Tracer diffusion of aromatic hydrocarbons in n-hexane up to the supercritical region

Tracer diffusion coefficients are determined with the Taylor-Aris dispersion method for benzene, toluene, p-xylene, mesitylene, naphthalene and phenanthrene in liquid n-hexane along the vapour-liquid coexistence curve from 333.2 to 485.4 K and in supercritical n-hexane at 507.4, 522.0, 533.3, 543.2K and several pressures. A rough-hard-sphere model is found to represent quite well (to within ± 6%) the observed tracer diffusivities in the density region where computer simulation results for D₁₂^SHS/D₁₂^E are available, i.e. 1.5 V/V₀ 3. Furthermore, Hildebrand's free volume model together with the excluded-volume effect provides the basis for a general linear relationship between D₁₂V_C12 and V for tracer diffusion in liquid n-hexane up to its critical temperature. For diffusion in the supercritical region two definitions of reduced tracer diffusivity, one based on the rough-hard-sphere theory of tracer diffusion and the other on the extension of Helfand-Rice corresponding state principle for self-diffusion, permit generalized correlations that are capable of representing the experimental results in both supercritical n-hexane and carbon dioxide to within ±4% on average.