Significant difference in the attractive energies of ethane and ethanol in supercritical CO2

Abstract

Vibrational Raman spectra of the CC stretching modes for ethane and ethanol were measured in supercritical carbon dioxide (scCO2) at the isotherm of reduced temperature Tr=T/Tc=1.02 (Tc is the critical temperature) as a function of fluid density, ρr=ρ/ρc, in the range of 0.01–1.5. The peak frequencies of the Raman spectra were analyzed using the perturbed hard-sphere theory, and the attractive and repulsive energy components between solute and solvent were quantified as a function of the fluid density. For ethane, the magnitude of the attractive energy was almost equal to that of the repulsive energy. In contrast, the magnitude of the attractive energy for ethanol was four times greater than that of the repulsive energy. The significant difference in the attractive energies of ethane and ethanol is attributed to the solvation of the CO2 molecules at the oxygen atom of ethanol, according to analyses of local density and quantum chemical calculations. These observations provide useful information to understand the entrainer effect in scCO2.