Self-diffusion and density of liquid hexafluorobenzene as a function of pressure and temperature

Measurements of self-diffusion coefficients to pressures of 2000 barFootnote† 1 bar ≡ 105 N m-2. and densities to 2600 bar, except where limited by freezing, have been made for liquid hexafluorobenzene C6F6 at eight temperatures from 15°C to 150°C. The N.M.R. spin echo technique was used for the self-diffusion measurements, and the densities were determined with a densitometer of the bellows-slidewire type. The density data have been fitted satisfactorily to a modified form of the Tait equation, and the self-diffusion results were reproduced by a polynominal in pressure for each isotherm. Several models of transport in liquids were evaluated. All models depend sensitively upon a quantity ν-ν0, the difference between the specific volume of the liquid and that of a hypothetical dense solid. The free volume model describes the data only if ν0 is allowed to depend upon pressure. The hard sphere model failed to describe our results for C6F6 at temperatures above 70°C even when the effective hard sphere diameter was treated as an adjustable parameter.