Generalized Debye-Hückel Theory for Ion Activities in Mixed-Salt and Mixed-Solvent Electrolyte Solutions

A generalized Debye-Hückel (GDH) theory is proposed for the thermodynamic modeling of ion activities in mixed-salt and mixed-solvent electrolyte solutions in a wide range of concentration, composition, temperature, and pressure. The theory is based on a molecular mean-field Poisson-Fermi model that treats ions and solvent molecules of any volume and shape with interstitial voids and takes account of ion-ion, ion-solvent, and solvent-solvent correlations, polarizability of solvent molecules and ions, and permittivity variations with ionic strength, temperature, polarizability, and location. The theory parameters (correlation length, relative permittivity, Born radius, solvation domain etc.) are either derived from physical principles or based on experiments. GDH is demonstrated how to fit experimental activity coefficient data sets and predict activity coefficients in a variety of compositions of single-salt, two-salt, three-salt, single-solvent, or two-solvent mixtures with a wide ranges of ionic valence, ionic strength, temperature, and pressure. Examples are also given to illustrate the novelty of GDH in terms of these properties and features, including comparisons with the traditional Debye-Hückel theory.