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

Self-diffusion coefficients of cations and solvent molecules were determined with H-1 NMR in mixtures of 1-n-butyl-3-methylimidazolium (Bmim(+)) tetrafluoroborate (BF4-), hexafluorophosphate (PF6-), trifluoromethanesulfonate (TfO-), and bis(trifluoromethylsulfonyl)imide (TFSI-) with acetonitrile (AN), gamma-butyrolactone (gamma-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 (gamma-BL), or negligibly (PC) at higher IL concentrations. This behavior was related to the different solvation patterns in the employed solvents. In BmimPF(6)-based systems, anionic diffusivities were followed via P-31 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.