Further reflections about the “Born” term used in thermodynamic models for electrolytes

Abstract

This paper is a follow-up of previous work about the “Born” term, which must not be confused with the famous Born equation for ion solvation energy. The “Born” term is an empirical formula that is often introduced in explicit-solvent models for electrolytes in conjunction with a contribution for ion-ion interactions taken from a model with implicit solvent. An inconsistency is found for the expression commonly used in the literature. The “Born” term is supposed to describe the effect of ion–solvent interactions. The relevance of the “Born” term for that purpose is further investigated by considering various different situations: The transfer of an ion from pure water into a pure molten salt is examined; then, the nonprimitive mean spherical approximation theory for mixtures of ions and dipoles (MSA-ID) is used to get further insight in the case of a waterlike solvent and, besides, in the limit of low density and low coupling where the MSA becomes exact. The various situations considered in this work further indicate that this term is unsuitable. The possibility of deriving a formula in the spirit of the original Born equation is also considered.