Resurrection of Hückel’s idea: Decoupling ion–ion and ion–water terms in activity coefficients via the state-dependent dielectric constant

Abstract

Although Hückel proposed the basic idea of using a concentration dependent dielectric constant, ϵ(c), to compute the activity coefficients of ions in electrolytes in 1925 (Hückel, 1925), a large amount of modeling studies appeared in the literature only after 2010 when we published our II+IW theory (Vincze et al., 2010) that splits the excess chemical potential into two terms corresponding to interactions between ions (II) and interactions between ions and water (IW). In this approach, the two terms are decoupled, which means that they can be computed independently with ϵ(c) being the only link between them. Here, we review our theory and other works based on Hückel’s suggestion by discussing several issues that are partly cornerstones of the theory, partly make it possible to put the theory into a larger context. These issues include the role of ϵ(c) and the ionic radii used in the II and IW term, the statistical mechanical methods to estimate the II term, the existence and interpretation of individual activities, phenomena associated with strong ionic correlations in multivalent electrolytes, and explicit-water models used in molecular dynamics simulations.