(776e) Multicomponent Mass Transport In Ionic Liquids

Authors: 
Vlugt, T., Delft University of Technology
Bardow, A., RWTH Aachen University
Liu, X., RWTH Aachen University


Ionic liquids 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 ionic liquids is therefore highly desirable. Previous research has
largely focused on self-diffusion in ionic liquids. For practical
applications, the understanding of mutual diffusion is by far more
important than self-diffusion. For describing mutual diffusion in
multicomponent systems, the Maxwell-Stefan approach is commonly
used. Unfortunately, it is difficult to obtain Maxwell-Stefan
diffusivities from experiments but they can be extracted from
Molecular Dynamics simulations, requiring extensive amounts of CPU
time. In this work, Maxwell-Stefan diffusivities were computed in
systems containing 1-alkyl-3-methylimidazolium chloride (CnmimCl, n =
2,4,8), water and/or dimethyl sulfoxide (DMSO). Molecular Dynamics
simulations using a classical force field were used. Our model very
well reproduces experimental self-diffusivities. The dependence of
Maxwell-Stefan diffusivities on mixture composition was investigated
in detail. Our results show that: (1) for solutions of ionic liquids
in water and DMSO, Maxwell-Stefan diffusivities exponentially decrease
with increasing ionic liquid concentration; (2) the Maxwell-Stefan
salt diffusivities are almost independent of the alkyl chain length in
contrast to the self-diffusion coefficients; (3) ionic liquids stay in
a form of isolated ions in CnmimCl-H2O mixtures, however, dissociation
into ion pairs is much less observed in CnmimCl-DMSO systems. This has
a very large effect on the concentration dependence of Maxwell-Stefan
diffusivities. Our research clearly shows that new models are needed
to describe multicomponent mass transport in ionic liquids.
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