Thermodynamic modeling of aqueous and mixed-solvent alkali chloride solutions using an ion-pairing equation of state

In this work, the Binding-MSA (BiMSA) theory is implemented in an equation of state for the first time. The proposed model, BiMSA-electrolyte polar perturbed chain statistical associating fluid Theory (BiMSA-ePPC-SAFT), is applied to aqueous and mixed solvent electrolyte systems (water + methanol and water + ethanol) to investigate the impact of ion pairs. In a first step, a comparison is made between the Bjerrum and Wertheim theories for the calculation of the ion-ion association strength. The results obtained show that the Bjerrum theory is more successful in describing the association of ions especially in mixed solvent systems. In a second step, different types of relative static permittivity (RSP) models are implemented and compared. The obtained results reveal that using the Bjerrum theory with a volume-dependent RSP yields an ion-ion association strength that strongly changes with salinity. The models are further analyzed focusing on the relative importance of the various types of association (ion-solvent, solvent-solvent, solvent-cosolvent and cosolvent-cosolvent). It was observed that in an aqueous solution, ion-solvent and solvent-solvent association bonds are stronger and more important than those of ion pairing. However, for the mixed solvent systems, in high alcohol concentration, ion pairs exhibit the strongest bond.