Influence of Cosolutes on Chemical Equilibrium: a Kirkwood–Buff Theory for Ion Pair Association–Dissociation Processes in Ternary Electrolyte Solutions

Abstract

We present an application of the Kirkwood–Buff theory, which is introduced to describe the influence of cosolutes on the chemical equilibrium between dissociated and associated ion pairs in ternary electrolyte solutions. Our approach makes use of Kirkwood–Buff integrals and the introduction of a local/bulk partition model. The cosolute species can be either charged or uncharged, and our approach is applicable for ideal and weak nonideal solutions in combination with low ion concentrations. As the main result, the theory reveals that differences in the local cosolute accumulation behavior around the ions induce a shift of the chemical equilibrium either to the associated or the dissociated state. The findings of our analysis are useful for a deeper understanding of electrolyte solutions in modern electrochemical storage devices. All results are verified by atomistic molecular dynamics simulations in terms of sodium chloride pairs in dimethylacetamide–water mixtures.