MOLECULAR CLUSTERING IN SUPERCRITICAL SOLUTIONS

Molecular explanations have recently identified massive solvent clusters surrounding the dissolved solute as the causative agent of the striking solvent properties of supercritical fluids. Such clusters were first inferred from the large negative values of the solute partial molar volume near the solvent critical point; these clusters were estimated to contain about 100 solvent molecules per solute molecule in excess of the bulk average. Subsequent evidence from visible and ultraviolet absorption and fluorescence spectroscopy and from chemical reaction studies have provided qualitative support for the notion of solvent-solute clusters. Other spectroscopic evidence suggests the formation of solute-solute aggregates, as well, near the solvent critical point. Such structures on a molecular level are describable in terms of the molecular distribution functions. We present results on the size and radial profile of solvent-solute clusters and on solute-solute aggregation for systems representative of typical dilute, attractive, supercritical solutions calculated from integral equation theories. The microstructure of theoretically-predicted dilute, repulsive, supercritical solutions exhibits a deficit of solvent molecules surrounding the solute. Solute partial molar volume and chemical potential are predicted to increase dramatically in such repulsive mixtures as density is increased toward the critical point at slightly supercritical temperatures.