonds in water and aqueous solutions and have thus been the topic of extensive studies. However, the microscopic origin and the corresponding temperature dependence of NQEs have been elusive and still remain the subject of ongoing discussion. Previous x-ray scattering investigations indicate that NQEs on the structure of water exhibit significant temperature dependence [Phys. Rev. Lett. 94, 047801 (2005)]. Here, by performing wide-angle x-ray scattering of H2O and D2O droplets at temperatures from 275 K down to 240 K, we determine the temperature dependence of NQEs on the structure of water down to the deeply supercooled regime. The data reveal that the magnitude of NQEs on the structure of water is temperature independent, as the structure factor of D2O is similar to H2O if the temperature is shifted by a constant 5 K, valid from ambient conditions to the deeply supercooled regime. Analysis of the accelerated growth of tetrahedral structures in supercooled H2O and D2O also shows similar behavior with a clear 5 K shift. The results indicate a constant compensation between NQEs delocalizing the proton in the librational motion away from the bond and in the OH stretch vibrational modes along the bond. This is consistent with the fact that only the vibrational ground state is populated at ambient and supercooled conditions.

National Category
Physical Sciences
Research subject
Chemical Physics
Identifiers
urn:nbn:se:su:diva-147088 (URN)10.1103/PhysRevLett.119.075502 (DOI)000407557300009 ()
Available from: 2017-10-16 Created: 2017-10-16 Last updated: 2020-05-05Bibliographically approved
2. Maxima in the thermodynamic response and correlation functions of deeply supercooled water
Open this publication in new window or tab >>Maxima in the thermodynamic response and correlation functions of deeply supercooled water
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2017 (English)In: Science, ISSN 0036-8075, E-ISSN 1095-9203, Vol. 358, no 6370, p. 1589-1593Article in journal (Refereed) Published
Abstract [en]

Femtosecond x-ray laser pulses were used to probe micrometer-sized water droplets that were cooled down to 227 kelvin in vacuum. Isothermal compressibility and correlation length were extracted from x-ray scattering at the low-momentum transfer region. The temperature dependence of these thermodynamic response and correlation functions shows maxima at 229 kelvin for water and 233 kelvin for heavy water. In addition, we observed that the liquids undergo the fastest growth of tetrahedral structures at similar temperatures. These observations point to the existence of a Widom line, defined as the locus of maximum correlation length emanating from a critical point at positive pressures in the deeply supercooled regime. The difference in the maximum value of the isothermal compressibility between the two isotopes shows the importance of nuclear quantum effects.

National Category
Physical Sciences
Research subject
Chemical Physics
Identifiers
urn:nbn:se:su:diva-151198 (URN)10.1126/science.aap8269 (DOI)000418448000058 ()
Available from: 2018-01-11 Created: 2018-01-11 Last updated: 2020-05-05Bibliographically approved
3. Coherent X-rays reveal the influence of cage effects on ultrafast water dynamics
Open this publication in new window or tab >>Coherent X-rays reveal the influence of cage effects on ultrafast water dynamics
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2018 (English)In: Nature Communications, ISSN 2041-1723, E-ISSN 2041-1723, Vol. 9, article id 1917Article in journal (Refereed) Published
Abstract [en]

The dynamics of liquid water feature a variety of time scales, ranging from extremely fast ballistic-like thermal motion, to slower molecular diffusion and hydrogen-bond rearrangements. Here, we utilize coherent X-ray pulses to investigate the sub-100 fs equilibrium dynamics of water from ambient conditions down to supercooled temperatures. This novel approach utilizes the inherent capability of X-ray speckle visibility spectroscopy to measure equilibrium intermolecular dynamics with lengthscale selectivity, by measuring oxygen motion in momentum space. The observed decay of the speckle contrast at the first diffraction peak, which reflects tetrahedral coordination, is attributed to motion on a molecular scale within the first 120 fs. Through comparison with molecular dynamics simulations, we conclude that the slowing down upon cooling from 328 K down to 253 K is not due to simple thermal ballistic-like motion, but that cage effects play an important role even on timescales over 25 fs due to hydrogen-bonding.

National Category
Physical Sciences
Research subject
Chemical Physics
Identifiers
urn:nbn:se:su:diva-156793 (URN)10.1038/s41467-018-04330-5 (DOI)000432115300021 ()29765052 (PubMedID)
Available from: 2018-05-30 Created: 2018-05-30 Last updated: 2020-05-05Bibliographically approved
4. Apparent power-law behavior of water's isothermal compressibility and correlation length upon supercooling
Open this publication in new window or tab >>Apparent power-law behavior of water's isothermal compressibility and correlation length upon supercooling
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2019 (English)In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 21, no 1, p. 26-31Article in journal (Refereed) Published
Abstract [en]

The isothermal compressibility and correlation length of supercooled water obtained from small-angle X-ray scattering (SAXS) were analyzed by fits based on an apparent power-law in the temperature range from 280 K down to the temperature of maximum compressibility at 229 K. Although the increase in thermodynamic response functions is not towards a critical point, it is still possible to obtain an apparent power law all the way to the maximum values with best-fit exponents of gamma = 0.40 +/- 0.01 for the isothermal compressibility and nu = 0.26 +/- 0.03 for the correlation length. The ratio between these exponents is close to a value of approximate to 0.5, as expected for a critical point, indicating the proximity of a potential second critical point. Comparison of gamma obtained from experiment with molecular dynamics simulations on the iAMOEBA water model shows that it would be located at pressures in the neighborhood of 1 kbar. The high value and sharpness of the compressibility maximum observed in the experiment are not reproduced by any of the existing classical water models, thus inviting further development of simulation models of water.

