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Proc Natl Acad Sci U S A. 2009 Sep 8;106(36):15214-8. doi: 10.1073/pnas.0904743106. Epub 2009 Aug 13.

The inhomogeneous structure of water at ambient conditions.

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  • 1Stanford Synchrotron Radiation Lightsource, P.O.B. 20450, Stanford, CA 94309, USA.

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 approximately 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 (nondirectional 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.

Comment in

  • The liquid water polymorphism. [Proc Natl Acad Sci U S A. 2009]
  • Is ambient water inhomogeneous on the nanometer-length scale? [Proc Natl Acad Sci U S A. 2010]
PMID:
19706484
[PubMed - indexed for MEDLINE]
PMCID:
PMC2741230
Free PMC Article

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