Attractive, weakly attractive, and repulsive near-critical systems

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Abstract
Mixtures composed of trace amounts of a solute in the vicinity of a solvent’s critical point exhibit one of three types of behavior: repulsive, weakly attractive, or attractive. Each regime is characterized by the signs of the diverging solute partial molar volume (and enthalpy), and of the correlations between solute and solvent concentration fluctuations at infinite dilution. Attractive behavior, in which the former quantity is negative and the latter positive, underlies all of the actual or proposed applications of supercritical fluids to processes ranging from precipitation polymerization to activated carbon regeneration. The presence of trace amounts of a solute within a solvent characterized by long-ranged density fluctuations gives rise to a cooperative phenomenon which, for attractive systems, involves the formation of a large solvent-rich region around each solute molecule. Likewise, for repulsive behavior, the cooperative phenomenon involves the formation of a large solvent-deficient region around each solute molecule. The calculated volume, enthalpy, and entropy changes per solvent molecule are very similar to the corresponding condensation (attractive behavior) and evaporation (repulsive behavior) effects for the pure solvent at subcritical temperatures. This suggests a qualitative description of near-critical cooperative phenomena as induced phase transitions.