Description of vapor-liquid phase equilibria of the H (sub 2) O-NaCl system between 100 and 900 degrees C with a thermodynamic model based on the mean spherical approximation

The Mean Spherical Approximation (MSA) of the ion-dipole mixture is used to describe the vapour-liquid equilibria of the H (sub 2) O-NaCl system. Comparison with experimental data reveals that the H (sub 2) O-NaCl mixture cannot be solely modelled by the MSA ion-dipole model. Discrepancies have been significantly reduced by taking into account NaCl ion pair formation in vapours at all temperatures and in high-temperature liquids (T>387 degrees C). A van der Waals interaction term must be included at lower temperatures (T<387 degrees C) and is believed to describe effects of the solvation of ions by water molecules or short-range interactions between anions and cations. The model has been fitted in the 100-850 degrees C temperature range, and represents experimental data with a good accuracy from dilute aqueous solutions to fused salts. This model provides insights on the effects of electrostatic interactions (ion-ion, ion-dipole, dipole-dipole) and stresses the importance of NaCl ion pair formation and other effects on the vapour-liquid equilibria of the H (sub 2) O-NaCl system.