Influence of solute-solvent asymmetry upon the behavior of dilute supercritical mixtures

Infinitely dilute supercritical mixtures may be classified into three categories: attractive, weakly attractive, and repulsive, according to the sign of the solute's partial molar properties, and of the excess number of solvent molecules surrounding a given solute molecule. These quantities are arbitrarily large near the solvent's critical point. Their sign is determined by differences in size, energy, and shape between solute and solvent (molecular asymmetry). The attractive or repulsive character of a van der Waals mixture is determined by the ratio of solute to solvent specific energies, referred to the respective molecular volumes. For a lattice-gas mixture with constant cell size, the boundaries between the three regimes are a function of chain length ratio, segment energy ratio, and solvent length. A simplified perturbed hard chain model predicts van der Waals-like behavior in the limit of unit chain length, and lattice-like behavior in the limit of constant segment size. The classification into attractive and repulsive behavior can also be expressed in terms of slopes of critical lines. Attractive behavior near the solvent's critical point is a necessary condition for supercritical solubility enhancement; repulsive behavior near the less volatile component's critical point is closely related to gas-gas immiscibility.