Near-Critical Behavior of Mutual Diffusion Coefficients for Five Solutes in Supercritical Carbon Dioxide

Abstract

The Taylor−Aris dispersion technique was employed to measure the binary diffusion coefficients of five solid solutes (phenanthrene, biphenyl, benzoic acid, 1,4-dichlorobenzene, and phenol) in supercritical carbon dioxide within the pressure range from 75 to 170 bar at 308.2 K. Measurements were made at very dilute concentrations, in the vicinity of the pure CO2 critical point and also near the solid/supercritical fluid lower critical end point. At 308.2 K (4 K above the critical temperature of the solvent), decreasing the pressure (density) of the fluid causes the binary diffusion coefficients to increase until a certain pressure is reached. With further decreases in pressure, approaching the critical pressure of CO2, the diffusion coefficients decrease sharply. The decrease in diffusion coefficient near the solvent critical point is discussed using the concept of the solvent density inhomogeneities in supercritical fluids. A crossover theory that considers both the critical singular contribution and background contribution of transport properties is used to describe semiquantitatively the observed decrease in the diffusion coefficient.