Modified Debye–Hückel–Onsager theory for electrical conductivity in aqueous electrolyte solutions: Account of ionic charge nonlocality.

This article presents a mean field theory of electrolyte solutions that extends the classical Debye–Hückel–Onsager (DHO) framework by incorporating ion-specific effects such as steric interactions, ion hydration, and the nonlocal spatial distribution of charge on solvated ions. Using an exponential (Slater-type) charge distribution function with a single adjustable parameter representing the charge smearing scale, the theory derives analytical expressions for ion mobility and electrical conductivity, including relaxation and electrophoretic contributions, and successfully approximates experimental conductivity data for various aqueous 1:1, 2:1, and 3:1 electrolyte solutions over a wide concentration and temperature range. The model accounts for concentration-dependent viscosity and dielectric decrement effects and highlights the importance of considering both excluded volume and charge nonlocality for accurate conductivity predictions, especially at higher concentrations. While the theory aligns with Kohlrausch's limiting law at low concentrations, it also explains deviations at higher concentrations due to ion-specific interactions, and it identifies limitations related to ion pairing and the assumption of identical charge form factors for ions.