Multicomponent Mass Transport In Ionic Liquids

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 (C_nmimCl, 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 C_nmimCl-H₂O mixtures, however, dissociation
into ion pairs is much less observed in C_nmimCl-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.