title: 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.
authors.affiliation: Stanford Synchrotron Radiation Lightsource, P.O.B. 20450, Stanford, CA 94309, USA.
authors.forename: C
authors.initials: C
authors.lastname: Huang
authors.forename: K T
authors.initials: KT
authors.lastname: Wikfeldt
authors.forename: T
authors.initials: T
authors.lastname: Tokushima
authors.forename: D
authors.initials: D
authors.lastname: Nordlund
authors.forename: Y
authors.initials: Y
authors.lastname: Harada
authors.forename: U
authors.initials: U
authors.lastname: Bergmann
authors.forename: M
authors.initials: M
authors.lastname: Niebuhr
authors.forename: T M
authors.initials: TM
authors.lastname: Weiss
authors.forename: Y
authors.initials: Y
authors.lastname: Horikawa
authors.forename: M
authors.initials: M
authors.lastname: Leetmaa
authors.forename: M P
authors.initials: MP
authors.lastname: Ljungberg
authors.forename: O
authors.initials: O
authors.lastname: Takahashi
authors.forename: A
authors.initials: A
authors.lastname: Lenz
authors.forename: L
authors.initials: L
authors.lastname: Ojamäe
authors.forename: A P
authors.initials: AP
authors.lastname: Lyubartsev
authors.forename: S
authors.initials: S
authors.lastname: Shin
authors.forename: L G M
authors.initials: LG
authors.lastname: Pettersson
authors.forename: A
authors.initials: A
authors.lastname: Nilsson
mesh_terms: D006860:Hydrogen Bonding; D008956:Models, Chemical; D008968:Molecular Conformation; D013052:Spectrometry, X-Ray Emission; D013696:Temperature; D014867:Water / Q000737:chemistry*
chemical_list: D014867:Water
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