Binary p-dihalobenzene systems - correlation of thermochemical and phase-diagram data

The excess Gibbs energy difference between the liquid and the mixed crystalline state, having space group P2(1)/a with Z = 2, of binary p-dihalobenzene systems (halo = Cl, Br, I) can be represented, as a function of temperature and mole fraction of the second component (the one with the larger molar volume), by the formula DELTA-G(E)(T,X) = A(1 - T/theta) X(1-X)[1+B(1-2X)] The constant-theta is a characteristic temperature; for the family of p-dihalobenzene systems it has the value 500 K. The constant B, which is a measure of the asymmetry of the thermodynamic mixing properties of a system, can be given as B = 0.2. The other binary characteristic, the constant A, stands in relation to the coefficient of crystalline isodimorphism epsilon-m. The latter expresses the degree of geometrical similarity of the unit cells of the two pure components. The relation between A and epsilon-m is A = -62(1-epsilon-m) kJ.mol-1