SELF-DIFFUSION IN GASES AND LIQUIDS

A systematic study of the self-diffusion coefficient in hard-sphere fl uids, Lennard-Jones fluids, and real compounds over the entire range o f gaseous and liquid states is presented. First an equation is propose d for the self-diffusion coefficient in a hard-sphere fluid based on t he molecular dynamics simulations of Alder et al. (J. Chem. Phys. 1970 , 53, 3813) and Erpenbeck and Wood (Phys. Rev. A 1991, 43, 4254). That expression, extended to the Lennard-Jones fluids through the effectiv e hard-sphere diameter method, represents accurately the self-diffusio n coefficients obtained in the literature by molecular dynamics simula tions, as well as those determined experimentally for argon, methane, and carbon dioxide. A rough Lennard-Jones expression which contains be sides the diameter sigma(LJ) and energy epsilon(LJ) the translational- rotational factor, A(D) (which could be correlated with the acentric-f actor), is adopted to describe the self-diffusion in nonspherical flui ds. The energy parameter is estimated using a correlation obtained fro m viscosity data, and the molecular diameter is obtained from the diff usion data themselves. The equation represents the self-diffusion coef ficients with an average absolute deviation of 7.33%, for 26 compounds (1822 data points) over wide ranges of temperature and pressure.