Multicomponent diffusion is more complicated than it is often realized. First of all there are different approaches to describe diffusion phenomena in liquids, two commonly used are the Fickian and the Maxwell-Stefan theory, which both contain different diffusion coefficients. E.g., a ternary mixture has four Fickian diffusion coefficients, instead of one in a binary mixture. The values of these diffusion coefficients in ternary mixtures depend not only on the order of the components but also on the velocity reference frame. Only the eigenvalues of the diffusion matrix are invariant with respect to these aspects.
To determine diffusion coefficients, experimental methods, molecular simulation, theoretical or empirical approaches are used. Nowadays, experimentally determined multicomponent diffusion coefficients are available only for a very limited number of ternary and higher mixtures. Numerous theoretical and empirical approaches to predict mutual diffusion coefficients in multicomponent mixtures often fail in predictive applications because they relate mutual diffusion coefficients to one-component properties or simplify the interactions between unlike molecules. In this context, molecular simulation offers a promising route for predicting mutual diffusion coefficients and for understanding the diffusion phenomena on a microscopic basis.
Different ternary mixtures have been investigated by means of equilibrium molecular dynamic simulations. The Green Kubo formalism was used to predict Onsager and Maxwell-Stefan diffusion coefficients. Moreover the chemical potential was calculated by MC simulations to test the thermodynamic behavior of the models compared to excess Gibbs energy models fitted to experimental data. A comparison of the resulting diffusion coefficients with available experimental data and the encountered problems will be discussed.
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