Potential-energy functions for the solvation of alkali-metal cations

Abstract

The purpose of this paper is to explore the use of several model potential-energy functions in order to try better to understand the nature of the forces which operate between simple monatomic cations and the molecules of solvent which surround them. This work builds upon and extends a recently reported initial treatment. In that report, the potential energy was modelled as the sum of an exponential (Born-type) repulsion and an ionic–dipolar attraction. In this paper we consider composite functions which consist variously of an exponential form of repulsion or an inverse R-12 repulsion, and attractions which are expressed in terms of the basic ionic–dipolar terms together with additional terms which depend upon the polarizabilities of the ion and the solvent. No more than two adjustable parameters are used. The force constants for the far-infrared-active vibrations of the caged ions are known. With the use of the equilibrium conditions for the cage of solvent, the values of the parameters can be determined. Although all of the functions examined show the effect of the polarizability of the ion on the force constants, a potential which consists of independent R^–6 and R^–12 terms together with other experimentally fixed ionic–dipolar terms shows most clearly the effect of polarizability. We also find, however, that the consideration of ionic polarizability alone is inadequate to explain in a simple manner the variation of the force constants for the alkali-metal cations.