A GLOBAL PHASE-DIAGRAM FOR THE ION-DIPOLE MODEL IN THE MEAN SPHERICALAPPROXIMATION

This work is a theoretical study of the phase relations of the ion-dip ole mixture, as estimated by the mean spherical approximation. Phase d iagrams have been calculated for different values of the model paramet ers (ion charge, dipole moment, dipole, and ion size) and classified a ccording to the nomenclature of van Konynenburg and Scott. The differe nt types of topologies have been mapped onto a global phase diagram in the space of only two dimensionless parameters, denoted respectively alpha(1) and alpha(2). The alpha(1) parameter measures the ratio of fo rce fields produced by dipoles and ions, whereas the alpha(2) paramete r is the ratio of the dipole diameter to the ion diameter. For mixture s of equal-sized ions and dipoles the following sequence of phase topo logy has been recognized with increasing alpha(1) parameter; type III, type IIIm, type IV, type II, type II with double azeotropy, type IV w ith double azeotropy, and type III-HA. More complex topologies are obt ained for alpha(2) values below and above 1 (gas-gas equilibria, heter oazeotropy, azeotropy). It is shown that the global phase diagram prov ides insights on the phase topology of water-salt and water-salt-gas s ystems. (C) 1998 American Institute of Physics. [S0021-9606(98)51725-4 ].