Interaction balance theory

The classification of electrolytes and prediction of their properties is a fundamental challenge in electrolyte thermodynamics. Understanding the balance of ionic interactions is key to accurately describing solution behavior. We introduce the interaction balance theory, a framework that links microscopic ionic interactions to macroscopic activity coefficients enabling the decomposition of intermolecular interactions, systematic analysis of the sources of non-ideality, and a quantitative classification of electrolytes. Application to the sodium halides NaF, NaCl, NaBr, and NaI in water and non-aqueous solvents shows that the theory captures their distinct intermolecular behaviors, distinguishing the relative contributions of long-range Coulombic forces, short-range repulsions, and solvent-mediated interactions and correlating these decompositions with the salts' solubility. Our results also show that the minimum in activity coefficient marks the end of a specific equilibrium between the cumulative short- and long-range forces, following the considered Ewald decomposition of the forces. This approach provides a clear, quantitative method to interpret experimental data, disentangle what essentially builds the non-ideality of systems, and to guide the development of thermodynamic models for electrolytes, highlighting which interactions dominate in different systems.