SOLVATION IN SUPERCRITICAL WATER

The aim of this work is to determine the solvation structure in supercritical water compared with that in ambient water and in simple supercritical solvents. Molecular dynamics studies have been undertaken of systems that model ionic sodium and chloride, atomic argon, and molecular methanol in supercritical aqueous solutions using the simple point charge model of Berendsen for water. Because of the strong interactions between water and ions, ionic solutes are strongly attractive in supercritical water, forming large regions of increased local water density around each ion comparable to the solvent structures surrounding attractive solutes in simple supercritical fluids. Likewise, the deficit of water molecules surrounding a dissolved argon atom in supercritical aqueous solutions is comparable to that surrounding repulsive solutes in simple supercritical fluids. Methanol appears to be a weakly attractive or even repulsive solute in supercritical water. Only a small number of excess water molecules (if any) surround a methanol molecule in supercritical water, and this becomes a deficit at higher density. The number of hydrogen bonds per water molecule in supercritical water was found to be about one-third the number in ambient water. The number of hydrogen bonds per water molecule surrounding a central particle in supercritical water was only weakly affected by the identity of the central particle - atom, molecule or ion. These results should be helpful in developing a qualitative understanding of important processes which occur in supercritical water.