The density dependence of the self-diffusion coefficient of liquid methane

Self-diffusion coefficients for liquid methane obtained by the NMR spin-echo technique are reported at 110, 140 and 160 K. The density range extends to the freezing line at 110 and 140 K, and to 30 mol/dm3 at 160 K. Using the hard sphere diameter as a single temperature dependent parameter it was found that the reduced diffusivity isotherms fall on a common curve when plotted against reduced density [n∗#620.26, 0.58⩽Tr⩽1.70]. Literature viscosities could also be correlated in this way using diameters obtained from the diffusion data. The experimental results are in good agreement with molecular dynamics calculations for the hard sphere fluid, except at densities above n∗ = 0.87. This difference at high densities is similar to deviations from the rough hard sphere model observed for more complex molecular liquids (CCl4, C6H6). The diffusion data are also compared with the results of molecular dynamics and other calculations for more sophisticated potential models.