National Category
Chemical Sciences Physical Sciences
Research subject
Chemical Physics
Identifiers
urn:nbn:se:su:diva-165683 (URN)10.1039/c8cp05862h (DOI)000454836700051 ()30489577 (PubMedID)
Available from: 2019-02-06 Created: 2019-02-06 Last updated: 2020-05-05Bibliographically approved
5. Intermediate range O-O correlations in supercooled water down to 235 K
Open this publication in new window or tab >>Intermediate range O-O correlations in supercooled water down to 235 K
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2019 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 150, no 22, article id 224506Article in journal (Refereed) Published
Abstract [en]

Wide angle x-ray scattering of supercooled water down to 234.8 K was studied using high energy x rays at the European Synchrotron Radiation Facility. The oxygen-oxygen pair distribution function (PDF) was calculated from the scattering pattern out to the 5th peak at an intermolecular distance, r approximate to 11 angstrom. We observe that the 4th peak and the 5th peak in the PDF increase in height upon supercooling. We also observe that the 4th peak position (r(4)) shifts to shorter distances upon supercooling consistent with previous studies, but we see a more rapid change at the lowest temperature. The running oxygen-oxygen coordination number is calculated for 5 different temperatures, and an isosbestic point at r(iso) = 3.31 +/- 0.05 angstrom was found corresponding to a coordination number of 4.39 +/- 0.15. The comparison of the PDF of the coldest water with that of amorphous ice shows distinct differences. We propose that there are 5-member pentamer rings in low density liquid-like structures giving rise to the sharp correlations at r approximate to 9 angstrom and r approximate to 11 angstrom.

National Category
Chemical Sciences Physical Sciences
Research subject
Chemical Physics
Identifiers
urn:nbn:se:su:diva-171133 (URN)10.1063/1.5100811 (DOI)000471692400035 ()31202250 (PubMedID)
Available from: 2019-08-16 Created: 2019-08-16 Last updated: 2020-05-05Bibliographically approved
6. Temperature dependent anomalous fluctuations in water: shift of approximate to 1 kbar between experiment and classical force field simulations
Open this publication in new window or tab >>Temperature dependent anomalous fluctuations in water: shift of approximate to 1 kbar between experiment and classical force field simulations
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2019 (English)In: Molecular Physics, ISSN 0026-8976, E-ISSN 1362-3028, Vol. 117, no 22, p. 3232-3240Article in journal (Refereed) Published
Abstract [en]

Here we report on the temperature dependence of the anomalous behaviour of water in terms of (i) its growth in tetrahedral structures, (ii) instantaneous spatial correlations from small angle x-ray scattering (SAXS) data, (iii) estimates of thermodynamic response functions of isothermal compressibility and (iv) thermal expansion coefficient. Water's thermal expansion coefficient is estimated for the first time at supercooled conditions from liquid water's structure factor. We used previously published data from classical force-fields of TIP4P/2005 and iAMOEBA to compare experimental data with molecular dynamics simulations and observe that these force-fields underestimate water's anomalous behaviour but perform better upon increasing pressure. We demonstrate that the molecular dynamics simulations can describe better the temperature dependent anomalous behaviour of ambient pressure water if simulated at 1 kbar. The deviation in anomalous fluctuations in the simulations is not restricted to approximate to 228 K but extends all the way to ambient temperatures.

Keywords
Supercooled water, molecular modelling, x-ray scattering, thermal expansion coefficient, isothermal compressibility
National Category
Chemical Sciences Physical Sciences
Research subject
Chemical Physics
Identifiers
urn:nbn:se:su:diva-173178 (URN)10.1080/00268976.2019.1649486 (DOI)000479874200001 ()
Available from: 2019-09-17 Created: 2019-09-17 Last updated: 2020-05-05Bibliographically approved
7. Experimental Observation of the Liquid-Liquid Transition in Bulk Supercooled Water
Open this publication in new window or tab >>Experimental Observation of the Liquid-Liquid Transition in Bulk Supercooled Water
(English)Manuscript (preprint) (Other academic)
National Category
Atom and Molecular Physics and Optics
Research subject
Chemical Physics
Identifiers
urn:nbn:se:su:diva-180842 (URN)
Available from: 2020-04-16 Created: 2020-04-16 Last updated: 2020-05-05Bibliographically approved
8. Enhancement and Maximum in the Isobaric Specific Heat Capacity Measurements of Deeply Supercooled Water using Ultrafast Calorimetry
Open this publication in new window or tab >>Enhancement and Maximum in the Isobaric Specific Heat Capacity Measurements of Deeply Supercooled Water using Ultrafast Calorimetry
(English)Manuscript (preprint) (Other academic)
National Category
Atom and Molecular Physics and Optics
Research subject
Chemical Physics
Identifiers
urn:nbn:se:su:diva-180844 (URN)
Available from: 2020-04-16 Created: 2020-04-16 Last updated: 2020-05-05Bibliographically approved
9. Anisotropic X-ray Scattering of Transiently Oriented Water
Open this publication in new window or tab >>Anisotropic X-ray Scattering of Transiently Oriented Water
(English)Manuscript (preprint) (Other academic)
National Category
Atom and Molecular Physics and Optics
Research subject
Chemical Physics
Identifiers
urn:nbn:se:su:diva-180845 (URN)
Available from: 2020-04-16 Created: 2020-04-16 Last updated: 2020-05-05Bibliographically approved
10. Double pulse X-ray speckle visibility on atomic length scales
Open this publication in new window or tab >>Double pulse X-ray speckle visibility on atomic length scales
(English)Manuscript (preprint) (Other academic)
National Category
Atom and Molecular Physics and Optics
Research subject
Chemical Physics
Identifiers
urn:nbn:se:su:diva-180846 (URN)
Available from: 2020-04-16 Created: 2020-04-16 Last updated: 2020-05-05Bibliographically approved

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