The density dependence of the self-diffusion coefficient of methane at −50°, 25° and 50°C

Abstract

Self-diffusion data for methane obtained by the NMR spin-echo technique are reported at −50°, 25° and 50°C over the density range 2 to 27 mol dm −3. The apparatus was calibrated using literature atmospheric pressure tracer data for water and benzene and the calibration was checked using high pressure tracer data for benzene obtained by a separate technique. The methane results are found to be somewhat lower than those of earlier workers, but are consistent with limiting values calculated using the recent numerical intermolecular potential of Matthews and Smith. The data suggest that the first “virial” coefficient in the density expansion changes sign just above −50°C, though measurements at lower densities than are accessible with the present apparatus would be needed to confirm this conclusion. Chandler's rough hard sphere model is applied to the high density data and these are consistent with there being little, if any, translational-rotational coupling for diffusive motion of CH 4. A relation is established between the hard sphere diameter and temperature allowing the calculation of D for liquid and dense gaseous methane in the region 90< T/K < 323 and ϱ > 20 mol dm −3. A comparison is made between recent molecular dynamics calculations of D and the experimental measurements.