Solute-solvent intermolecular interactions in supercritical Xe, SF6, CO2, and CHF3 investigated by Raman spectroscopy: greatest attractive energy observed in supercritical Xe.

Vibrational Raman spectra of the C=C stretching modes of cis- and trans-1,2-dichloroethylene (C(2)H(2)Cl(2)) were measured in supercritical Xe, SF(6), CO(2), and CHF(3). The spectra were collected over a wide range of densities of supercritical fluids at a fixed solute mole fraction and isotherm of T(r) = T/T(c) = 1.02. In all fluids, as the density increased, the peak frequencies of the C=C stretching modes shifted toward the low-energy side. By analyzing these density dependencies using the perturbed hard-sphere theory, the shifted amounts were characterized into attractive and repulsive components. The attractive shifts of both isomers were almost equivalent in supercritical CHF(3), CO(2), and SF(6), whereas they were significantly larger in supercritical Xe. The attractive shifts obtained experimentally were compared with the ones calculated on the basis of dispersion, dipole-dipole, dipole-induced-dipole, and dipole-quadrupole interactions between solute and solvent molecules. The experimental attractive shifts in supercritical Xe were 2-3 times greater than the calculated shifts. The large attractive shifts were ascribed to both an anisotropic solvation structure and to a strong interaction (charge transfer) between Xe and C(2)H(2)Cl(2) molecules.