Size Matters! On the Way to Ionic Liquid Systems without Ion Pairing

Several, partly new, ionic liquids (ILs) containing imidazolium and ammonium cations as well as the medium-sized [NTf2]− (0.230 nm3; Tf=CF3SO3−) and the large [Al(hfip)4]− (0.581 nm3; hfip=OC(H)(CF3)2) anions were synthesized and characterized. Their temperature-dependent viscosities and conductivities between 25 and 80 °C showed typical Vogel–Fulcher–Tammann (VFT) behavior. Ion-specific self-diffusion constants were measured at room temperature by pulsed-gradient stimulated-echo (PGSTE) NMR experiments. In general, self-diffusion constants of both cations and anions in [Al(hfip)4]−-based ILs were higher than in [NTf2]−-based ILs. Ionicities were calculated from self-diffusion constants and measured bulk conductivities, and showed that [Al(hfip)4]−-based ILs yield higher ionicities than their [NTf2]− analogues, the former of which reach values of virtually 100 % in some cases.From these observations it was concluded that [Al(hfip)4]−-based ILs come close to systems without any interactions, and this hypothesis is underlined with a Hirshfeld analysis. Additionally, a robust, modified Marcus theory quantitatively accounted for the differences between the two anions and yielded a minimum of the activation energy for ion movement at an anion diameter of slightly greater than 1 nm, which fits almost perfectly the size of [Al(hfip)4]−. Shallow Coulomb potential wells are responsible for the high mobility of ILs with such anions.