SAFT-VRE: Phase behavior of electrolyte solutions with the statistical associating fluid theory for potentials of variable range

A modification of the statistical associating fluid theory has recently bee n developed to model non-conformal fluids with attractive potentials of var iable range (SAFT-VR) which gives a very good description of the phase beha viour of water and its mixtures with nonelectrolytes. In the present paper we extend the SAFT-VR approach to deal with strong-electrolyte solutions (S AFT-VRE). The water molecules are modeled as hard spheres with four attract ive short-range sites to describe the hydrogen-bonding association. The ele ctrolyte molecules are modeled with two hard spheres of different size whic h describe the anion and cation respectively. The mean-spherical approximat ion (MSA) is used for the restricted primitive model (RPM) to account for t he long-range Coulombic ion-ion interactions, while the long-range water-wa ter and ion-water attractive interactions are modeled as square-well disper sive interactions treated via a second-order high-temperature expansion in the spirit of the SAFT-VR approach. We have studied nine single-salt aqueou s solutions and one mixed-salt system of characteristic strong electrolytes (alkali halides) in the temperature range between 273 and 373 K. Using onl y one transferable fitted parameter per ion, the experimental vapor pressur es and densities are very well described by the SAFT-VRE theory. As a limit of the MSA, the Debye-Huckel (DH) expression is used to describe the ion-i on interactions in one of the solutions. Due to the excellent description o f the solvent in the SAFT-VR approach, the experimental vapor pressure for an aqueous solution of sodium chloride is also very well described with thi s simple approach.