A model of solvent structure around ions

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Abstract
The nature of solvent structure around ions is considered using a model of hard spheres with embedded point charges in a solvent of hard spheres with embedded point dipoles. The statistical mechanics of this model is treated in the mean spherical approximation which is a natural extension of the Debye–Hückel theory of electrolytes to include discrete charges and dipoles of finite size. Our results include (i) a modified expression for the Born energy which had been used empirically to fit solubility data, (ii) explicit forms for the polarization density about an ion from which we can deduce the orientation order of the dipolar solvents and the validity or otherwise of the concept of a ’’local’’ dielectric constant near charged bodies, and (iii) the form of the interaction free energy (potential of mean force) between ions at separations comparable to the solvent size. In presenting these results which are given in detail in Section IV only a familiarity with the general description of this model given in Section II is assumed; it will not be necessary to refer to the mathematical detail in Section III.

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