Maxwell-Stefan Diffusivities in Binary Mixtures of Ionic Liquids with Dimethyl Sulfoxide (DMSO) and H2O

Abstract

Ionic liquids (ILs) are promising solvents for applications ranging from CO2 capture to the pretreatment of biomass. However, slow diffusion often restricts their applicability. A thorough understanding of diffusion in ILs is therefore highly desirable. Previous research largely focused on self-diffusion in ILs. For practical applications, mutual diffusion is by far more important than self-diffusion. For describing mutual diffusion in multicomponent systems, the Maxwell–Stefan (MS) approach is commonly used. Unfortunately, it is difficult to obtain MS diffusivities from experiments, but they can be directly extracted from molecular dynamics (MD) simulations. In this work, MS diffusivities were computed in binary systems containing 1-alkyl-3-methylimidazolium chloride (CnmimCl, n = 2, 4, 8), water, and/or dimethyl sulfoxide (DMSO) using MD. The dependence of self- and MS diffusivities on mixture composition was investigated. Our results show the following: (1) For solutions of ILs in water and DMSO, self-diffusivities decrease strongly with increasing IL concentration. For DMSO-IL, a single exponential decay is observed. (2) In both water–IL and DMSO–IL, MS diffusivities vary by a factor of 10 within the concentration range which is, however, still significantly smaller than the variation of the self-diffusion coefficients. (3) The MS diffusivities of the IL are almost independent of the alkyl chain length. (4) ILs stay in a form of isolated ions in CnmimCl–H2O mixtures; however, dissociation into ions is much less observed in CnmimCl–DMSO systems. This has a large effect on the concentration dependence of MS diffusivities. (5) Recently, we proposed a new model for predicting the MS diffusivity at infinite dilution, that is, Đijxk→1 (Ind. Eng. Chem. Res.2011, 50, 4776–4782). This quantity describes the friction between components i and j when both are infinitely diluted in component k. In contrast to earlier empirical models, our model is based on the linear response theory and the Onsager relations which allows a clear interpretation of the results. The key assumption in the model is that velocity cross-correlations are neglected. The present study clearly shows that velocity cross-correlation functions in ILs cannot be neglected and that the dissociation of ILs into ions has a very strong influence on diffusion.