Evaluation of the effect of segment diameter and combining rules in eSAFT-VR Mie modeling of aqueous electrolyte solutions

Electrolyte solutions are central to many industrial, geochemical, and biological processes, yet their thermodynamic modeling remains challenging. This work assesses the predictive performance of the eSAFT–VR Mie equation of state by focusing on two key modeling parameters: (i) the distance of closest approach, comparing the ion segment diameter ( σ ) with the effective hard–sphere diameter ( d ), and (ii) the choice of ion–ion combining rules, both dispersion–energy–based formulations, namely the Hudson–McCoubrey (CR1) and the modified Lennard–Jones (CR2) rules. Predictions of mean ionic activity coefficients (MIAC) and liquid densities were evaluated for 57 aqueous salts without additional parameter fitting. Results show that density is relatively insensitive to the choice of parameters, whereas MIAC exhibits salt– and concentration–dependent sensitivity, particularly for multivalent systems. The comparison of CR1 and CR2 highlights that no single combining rule performs universally best, with accuracy depending on the ion type and charge density. These findings provide guidance for selecting the parameters and improving predictive electrolyte models.