The inhomogeneous structure of water at ambient conditions
- C. Huanga,
- K. T. Wikfeldtb,
- T. Tokushimac,
- D. Nordlunda,
- Y. Haradac,d,
- U. Bergmanna,
- M. Niebuhra,
- T. M. Weissa,
- Y. Horikawac,e,
- M. Leetmaab,
- M. P. Ljungbergb,
- O. Takahashif,
- A. Lenzg,
- L. Ojamäeg,
- A. P. Lyubartsevh,
- S. Shinc,i,
- L. G. M. Petterssonb and
- A. Nilssona,b,1
- aStanford Synchrotron Radiation Lightsource, P.O.B. 20450, Stanford, CA 94309;
- bFYSIKUM, AlbaNova, and
- hDivision of Physical Chemistry, Stockholm University, S-10691 Stockholm, Sweden;
- cRIKEN/SPring-8, Sayo-cho, Sayo, Hyogo 679-5148, Japan;
- dDepartment of Applied Chemistry, University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-8656, Japan;
- eDepartment of Physical Science, Hiroshima University, Higashi-Hiroshima 739-8526, Japan;
- fDepartment of Chemistry, Hiroshima University, Higashi-Hiroshima 739-8526, Japan;
- gDepartment of Chemistry, Linköping University, S-581 83 Linköping, Sweden; and
- iInstitute for Solid State Physics, University of Tokyo, Kashiwanoha, Kashiwa, Chiba 277-8581, Japan
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Edited by H. Eugene Stanley, Boston University, Boston, MA, and approved July 7, 2009 (received for review May 7, 2009)
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 CCl4, 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.
- density fluctuations
- liquid–liquid hypothesis
- small angle X-ray scattering
- water structure
- X-ray spectroscopy
Footnotes
- 1To whom correspondence should be addressed. E-mail: nilsson{at}slac.stanford.edu
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Author contributions: A.N. designed research; C.H., K.T.W., T.T., D.N., Y. Harada, U.B., M.N., T.M.W., Y. Horikawa, M.L., M.P.L., O.T., A.L., L.O., A.P.L., S.S., and L.G.M.P. performed research; C.H., T.T., M.N., and D.N. analyzed data; and C.H., K.T.W., L.G.M.P., and A.N. wrote the paper.
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The authors declare no conflict of interest.
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This article is a PNAS Direct Submission.
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See Commentary on page 15097.
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This article contains supporting information online at www.pnas.org/cgi/content/full/0904743106/DCSupplemental.
