SOLVENT STRUCTURAL-CHANGES IN REPULSIVE AND ATTRACTIVE SUPERCRITICAL MIXTURES - A MOLECULAR-DISTRIBUTION STUDY

By examination of the molecular distribution functions, we are able to determine the long-range solvent structures that lead to ''repulsive'' or ''attractive'' supercritical-fluid behavior. Integral equation theories are used to obtain these distribution functions. Depletion of solvent molecules about the solute molecule over 20 molecular diameters is responsible for the positive growth of the partial molar volumes in repulsive mixtures; just as enrichment of solvent molecules over long ranges causes the collapse of the partial molar volumes in attractive mixtures. In fact, there is a competition between the ''compressibility'' of the solvent near the critical point and the ''affinity'' of the solvent toward the solute species. The macroscopic properties derive their major contribution from these long-range solvent-solute correlations, although there is evidence of local rarefaction or densification of solvent molecules in the first solvation shell.