Discontinuous model for hydration of monatomic ions

Abstract

Calculations have been made of the free energies of hydration of monatomic cations and anions. The water molecules were assumed to consist of three invariant point charges, and each charge-charge interaction was calculated. In addition, each water molecule was considered to have a polarizable dipole, situated on the bisector of the H—O—H angle, and the individual ion-(induced dipole) interactions were computed. The London equation was used to calculate the dispersion forces for the ion and each water molecule, and for nearest-neighbour water molecules. Repulsion forces were calculated on the basis of a Lennard-Jones potential, the constant being evaluated so as to give the experimental intermolecular distance. The calculated free energies were minimized with respect to rotation of the water molecules about their axes of symmetry. Calculations were made for both tetrahedral and octahedral hydration; as expected, the agreement with experiment is better for tetrahedral hydration with the small cations, and for octahedral hydration with the larger ones. Some sources of error are considered.