Determination of diffusion coefficients for supercritical fluids

A review of the diffusion coefficients for solutes in supercritical fluids as reported in the literature is presented together with the correlation methods applied by the authors for modeling the experimentally determined data. Supercritical carbon dioxide has been the preferred solvent in most of the systems investigated, although other solvents at elevated pressure have also been used. The influence of pressure, temperature, density, and viscosity on the binary diffusion coefficients is discussed, and some general trends have been established. A number of experimental methods for determining diffusion coefficients in supercritical fluids have been reported in the literature. The methods are described, their advantages and disadvantages are discussed and some examples of their application are given. Predictive equations based on the Stokes-Einstein model, the Rough-Hard-Sphere theory, and other methods for the calculation of diffusion coefficients in supercritical fluids at infinite dilution are reviewed. The review also looks at the ternary systems reported in the literature. The latter are discussed in terms of temperature, pressure, the type of modifier employed, amount of modifier, and solute-modifier interactions. Several equations have been proposed for correlating and predicting the diffusion coefficients in ternary systems.