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The inhomogeneous structure of water at ambient conditions.


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.

SUBMITTER: Huang C 

PROVIDER: S-EPMC2741230 | biostudies-literature | 2009 Sep

REPOSITORIES: biostudies-literature

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The inhomogeneous structure of water at ambient conditions.

Huang C C   Wikfeldt K T KT   Tokushima T T   Nordlund D D   Harada Y Y   Bergmann U U   Niebuhr M M   Weiss T M TM   Horikawa Y Y   Leetmaa M M   Ljungberg M P MP   Takahashi O O   Lenz A A   Ojamäe L L   Lyubartsev A P AP   Shin S S   Pettersson L G M LG   Nilsson A A  

Proceedings of the National Academy of Sciences of the United States of America 20090813 36


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 spectroscop  ...[more]

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