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 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...

Microscopic properties of liquid water from combined ab initio molecular dynamics and energy decomposition studies

“…It has been suggested that a large fraction of molecules form only two strong HBs: one acceptor and one donor bond 2,152-156 . However, the"rings and chains" structure of liquid water 2 , as well as the inhomogeneous two-state model 152,157 implied by such an interpretation, have been challenged on many fronts 143,147,[158][159][160][161][162][163][164][165][166][167] and are a matter of an ongoing debate 80,143,147,[155][156][157][158][159][160][161][162][163][164][166][167][168][169][170][171][172] .…”

Two-Dimensional Infrared Spectroscopy of Supercooled Water

“…In liquid water, depending on the temperature, this may give rise to different hydrogen bonding motifs, which at ambient conditions seem to coexist as structural and dynamic heterogeneities. [3][4][5] Going down to the supercooled regime, these heterogeneities appear to be more pronounced, [6][7][8] and it is hypothesized that they can be understood as the residual of two distinct liquid phases. 9,10 One way to acquire information about this hydrogen bonding network is to monitor the changes of the OH (or OD) stretch frequency.…”

The OH stretch vibration of liquid water reveals hydrogen-bond clusters

“…a low-enthalpy, low-density form, and a highentropy, high-density form of water, which ultimately leads to the suspected liquid-to-liquid phase transition at low enough temperatures. Nevertheless, there has been experimental evidence that these two forms still exist at ambient temperatures or even up to the boiling point, in the sense of fluctuating and quickly inter-converting domains [8]. In light of this discussion, cluster 1 would be the low-enthalpy, low-density form, as judged form its temperature dependence (Tab.…”

“…In Ref. [8] it has been argued that water domains have a spatial correlation length of ≈10Å, but it has also been shown that SPC water cannot reproduce this large size. Our study nevertheless indicates that correlations exist up to at least the second solvation shell (≈5Å).…”

“…the directionality of hydrogen bonding that favors a lowdensity, tetrahedral (ice-like) form of water in order to minimize enthalpy, and maximizing entropy by nondirectional hydrogen bonds and disorder, resulting in a closer packing. Furthermore, recent small angle x-ray scattering evidenced that water is structurally inhomogeneous with spatial correlations over ≈10Å [8]. Even though these mesaurements were performed between room temperature and close to the boiling point, the experiments were discussed in the context of the liquid-to-liquid phase transition of water in the deeply supercooled regime, i.e.…”