EPC-SAFT advanced – Part II: Application to Salt Solubility in Ionic and Organic Solvents and the Impact of Ion Pairing

Abstract

The applications of electrolyte thermodynamic models to non-aqueous systems is of great value to reduce experimental effort and gain inside into molecular interactions. A large-scale application is for example the design of advanced battery electrolytes. For non-aqueous electrolyte systems, the Born term was found to be important, as it accounts for the transfer of ions from water into non-aqueous medium. In part one of this study [Bülow et al., Fluid Phase Equilibria 2021, 112967] the Born term was combined with a concentration-dependent dielectric constant within the ePC-SAFT framework (electrolyte Perturbed-Chain Statistical Associating Fluid Theory). In the present work, the Bjerrum treatment for ion pairing was included in the Debye-Hückel framework within ePC-SAFT. The approach was validated by experimental data for the dissociation of salts in organic solvents derived from conductivity measurements. Further, solubility was modeled of alkali halides in organic solvents and in ionic liquids. Modeling solubility required access to the solubility product KSP, which does not depend on the solvent. The approach within this work was to first determine KSP using experimental solubility data in water and the respective ePC-SAFT predicted activity coefficients prior to predict activity coefficients in non-aqueous medium, finally yielding solubility. The so-determined solubility values were found to be in reasonable agreement with the experimental data without fitting model parameters to any data of the non-aqueous solutions. The solubility product requires the solid form of the precipitating salt to be equal for all solvents; as alkali salts precipitate from aqueous solutions as hydrates, the method cannot be applied. Therefore, a methodology is presented to extrapolate the high-temperature KSP of anhydrates to lower temperature. Using the so-extrapolated KSP allowed predicting solubility of non-solvates in other solvents.