Maxwell-Stefan diffusivities in liquid mixtures: Using molecular dynamics for testing model predictions

Multicomponent diffusion is ubiquitous in (bio)chemical processes. The Maxwell-Stefan (M-S) framework provides a sound theoretical basis for describing transport diffusion as it correctly accounts for the gradient in chemical potential as driving force. Unfortunately, M-S diffusivities Đij cannot be measured directly in experiments. The use of predictive models based on easily measurable quantities like Fick- or self-diffusivities in diluted systems is therefore desirable. In this study, equilibrium molecular dynamics (EMD) simulations are used to study M-S diffusivities in liquid mixtures containing n-hexane, cyclohexane and/or toluene. Predictive models for estimating M-S diffusivities Đijxk→1 in ternary systems are investigated. The following analysis are carried out. First, these predictive models are used to calculate the self-diffusivity in the infinite dilution limit using the well-known Vignes approximation. The predicted self-diffusivity is compared to the self-diffusivity directly calculated from EMD simulations. Second, we investigated the quality of the Vignes approximation using diffusivities obtained from EMD simulations. Third, we directly compared the predictive models for Đijxk→1 with EMD simulations. Our results show that: (1) predicted self-diffusivities are not very sensitive to the choice of the predictive model for Đijxk→1; (2) the Vignes equation results in only reasonable predictions for M-S diffusivities, yielding errors of 13% on average; (3) the interaction between solutes and solvent cannot be neglected in predictive models for Đijxk→1; (4) present predictive models for calculating Đijxk→1 from binary data results in errors of 8% for the systems under investigation.