STABILITY OF FLUIDS WITH MORE THAN 2 COMPONENTS .1. GENERAL THERMODYNAMIC THEORY AND CONCENTRATION-CONCENTRATION STRUCTURE FACTOR

A complete theoretical basis is presented for integral equation invest igations on the phase stability of simple fluid mixtures with any numb er of components. Thermodynamic criteria for local stability in terms of the Helmholtz free energy are reviewed accurately and compared with those expressed through the Gibbs free energy. The equivalence of sev eral forms of stability condition is proved by means of new thermodyna mic identities. Each form of stability criterion consists of a set of inequalities, and the reducibility of each set to only one necessary c ondition, namely to the most restrictive inequality, is emphasized. As a main result of the paper, Kirkwood-Buff theory is used and thermody namic stability criteria for an M component fluid are related to micro scopic structural properties, namely to correlation functions which ca n be calculated from integral equation theories. To complete such an e xtension of known formulas for binary mixtures, a generalization S-CC( (M))(k) is proposed of the concentration-concentration structure facto r S-CC(k) introduced by Bhatia and Thornton for two-component systems. The divergence of its long wavelength limit, S-CC((M))(k = 0), signal s phase instability.