AN EVALUATION OF DENSITY CORRECTIONS FOR ESTIMATING DIFFUSIVITIES IN LIQUIDS AND LIQUID-MIXTURES

In this work we analyse the effect of density correction on the estima tion of diffusion coefficients in liquids and liquid mixtures using th e Carnahan-Starling (1969, J. Chem. Phys. 51, 635-636) pair correlatio n function and the correlation of Speedy (1987, Molec. Phys. 62, 509-5 15) and Harris (1992, Molec. Phys. 77, 1153-1167) which have been prop osed as models of self-diffusion coefficient of hard-sphere fluids. Th e hard-sphere diameters of nine liquids were estimated by fitting the experimental self-diffusion coefficients with the smooth-hard-sphere t heory and the estimated diameters were used for predicting diffusiviti es in 13 binary and eight ternary systems. This theory with the densit y- and temperature-dependent hard-sphere diameter obtained from the We eks-Chandler-Andersen (Weeks et al., 1971, J. Chem. Phys. 54, 5237-524 7) perturbation theory of liquids is shown to be an excellent approach for predicting diffusivities in liquids and liquid mixtures. The calc ulations involved nonideal mixtures as well as systems with high molec ular asymmetry. The predicted diffusivities are in good agreement with the experimental data for the binary and also for the ternary systems . The present results are much better than the estimates of Guo and Ke e (1991, Chem. Engng Sci. 46, 2133-2141) for the mutual diffusion coef ficient at infinite dilution for any of the density correction methods . It seems, then, that the coupling parameter of the rough hard-sphere theory has no essential role for predicting diffusion coefficients, e ven for large molecules, if a suitable hard-sphere diameter is used. T he methodology proposed here only makes use of pure component informat ion and density of mixtures. The simple algebraic relations proposed a re without any binary adjustable parameters and can be readily used fo r estimating diffusivities in multicomponent liquid mixtures. (C) 1997 Elsevier Science Ltd.