Prediction of binary diffusion coefficients in non-ideal mixtures from NMR data: Hexane-nitrobenzene near its consolute point

Pulsed field gradient nuclear magnetic resonance was used to measure the tracer diffusivity of the species in mixtures of nitrobenzene and n-hexane close to the consolute point. Measurements are reported over the full range of composition at 21 °C (the consolute temperature is 19.4 °C), and at several compositions including the consolute composition (x1=0.422) over the range 21–35 °C. These NMR-derived tracer diffusivities are compared with literature values for the binary diffusion coefficient under the same conditions. It is shown that it is possible to calculate the binary diffusion coefficient, even very close to the consolute point, from the NMR-derived tracer diffusivities using a fairly simple thermodynamic correction factor, of a form similar to those reported in the literature based on critical point scaling laws. The necessary thermodynamic parameters are calculated by fitting vapour–liquid equilibrium data for the system under the same conditions, which is available in the literature. The ability to predict binary diffusion coefficients from NMR measurements has significant potential, for example in studying mass transport in porous solids or packed beds, situations where conventional diffusion measurements are impossible to make.