Polar solvent structure in the theory of ionic solvation

Abstract

The description of ionic solvation in terms of modern electrodynamics of condensed media with spatial dispersion enables us rigorously to take account of the structure of a polar solvent. The basic physical principles of the dielectric formulation of polar systems are considered. On this basis the phenomenological theory of ionic solvation, which involves the collective interactions of the solvent particles and the statistical character of the structure of the solvent, is developed. It is shown that the free energy of solvation is less than given by the Born formula, due to the correlation of polarization fluctuations in space; these correlations are defined by the characteristic correlation ranges. It is shown that the so-called effective decrease of the dielectric constant near the ion in solvation phenomena may follow as a consequence of the structure of a pure liquid without taking dielectric saturation into consideration. A theoretical basis is given to the phenomenon of the reduction of the solvation of the solute with the strengthening of the structure of a pure solvent. The expression for the free energy of Born charging differs considerably from the Born formula; however, it reduces to the Born formula when the ionic radius is much greater that all the correlation ranges in the solvent. An analytic expression for the free energy of solvation is obtained on the basis of resonable approximations. To compare the theory with experiment, the hydration of a alkali cations and halide anions is considered. The single-ion free energies of hydration are calculated and compared with the experimental free energies for the alkali halide hydration at infinite dilution. Without use of adjustable parameters the results are essentially in quantitative agreement with experiment.