Noncritical behavior of density fluctuations in supercooled water

Xie, Yonglin; Ludwig, Karl; Morales, G.; Hare, David E.; Sorensen, C. M.

doi:10.1103/physrevlett.71.2050

Cited by 118 publications

(101 citation statements)

References 22 publications

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“…18, and find a similar temperature dependence albeit shifted in absolute value. It is thus clear that the observed fluctuations in ambient water should be different from random thermal motion and instead similar to what has been discussed in the supercooled regime (10)(11)(12).…”

Section: Figurementioning

confidence: 76%

The inhomogeneous structure of water at ambient conditions

Huang

Wikfeldt

Tokushima

et al. 2009

Proc. Natl. Acad. Sci. U.S.A.

562

610

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Small-angle X-ray scattering (SAXS) is used to demonstrate the presence of density fluctuations in ambient water on a physical length-scale of Ϸ1 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 L iquid water shows many anomalies in its thermodynamic properties such as compressibility, density variation and heat capacity (1-4). In the low-temperature regime, below the freezing point, these properties deviate strongly from normal and theories, related to a liquid-liquid phase transition between high and low density water, have been proposed to account for these anomalies (5). Although the anomalies are extreme in the supercooled region they are also present at ambient conditions where most of waters' physical, chemical and biological processes of importance occur. In contrast, water at ambient conditions has traditionally been considered as a homogeneous distribution of near-tetrahedral hydrogen-bonded (H-bonded) structures with thermal fluctuations increasing with temperature. This picture has been challenged by recent studies based on X-ray Raman (XRS) and conventional X-ray absorption spectroscopy (XAS) (6), and X-ray emission spectroscopy (XES) (7), suggesting two distinct local structures with tetrahedral as a minority and a highly hydrogen-bond (H-bond) distorted asymmetrical as the majority. In particular the proposed predominant asymmetrical structure has caused intense debate in the last years (see refs. 7 and 8 for detailed discussion).SAXS and small-angle neutron scattering (SANS) provide the most direct probes of density variations or fluctuations on large length scales in a liquid. Through an enhancement of the structure factor at low momentum transfer, Q, small deviations from the average electron density at different length scales can be reliably identified (9). Previous SAXS studies of w...

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Section: Figurementioning

confidence: 76%

The inhomogeneous structure of water at ambient conditions

Huang

Wikfeldt

Tokushima

et al. 2009

Proc. Natl. Acad. Sci. U.S.A.

562

610

View full text Add to dashboard Cite

show abstract

“…16, or (2) S N ðQÞ ¼ S N ð0Þ over the entire small-Q region measured (21,27). The values of S N ð0Þ used in the Lorentzian function fitting procedure for experiment were calculated using Eq.…”

Section: Methodsmentioning

confidence: 99%

“…The normal component of scattering is either assumed to be Q-independent, i.e., S N ðQÞ ¼ S N ð0Þ over the small-Q region (21,27), where…”

Section: Figurementioning

confidence: 99%

Small-angle scattering and the structure of ambient liquid water

Clark

Hura²,

Teixeira³

et al. 2010

Proc. Natl. Acad. Sci. U.S.A.

191

184

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Structural polyamorphism has been promoted as a means for understanding the anomalous thermodynamics and dynamics of water in the experimentally inaccessible supercooled region. In the metastable liquid region, theory has hypothesized the existence of a liquid-liquid critical point from which a dividing line separates two water species of high and low density. A recent smallangle X-ray scattering study has claimed that the two structural species postulated in the supercooled state are seen to exist in bulk water at ambient conditions. We analyze new small-angle X-ray scattering data on ambient liquid water taken at third generation synchrotron sources, and large 32,000 water molecule simulations using the TIP4P-Ew model of water, to show that the small-angle region measures standard number density fluctuations consistent with water's isothermal compressibility temperature trends. Our study shows that there is no support or need for heterogeneities in water structure at room temperature to explain the small-angle scattering data, as it is consistent with a unimodal density of the tetrahedral liquid at ambient conditions. anomalous scattering | density distribution | isothermal compressibility | structural polyamorphism W ater appears to be a unique liquid relative to other fluids in exhibiting several anomalous features of structure, thermodynamics, and dynamics. Both experiment and molecular dynamics simulation have shown that water in the metastable supercooled region exhibits response functions and transport properties that appear to diverge near −45°C (1). Below its glass transition temperature, experimental evidence shows that there are polyamorphic states of water, in particular the formation of low-density amorphous and high-density amorphous glasses (2, 3). This polyamorphism has been promoted as a means for understanding the anomalous thermodynamics and dynamics of water, such as the large increase in its isothermal compressibility with decreasing temperature, when extrapolated into the experimentally inaccessible supercooled liquid region (3, 4). Polyamorphism underlies one of the main assumptions of the second critical point hypothesis (5), i.e., the postulated existence of a liquid-liquid critical point from which a dividing line separates two fluctuating species of high and low-density liquids (HDL and LDL).However, recent studies have shown that while there may be low-temperature criticality in the ST2 water model, there is no need to invoke an explanation for a second critical point in water based on structural polyamorphism (6). Furthermore, it is also possible to explain the existing anomalies without invoking an additional thermodynamic singularity, in the so-called singularityfree interpretation (4, 7). Sastry and coworkers have shown that a finite increase in isothermal compressibility upon lowering the temperature of a liquid that expands upon cooling (like water) is a thermodynamic necessity, and was illustrated for a water-like lattice model that has no singularities (7). This diversity in ...

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“…Do water molecules make the transition from liquid to nascent crystal one by one or in groups? The hydrogen bonding network does become increasingly cooperative with decreasing temperature (Hare and Sorensen 1990;Rice and Sceats 1981), but X-ray scattering data showed that the correlation lengths of density fluctuations in water are virtually independent of temperature (Xie et al 1993). However, those same experiments showed that the number of water molecules incorporated into clusters increased as the temperature decreased.…”

Section: Homogeneous Nucleationmentioning

confidence: 99%