Difference of solute-solvent interactions of cis- and trans-1,2-dichloroethylene in supercritical CO2 investigated by raman spectroscopy

Vibrational Raman spectra of CC stretching modes of both cis- and trans-1,2-dichloroethylene (C2H2Cl2) were measured as a function of density in supercritical carbon dioxide (CO2). Measurements were performed with solute mole fraction of 0.01 at an isotherm of T r = T/ T c = 1.02. As the density of CO2 increased, peak frequencies of the CC stretching modes shifted toward the low energy side. By analyzing these density dependences using perturbed hard-sphere theory, we decomposed the shifted amounts into attractive and repulsive components. The amounts of repulsive shifts were almost equivalent, whereas those of the attractive shifts of trans-C2H2Cl 2 were larger than those of cis-C2H2Cl2 at all densities. This means that the nonpolar solute, trans-C2H2Cl2, shows stronger solute-solvent interactions than those of the polar solute cis-C2H2Cl2. The difference of attractive interactions between these isomers is the greatest at a density where local density enhancement of supercritical CO2 reaches the maximum.