Composition-dependence of relative static permittivity in ePPC-SAFT for mixed-solvent alkali halides

This work investigates the impact of the solvent and salt composition dependence of the relative static permittivity (RSP), a key input of the primitive electrolyte model, on the ePPC-SAFT model for mixed-solvent electrolyte solutions. Systematic deviations of mean ionic activity coefficient (MIAC) are observed as the aqueous ePPC-SAFT model is extended to mixed-solvent electrolyte solutions. To accurately represent the thermodynamic properties, approaches are proposed to correct the model's co-solvent and/or salt composition dependence without changing the aqueous part of the model: the RSP co-solvent composition dependence, the salt composition dependence in the co-solvent, or both are corrected. The obtained model is parameterized in an ion-specific approach based on critically evaluated databases of MIAC, vapor-liquid equilibria, and density. All 3 approaches accurately represent the thermodynamic properties, significantly improving upon the original RSP model. However, in an analysis of the contribution of the terms to the ionic and solvent activity coefficients, unphysical increase of the MSA contribution to MIAC is observed when only the salt composition dependence is corrected in the RSP model. Furthermore, unphysical vapor pressure elevation is predicted in the (alcohol + salt) binary mixtures when both co-solvent and salt composition dependence are corrected. Therefore, it is recommended to correct for the RSP co-solvent composition dependence only. In addition, the behavior of MIAC with alcohol composition and salt type, the effective RSP calculated using the model, and the extrapolation of the ePPC-SAFT to non-aqueous electrolyte solutions are discussed.