Predicting the diffusion coefficients in supercritical fluids

In this paper a semiempirical expression is proposed for the calculation of the diffusion coefficients of solutes in supercritical fluids at infinite dilution. This equation contains two terms: the first is due to the long-range correlation and the second to a background contribution. Stokes-Einstein type equations are employed for both contributions, and an expression is derived for the correlation length which appears in the first term. The correlation length depends on a thermodynamic factor, which is calculated using the Peng-Robinson equation of state. The equation was tested with the experimentally determined diffusivity data for 33 systems. The solutes involved include long-chain, polar, and complex molecules. The average absolute deviation for 598 data points is 5.71%, which is comparable to the experimental uncertainty.