ION HYDRATION IN SUPERCRITICAL WATER

A molecular thermodynamic model is presented for the prediction of the chemical potential of an ion in supercritical water, and also the standard free energy, internal energy, and entropy of hydration. The reference state consists of ion-water clusters in the gas phase at low pressure, containing integer numbers of water molecules from 1 to 6. The chemical potentials of the ion-water clusters are examined as a function of the relevant intermolecular forces, i.e., van der Waals repulsive and attractive forces, hydrogen bonding between the clusters and additional water molecules in the bulk, and Born electrostatic solvation. The hydrogen bonding stabilizes an ion-water cluster to a much greater extent than it is destabilized by repulsive forces. Consequently, hydrogen bonding increases the mean hydration number and the effective Born radius.