Small-angle scattering and the structure of ambient liquid water

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 small-angle 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.