A global phase diagram for the ion-dipole model in the mean spherical approximation

This work is a theoretical study of the phase relations of the ion-dipole mixture, as estimated by the mean spherical approximation. Phase diagrams have been calculated for different values of the model parameters (ion charge, dipole moment, dipole, and ion size) and classified according to the nomenclature of van Konynenburg and Scott. The different types of topologies have been mapped onto a global phase diagram in the space of only two dimensionless parameters, denoted respectively α₁ and α₂. The α₁ parameter measures the ratio of force fields produced by dipoles and ions, whereas the α₂ parameter is the ratio of the dipole diameter to the ion diameter. For mixtures of equal-sized ions and dipoles the following sequence of phase topology has been recognized with increasing α₁ parameter; type III, type IIIm, type IV, type II, type II with double azeotropy, type IV with double azeotropy, and type III-HA. More complex topologies are obtained for α₂ values below and above 1 (gas-gas equilibria, heteroazeotropy, azeotropy). It is shown that the global phase diagram provides insights on the phase topology of water-salt and water-salt-gas systems.