Translational Diffusion in Mixtures of Imidazolium ILs with Polar Aprotic Molecular Solvents

Self-diffusion coefficients of cations and solvent molecules were determined with 1H NMR in mixtures of 1-n-butyl-3-methylimidazolium (Bmim+) tetrafluoroborate (BF4 –), hexafluorophosphate (PF6 –), trifluoromethanesulfonate (TfO–), and bis(trifluoromethylsulfonyl)imide (TFSI–) with acetonitrile (AN), γ-butyrolactone (γ-BL), and propylene carbonate (PC) over the entire composition range at 300 K. The relative diffusivities of solvent molecules to cations as a function of concentration were found to depend on the solvent but not on the anion (i.e., IL). In all cases the values exhibit a plateau at low IL content (x IL < 0.2) and then increase steeply (AN), moderately (γ-BL), or negligibly (PC) at higher IL concentrations. This behavior was related to the different solvation patterns in the employed solvents. In BmimPF6-based systems, anionic diffusivities were followed via 31P nuclei and found to be higher than the corresponding cation values in IL-poor systems and lower in the IL-rich region. The inversion point of relative ionic diffusivities was found around equimolar composition and does not depend on the solvent. At this point, a distinct change in the ion-diffusion mechanism appears to take place.