Equation Of State For Mixed-Solvents Electrolyte Solutions

Theoretical and engineering models have been made great progress in predicting the themodynamic properties of electrolyte solutions in recent years. Many types of phase behavior have been reasonably determined including gas-liquid, liquid-liquid and liquid-solid equilibria, which is essential to the design of industrial processes. We had successfully developed a class of theoretical models based on Mean Spherical Approximation (MSA) and perturbation theory (PT). These models have been established in frame of statistical mechanics, by which the activity coefficient, osmotic coefficient and density of electrolyte solutions can be predicted simultaneously with a few molecule-based parameters. At present, most of the thermodynamic models are based on semi-empirical equations for electrolyte in mixed solvents, such as Pitzer and UNIFAC. To the best of our knowledge, the theoretical models derived from statistical mechanics have been not reported in literature.In this work, we established an equation of state (EOS) based on Helmholtz free energy, in combination of perturbation theory, low density expansion of non-primitive MSA and the latest progress of Statistical Associating Fluid Theory (SAFT). The equation of state was successfully applied to various electrolyte solutions, including aqueous, alcohol and water-alcohol mixed solvents. By introducing a suitable mixing rule, the activity coefficient, saturated vapor pressure and bubble point can be well predicted within certain temperature and pressure ranges by the EOS simultaneously, without any explicit expressions of dielectric constant.There are two adjustable parameters for salt-alcohol systems:one is cation-dependent and the other is salt dependent. The EOS was successfully applied in different temperatures by introducing a single temperature coefficient. For mixed-solvent electrolytes, such as salt-water-alcohol, only one additional parameter was added. The EOS-predicted properties agreed well with the published experimental data. In summary, the EOS has solid statistical mechanics foundation and good capability of extrapolation and prediction. The limitation of the EOS was also discussed. Further advices on developing more reliable models of electrolyte solutions were also given.