The inhomogeneous structure of water at ambient conditions

TLDR

Water exhibits density fluctuations that increase with decreasing temperature, unlike normal liquids such as CCl₄, a behavior also seen in molecular dynamics simulations. The study aims to explain SAXS contrast in water as arising from fluctuations between tetrahedral‑like low‑density and hydrogen‑bond‑distorted high‑density structures. A temperature‑dependent model is proposed in which water fluctuates between these two local structures, driven by competing enthalpic and entropic forces. SAXS measurements show 1 nm‑scale density fluctuations whose magnitude grows as temperature decreases, and experimental evidence indicates that the pronounced hydrogen‑bonding differences predicted for deeply supercooled water persist from ambient to near‑boiling 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 ≈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.