Kinetic Theory of Dense Fluids. X. Measurement and Interpretation of Self-Diffusion in Liquid Ar, Kr, Xe, and CH4

The self-diffusion coefficients of Ar, Kr, Xe, and CH4 have been measured as functions of temperature and pressure and the data used to examine critically the current theories of diffusion. It is found that: (1) The self-diffusion coefficients of Ar, Kr, and Xe fit one corresponding states relationship, and this relationship differs from that needed to fit the observed diffusion coefficients of CH4. For Ar, Kr, and Xe: logD̃=0.05+0.07p̃−(1/T̃)(1.04+0.1p̃),with D̃, T̃, and p̃ the reduced diffusion coefficient, temperature and pressure. (2) The free volume theory of diffusion is inadequate to describe our observations. (3) The volume of activation for CH4 is one third of the molar volume, while for Ar, Kr, and Xe it is one molar volume or larger. This observation is related to the steepness of the intermolecular repulsion, as is the deviation of CH4 from the corresponding states curve determined by Ar, Kr, and Xe. (4) The dominant temperature dependence of D arises from the correlation between successive increments in momentum. In describing molecular motion it is necessary to include negative portions of the momentum autocorrelation function. A simple fluid continuum model appears to be accurate in describing momentum correlations. Calculations of D and (d lnD/dT) are in good agreement with experiment. (5) The dense square-well fluid provides a zeroth-order approximation to real fluids. The computed temperature dependence of D is in quantitative agreement with experiment, but the computed values of D are in error by about 30%.