Near-critical behavior of mutual diffusion coefficients for five solutes in supercritical carbon dioxide

The Taylor-Aris dispersion technique was employed to measure the binary dif fusion coefficients of five solid solutes (phenanthrene, biphenyl, benzoic acid, 1,4-dichlorobenzene, and phenol) in supercritical carbon dioxide with in 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 po int and also near the solid/supercritical fluid lower critical end point. A t 308.2.K (4 K above the critical temperature of the solvent), decreasing t he 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 coeffici ents decrease sharply. The decrease in diffusion coefficient near the solve nt critical point is discussed using the concept of the solvent density inh omogeneities in supercritical fluids. A crossover theory that considers bot h the critical singular contribution and background contribution of transpo rt properties is used to describe semiquantitatively the observed decrease in the diffusion coefficient.