Fundamental behavior of benzene–CO2 mutual diffusion coefficients in the critical region of CO2

At higher pressures than the critical pressure of a mixture, we observed that the mutual diffusion coefficients of benzene in supercritical carbon dioxide sharply decreased to approach zero near the critical pressure. We could explain the phenomena based on the knowledge of irreversible thermodynamics that mutual diffusion coefficients are proportional to gradient of chemical potential. The calculated results combined with an equation of state for gradient of chemical potential show V-shape diffusion coefficients around the mixture critical pressure at a constant temperature. To know the behavior at wide range of pressures including the critical point, we carried out the measurement of mutual diffusion coefficients based on the Taylor dispersion method ranging wider pressures and low composition with 1 μl injection volume so as to be avoid from the vapor–liquid region. The experimental results also exhibited a V-shape near the critical point and this corresponded closely to that predicted by an equation of state and irreversible thermodynamic theory.