On the analogy between the restricted primitive model and capacitor circuits: Semi-empirical alternatives for over- and underscreening in the calculation of mean ionic activity coefficients

Abstract The analogy between the restricted primitive model and capacitor circuits, originally described decades ago for the Mean Spherical Approximation, is explored to demonstrate its transferability in linearized electrolyte theories. On this basis, we offer an explanation of why treating the salt diameter as a free adjustable parameter blurs differences between electrolyte theories in the calculation of mean ionic activity coefficients. Furthermore, a capacitor circuit analogy with an approximation of the Dressed Ion Theory is applied to develop a modified closest approach parameter “b” for the Pitzer-Debye-Huckel term. This modification is able to account for the qualitative effects of over- and underscreening in the calculation of mean ionic activity coefficients. This is achieved by defining “b” as a semi-empirical function that allows close resemblance with the multiple decay-length extension of the Debye-Huckel theory for high dielectric constant values and that lies close to recommended literature values of “b” for low dielectric constant values. Finally, as proof of principle, this modified semi-empirical Pitzer-Debye-Huckel term is implemented in the predictive COSMO-RS-ES model, an own reimplementation of the COSMO-RS theory developed for thermodynamic property calculations of electrolyte systems. It is shown that the modified semi-empirical Pitzer-Debye-Huckel term is an effective replacement for the recently published version of COSMO-RS-ES with explicit considerations for ion pairing. This reduces the modelling complexity by implicitly considering ion pairing and improves overall qualitative performance for the prediction of salt solubilities in mixed-solvent systems and even mean ionic activity coefficients in non-aqueous media.