The interatomic structure of water at supercritical temperatures

Abstract

LIQUID water, the medium in which life both began and persists, is in many ways a most unusual fluid. Much is known about the macroscopic properties of the condensed and gaseous states of water, but our understanding of the microscopic forces that define water structure remains incomplete¹. This structure, described in terms of correlations between the pairs of atoms O–H, O–O and H–H (ref. 2), can be studied by neutron diffraction techniques involving isotopic substitution³. In particular, the signature of hydrogen bonding is apparent in neutron diffraction experiments as a peak in the pair correlation function of O and H (g_OH (r)) at about 1.9 Å (refs 3, 4). Here we extend such studies into the supercritical regime of water, in which there is no longer any distinction between the liquid and vapour phases. We find that at the supercritical temperature of 400 °C almost all hydrogen bonding is broken down, even though the thermal energy k_BT is considerably less than the energy of the hydrogen bond. Our results are markedly different from the predictions of computer simulations under comparable conditions⁵ using a common model of water^5–7. Our results provide a sensitive test of models of water structure more generally, and give some indication of the environment that solute molecules will experience in extraction and reaction processes that employ supercritical water as a solvent⁸.