Structural investigation on supercritical carbon dioxide and its mixture with alcohol

Small-angle neutron scattering (SANS) experiments were carried out for three different systems, pure CO₂, CO₂ + methanol (1 mol%) and CO₂ + ethanol (1 mol%), along the isotherm of 311.15 K in their supercritical states. All the SANS profiles were well analyzed with Ornstein-Zernike equation. With addition of the alcohols, the maximum values of the correlation length, ξ, and the scattering intensity at Q = 0, I(0), became larger, and the pressure at which ξ and I(0) became maxima was shifted to be lower. These changes can be explained qualitatively with van der Waals equation of state by regarding CO₂ + alcohol mixture as a system consisting of a single kind of virtual molecules. From this analysis, both the molecular attractive interaction and volume are suggested to be increased in the mixture. These changes were considered to be strongly connected with the increase of solubility and selectivity on entrainer effect.