The inhomogeneous structure of water at ambient conditions.
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.
Huang
C
C
Stanford Synchrotron Radiation Lightsource, P.O.B. 20450, Stanford, CA 94309, USA.
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
D006860:Hydrogen Bonding; D008956:Models, Chemical; D008968:Molecular Conformation; D013052:Spectrometry, X-Ray Emission; D013696:Temperature; D014867:Water / Q000737:chemistry*
D014867:Water
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