Structure of Aqueous Solutions of Ions and Neutral Solutes at Infinite Dilution at a Supercritical Temperature of 683 K

We discuss the structure of alkali metal ions, halide ions, and uncharged solutes at infinite dilution in supercritical water solutions, at solvent densities of 0.35, 0.20, and 0.997 g cm-3 at a temperature of 683 K using the SPC/E model for water. This model has critical constants (Tc = 640 K, ρc = 0.29 g cm-3) which compare well with the corresponding values (Tc = 647 K, ρc = 0.322 g cm-3) for real water. The solute−water pair correlation functions are qualitatively different for the charged and uncharged solutes at 683 K at both 0.35 and 0.20 g cm-3 solvent densities, with water expelled from the immediate vicinity of the uncharged solute but retained and compressed in the neighborhood of a small ion. Increasing the solvent density to 0.997 g cm-3 at 683 K leads to dramatic changes in the solvent structure around an uncharged solute, with the formation of hydrogen-bonded cages analogous to those observed at room temperature (298 K) at the same solvent density. The primary hydration numbers of the ions at 683 K and solvent density of 0.35 g cm-3 are nearly the same as the corresponding values at room temperature at a solvent density of 0.997 g cm-3. The partial molar volumes of the ions and uncharged species at the supercritical temperature are different in sign and are explained in terms of a simple model. The dynamics of ions and uncharged solutes under the same supercritical conditions are discussed in the companion to this paper.