Density and temperature effects on motional dynamics of SF6 in the supercritical dense fluid region

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Abstract
The NMR19F spin–lattice relaxation times and self‐diffusion constants have been measured in the dense fluid of SF6 as a function of pressure and temperature over the density range 0.4⩽ ρ/ρ c ⩽2.6 and the temperature range 0.8⩽T/T c ⩽1.5. Densities have also been determined under the same experimental conditions. The relaxation mechanism for the fluorine nuclei is dominated by the spin–rotation interactions and therefore the analysis of the T 1 data yields the angular momentumcorrelation time τ J . On the other hand, the self‐diffusion data provide information about the behavior of the linear momentum correlation time τ P , in SF6. Interpretation of the experimental data based on the hard sphere model of liquids allows us to draw several conclusions of general character about the molecular dynamics of the spherical molecule of SF6 in the dense fluid region: (a) The angular momentum relaxation can be approximated as occurring through a sequence of uncorrelated binary collisions; (b) the estimated hard sphere diameter is temperature dependent and in reasonable agreement with the theoretical prediction; (c) for fluid densities lower than twice the critical density one finds considerable deviation between experiment and theory; (d) due to many body correlations the linear momentum behavior differs from the Enskog theory predictions in agreement with the results of the molecular dynamics calculations.