Modeling aqueous solutions with ePC-SAFT including a relative static permittivity model connected to Wertheim's Association Theory

A different approach for modeling electrolyte solutions is presented using PC-SAFT equation-of-state (EOS) as background. The electrostatic interactions are modeled by including a long-range term according to the complete Debye-Huckel (DH) framework. The Born term is attached to account for the solvation energy. An ion-solvent association term, based on the classical Wertheim's Association Theory, is also included. Within the ion-solvent term, it is possible to access the fraction of solvent molecules not bonded to ions, which is required in Maribo-Mogensen et al. (2013) relative static permittivity (RSP) model used in this work. Parameters for the ions Na⁺, K⁺, Cl⁻, and Br⁻ were regressed against literature data in an extended range of temperatures, pressures, and compositions. Three different ion site arrangements were studied. By the analysis of the individual ionic activity coefficients (IIAC's), it was possible to determine that the methodology better represents experimental data by adopting 2 association sites for the ions. The results for density, mean ionic activity coefficients (MIAC), and RSP are presented. Very good agreement with experimental data of MIAC's for the aqueous solutions of NaCl, NaBr, KCl, and KBr were obtained, even in high temperatures, pressures, and near the solubility limit of the salts. The excellent MIAC's results were obtained with reasonable agreement of the solution's RSP's, which very few electrolyte EOS's could achieve. An analysis of density deviations indicated that the model underpredicts the increase of density related to the addition of the salts in the solutions, especially in conditions near the critical point of water or at high salt concentration.