CRITICAL-BEHAVIOR OF FLUID BINARY-MIXTURES - INTERMOLECULAR PARAMETERS AND THERMODYNAMIC PROPERTIES

Direct correlation functions for binary fluid mixtures of Lennard-Jones spheres were calculated with the Percus-Yevick integral equation. Calculations were restricted to infinitely dilute solutions. The dependence of V2infinity and (partial derivative p/partial derivative x)V,T(infinity) on the relative values of the molecular parameters was established, including the near-critical region. It is clear that V2infinity diverges strongly as the critical point is approached and that its amplitude, (partial derivative p/partial derivative x)V,T(infinity), is well behaved and changes smoothly in the critical region. The presence of azeotropes may be established on the basis of the signs of the initial slopes (partial derivative p/partial derivative x)V,T(infinity). Differences between the results of this study and others which employ a van der Waals equation of state are shown to arise essentially in the different combining rules used to express the cross energy parameter epsilon-12. The way (partial derivative p/partial derivative x)V,T(infinity) varies with the ratio of molecular parameters is calculated showing that very strong cross interactions imply a tendency to colinearity of (partial derivative p/partial derivative T)CRL(infinity) with the extension of the solvent's vapor pressure curve.