Prediction of the mutual diffusivity in binary liquid mixtures containing one dimerising species, from the tracer diffusion coefficients

Abstract

In a recent publication (Moggridge, 2012), it was shown that a simple equation could be used to accurately predict the mutual diffusion coefficients in a wide range of non-ideal binary mixtures from their tracer diffusion coefficients and a thermodynamic correction factor. The physical basis of the equation is that dynamic concentration fluctuations in the liquid mixture result in a reduction of the mean thermodynamic correction factor relative to the hypothetical case in which no such fluctuations occur.Here, the analysis is extended to cases where strong molecular association occurs, in the form of dimerisation of a polar species, mixed with a non-polar component. This requires a modification of the average molecular mobility part of the predictive equation for mutual diffusivity, in the form of doubling the tracer diffusivity of the dimerised species — on the basis that each labelled molecule which moves carries with it a second unlabelled molecule of the same species. In this paper, it is demonstrated that such a modified equation is valid for four alcohol-non polar mixtures, in which the alcohol is close to fully dimerised beyond a mole fraction of about 0.2. It is of significance that the form of the thermodynamic correction factor required in these systems remains unchanged, raising the possibility of its more general validity for non-ideal liquid mixtures.The prediction of mutual diffusion coefficients from tracer diffusivities has practical implications, particularly where measurement of the mutual diffusion coefficient is difficult; tracer diffusivities can be easily measured by pulsed field gradient NMR or estimated using molecular simulation calculations.