Observation of chloride-ion hydration in high-temperature liquid and supercritical water by spherical harmonic expansion analysis

The structure of the Cl− hydration shell in high-temperature (77 and 197 °C) and supercritical (375 °C) water at moderate densities (roughly twice the critical density of 0.356 g/cm3) as well as in water at an ambient temperature, is investigated by means of neutron diffraction with isotope substitution on Cl− in aqueous 3 and 9 m solutions of LiCl in D2O. These relatively high densities (compared to the critical density) are used here to maintain the salt solubility at a sufficiently high value that a measurable signal is obtained in the isotope difference. The data are subjected to an empirical potential structure refinement which enables individual site-site radial distribution functions to be estimated. The same structure refinement is also used to estimate the spherical harmonic coefficients of the Cl-water orientational pair correlation function, and these are used in turn to reconstruct the local environment around Cl− in solution. A marked disordering of the Cl− hydration shell is observed as the critical point is approached. The orientational correlation functions of the water-molecule dipole vectors around Cl− in ambient water show a strong peak at a distance of about 3.2 Å, and centered at an angle of about 126°±30°, with respect to the Cl-O(D2O) axis, confirming an almost linear Cl⋯D-O hydrogen bond but with a significant spread of orientations about this value. At the higher-temperatures the peak remains at about the same position, but becomes progressively weaker in amplitude and with a larger spread of angles, suggesting a gradual decrease in orientational correlation of the water molecules around Cl−. At the supercritical temperature, the peak is even weaker, and there is evidence for a broader hydration shell developing at a distance of r≈3.9 Å in which the water molecules are apparently randomly oriented: they apparently become more loosely packed than at lower temperatures, without being dispelled from the ion hydration shell.