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The inhomogeneous structure of water at ambient conditions

Huang, Congcong ; Wikfeldt, K. T. ; Tokushima, T. LU ; Nordlund, D. ; Harada, Yoshihisa ; Bergmann, Uwe ; Niebuhr, M. ; Weiss, T. M. ; Horikawa, Yuka and Leetmaa, Mikael , et al. (2009) In Proceedings of the National Academy of Sciences of the United States of America 106(36). p.15214-15218
Abstract

Small-angle X-ray scattering (SAXS) is used to demonstrate the presence of density fluctuations in ambient water on a physical length-scale of ≈1 nm; this is retained with decreasing temperature while the magnitude is enhanced. In contrast, the magnitude of fluctuations in a normal liquid, such as CCl 4, exhibits no enhancement with decreasing temperature, as is also the case for water from molecular dynamics simulations under ambient conditions. Based on X-ray emission spectroscopy and X-ray Raman scattering data we propose that the density difference contrast in SAXS is due to fluctuations between tetrahedral-like and hydrogen-bond distorted structures related to, respectively, low and high density water. We combine our... (More)

Small-angle X-ray scattering (SAXS) is used to demonstrate the presence of density fluctuations in ambient water on a physical length-scale of ≈1 nm; this is retained with decreasing temperature while the magnitude is enhanced. In contrast, the magnitude of fluctuations in a normal liquid, such as CCl 4, exhibits no enhancement with decreasing temperature, as is also the case for water from molecular dynamics simulations under ambient conditions. Based on X-ray emission spectroscopy and X-ray Raman scattering data we propose that the density difference contrast in SAXS is due to fluctuations between tetrahedral-like and hydrogen-bond distorted structures related to, respectively, low and high density water. We combine our experimental observations to propose a model of water as a temperature-dependent, fluctuating equilibrium between the two types of local structures driven by incommensurate requirements for minimizing enthalpy (strong near-tetrahedral hydrogen-bonds) and maximizing entropy (non-directional H-bonds and disorder). The present results provide experimental evidence that the extreme differences anticipated in the hydrogen-bonding environment in the deeply supercooled regime surprisingly remain in bulk water even at conditions ranging from ambient up to close to the boiling point.

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publishing date
type
Contribution to journal
publication status
published
keywords
Density fluctuations, Liquid-liquid hypothesis, Small angle X-ray scattering, Water structure, X-ray spectroscopy
in
Proceedings of the National Academy of Sciences of the United States of America
volume
106
issue
36
pages
5 pages
publisher
National Academy of Sciences
external identifiers
  • scopus:70349326166
  • pmid:19706484
ISSN
0027-8424
DOI
10.1073/pnas.0904743106
language
English
LU publication?
no
id
22eb7589-9403-4382-b91b-6f13633dd25e
date added to LUP
2024-04-22 14:25:44
date last changed
2024-05-06 15:30:27
@article{22eb7589-9403-4382-b91b-6f13633dd25e,
  abstract     = {{<p>Small-angle X-ray scattering (SAXS) is used to demonstrate the presence of density fluctuations in ambient water on a physical length-scale of ≈1 nm; this is retained with decreasing temperature while the magnitude is enhanced. In contrast, the magnitude of fluctuations in a normal liquid, such as CCl <sub>4</sub>, exhibits no enhancement with decreasing temperature, as is also the case for water from molecular dynamics simulations under ambient conditions. Based on X-ray emission spectroscopy and X-ray Raman scattering data we propose that the density difference contrast in SAXS is due to fluctuations between tetrahedral-like and hydrogen-bond distorted structures related to, respectively, low and high density water. We combine our experimental observations to propose a model of water as a temperature-dependent, fluctuating equilibrium between the two types of local structures driven by incommensurate requirements for minimizing enthalpy (strong near-tetrahedral hydrogen-bonds) and maximizing entropy (non-directional H-bonds and disorder). The present results provide experimental evidence that the extreme differences anticipated in the hydrogen-bonding environment in the deeply supercooled regime surprisingly remain in bulk water even at conditions ranging from ambient up to close to the boiling point.</p>}},
  author       = {{Huang, Congcong and Wikfeldt, K. T. and Tokushima, T. and Nordlund, D. and Harada, Yoshihisa and Bergmann, Uwe and Niebuhr, M. and Weiss, T. M. and Horikawa, Yuka and Leetmaa, Mikael and Ljungberg, Mathias P and Takahashi, O. and Lenz, A. and Ojamäe, Lars and Lyubartsev, A. P. and Shin, S. and Pettersson, L. G.M. and Nilsson, A.}},
  issn         = {{0027-8424}},
  keywords     = {{Density fluctuations; Liquid-liquid hypothesis; Small angle X-ray scattering; Water structure; X-ray spectroscopy}},
  language     = {{eng}},
  month        = {{09}},
  number       = {{36}},
  pages        = {{15214--15218}},
  publisher    = {{National Academy of Sciences}},
  series       = {{Proceedings of the National Academy of Sciences of the United States of America}},
  title        = {{The inhomogeneous structure of water at ambient conditions}},
  url          = {{http://dx.doi.org/10.1073/pnas.0904743106}},
  doi          = {{10.1073/pnas.0904743106}},
  volume       = {{106}},
  year         = {{2009}},
}