DESCRIPTION OF VAPOR-LIQUID PHASE-EQUILIBRIA OF THE H2O-NACL SYSTEM BETWEEN 100-DEGREES-C 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 us ed to describe the vapour-liquid equilibria of the H2O-NaCl system. Co mparison with experimental data reveals that the H2O-NaCl mixture cann ot be solely modelled by the MSA ion-dipole model. Discrepancies have been significantly reduced by taking into account NaCl ion pair format ion in vapours at all temperatures and in high-temperature liquids (T> 387 degrees C). A van der Waals interaction term must be included at l ower temperatures (T<387 degrees C) and is believed to describe effect s of the solvation of ions by water molecules or short-range interacti ons between anions and cations. The model has been fitted in the 100-8 50 degrees C temperature range, and represents experimental data with a good accuracy from dilute aqueous solutions to fused salts. This mod el provides insights on the effects of electrostatic interactions (ion -ion, ion-dipole, dipole-dipole) and stresses the importance of NaCl i on pair formation and other effects on the vapour-liquid equilibria of the H2O-NaCl system