A new theoretical treatment has been developed for predicting the thermodynamic properties of electrolytes up to and beyond the critical temperature of water (973 K and at pressures up to 1000 MPa). The model is based upon the classical Born equation corrected for non-Born hydration effects. The temperature and pressure behavior of electrolytes can now be accurately predicted from existing low temperature data. Only two constants are needed for each electrolyte at all temperatures and pressures, where data exist to test the theory.

The unified theory of electrolytes (J. Phys. Chem. B 2009, 113, 2398-2404) for predicting the standard state thermodynamic properties of aqueous electrolytes has been extended to include mixed solvent systems. The solubility of solid sodium chloride in mixed solvents (methanol/water concentration up to 75% w/w) was also measured up to 466 K and pressures near 7 MPa. The present model, together with a simple modification of Pitzer's thermodynamic treatment of aqueous solutions, allows a priori prediction of solubility of electrolytes in aqueous/organic systems to extreme temperatures and pressures. Solubility is predicted for sodium chloride and potassium chloride in mixed solvents (methanol/water, ethanol/water) over a wide range of temperatures and compositions from the extension of the unified theory of electrolytes to mixed solvents. Comparisons indicate good agreement in all cases to well within the uncertainties of the experimental data. The stoichiometric activity coefficients of saturated solution of sodium chloride in methanol/water mixed solvents were calculated up to 473.15 K. The stoichiometric activity coefficients, as a function of temperature at all concentrations (0 ≤ m ≤ m(sat)) and the entire range of mole fraction of methanol, were also calculated up to 473.15 K. The novelty of the present approach is that no additional parameters are required to account for the medium effect.

Recently a theoretical treatment (J. Phys. Chem. B 2009, 113, 2398-2404) was developed for predicting the standard state thermodynamic properties of electrolytes up to and beyond the critical temperature of water (1273 K and at pressures up to 1000 MPa). In general, the model requires sufficient data at 298.15 K including the Gibbs free energy of hydration and at two higher temperatures to fix two constants for each electrolyte. This communication describes an extension of this "two constant" theory to thermodynamic properties of polyatomic ions for which no accurate data for the Gibbs free energy of hydration exits at 298.15 K.

Solid sodium chloride solubility is measured at temperatures from 297 to 467 K at pressure of 6.45 MPa in ethane-1,2-diol/water mixed solvent systems and at a concentration of cosolvent up to 75% w/w. The corresponding solubility is also predicted from an extension of the unified theory of electrolytes to mixed solvents (J. Phys. Chem. B201211690339042) for sodium and potassium chloride up to 473.15 K and over the whole composition range of the cosolvent. A comparison of the predicted solubility with the corresponding experimental values from this study and the available literature data indicates good agreement in all cases to well within the uncertainties of the experimental data. Also, for the saturated solution of sodium and potassium chloride in ethane-1,2-diol/water mixed solvent systems, the stoichiometric activity coefficient values are estimated up to 473.15 K. These stoichiometric activity coefficients, over the complete range of mole fraction of the cosolvent, are then extended to all concentrations (0 ≤ m ≤ m sat), and the results are compared with the available data in the literature.

In this communication, we report the first calorimetric data for the standard state enthalpies of a solution of sodium chloride obtained from high dilution, down to (10(-3) m), integral heats of solution measurements to 596.30 K. Although there are no comparable thermodynamic data available at such high dilutions in the literature, the present results for NaCl(aq) can be used for many thermodynamic studies by others to achieve a complete thermodynamic description of this key electrolyte over very wide ranges of concentration. From the recently developed unified theory of electrolytes, the experimental data from this study were used to predict Gibbs free energies of hydration of sodium chloride up to 1100 K. These Gibbs free energies of hydration at different pressures and densities compare well with reported values obtained from ab initio calculations by others.