MOLECULAR-BASIS FOR THE BORN MODEL OF ION SOLVATION

Statistical mechanical theory of aqueous solutions is used to provide an interpretation for the Born model of ion hydration at the molecular level. The analysis is based on an asymptotic solution to the extended RISM integral equation in the limit of a large spherical solute. It is shown that a construction of the appropriate direct correlation function yields the Born model for the electrostatic free energy of solvation. In this approximation the free energy of solvation equals half the average solute-solvent interaction energy. Results of the RISM-HNC calculations for microscopic ions as a function of charge are presented, and the Born radius of hydration is interpreted in terms of the first peak in the solute-solvent radial distribution function: in water this leads to different radii for positive and negative ions. Analysis indicates that the Born model gives the dominant contribution to the free energy of charging a microscopic ion in water but that it is not appropriate for the calculation of solvation enthalpies because of their sensitivity to the temperature dependence of the Born radius. © 1990 American Chemical Society.