Equilibria between fluid phases in mixtures of non-electrolytes
At low temperatures gas—liquid and liquid—liquid phase equilibria are determined by the balance between the like and unlike intermolecular potentials in a mixture For molecularly simple systems it is now clear that the unlike characteristic energies are weaker than the geometric means of the like and there is evidence that this difference increases with difference of molecular size In mixtures of aliphatic fluorocarbons and hydrocarbons the energy difference is large enough to lead to liquid—liquid equilibria and to upper critical solution temperatures (UCST). If the molecules are more complicated, by virtue of either their polarity or their ability to form charge-transfer complexes, then the effective unlike interactions can be smaller or larger than the geometric mean. The former case is understood, at least seniquantitatively, and can lead again to UCST. The latter can lead in special circumstances to lower critical points (LCST) Here our understanding is less satisfactory since only lattice models have been studied theoretically. At higher temperatures and pressures both simple and complicated systems have critical lines some of which can unambiguously be called gas—liquid lines and some liquid—liquid but many of which defy such simple classification The exploration of the kinds of critical line which can occur has recently been an active field of experimental research and It iS probable that there is now little really new to be discovered for binary mixtures. However, the precise behaviour of the therm dynamic functions on critical lines is a field which is ripe for further study since Griffiths and Wheeler have set up a classical thermodynamic scheme of classification This leads to many precise predictions which now await con firation or disproof. Our attempts to understand the critical behaviour of mixtures in terms of the intermolecular forces has proceeded along two lines First the great variety of critical lines has proved surprisingly easy of at least semiquantitative interpretation. Even van der Waals' equation, with suitable choice of the parameters, can represent most of the phenomena, and one can do better by using more accurate equations On the other hand, the nature of the singu laritics tt critical points in both one and two-component systems has proved one of the most formidable problems in statistical thermodynamics. Such progress as has been made is reviewed.