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 nm(3); Tf=CF3SO3-) and the large [Al-(hfip)(4)](-) (0.581 nm(3); hfip=OC(H)(CF3)(2)) anions were synthesized and characterized. Their temperature-dependent viscosities and conductivities between 25 and 80 degrees 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.