Solute Diffusion in Supercritical Solvents – Diffusion Coefficients D ₁₂ ^∞ and Diffusion–Viscosity Relationship for the Aromatic Model Solute Biphenyl in Carbon Dioxide

Abstract

The temperature and density dependence of the mutual diffusion coefficient D 12 ∞ of biphenyl at infinite dilution in supercritical carbon dioxide has been measured by the Taylor dispersion method at several temperatures from 309 K to 331 K and at pressures between about 8 MPa and 20 MPa, corresponding to densities from about 500 kg m−3 to 850 kg m−3. The observed diffusion coefficients are of the order of 10−8 m2 s−1. They decrease about linearly with increasing density and increase slightly with increasing temperature. The experimental data show a density and temperature dependence of the quantity D 12 ∞η/T, indicating a breakdown of the Stokes–Einstein relation. The use of empirical fractional power laws for describing the self-diffusion–viscosity relationship is discussed.