Effects of cations and anions on transport properties in tetrafluoroborate-based ionic liquids

The self-diffusion coefficient, diffusion radius, dissociation and association degree, and transport numbers of various cations (methylpyrrolidinium, 1-methyl-2-oxopyrrolidinium, methylpiperidinium, ethylmorpholinium, and methylmorpholinium) and tetrafluoroborate anion in ionic liquids are measured by observing 1H and 19F nuclei using the pulsed-field gradient spin-echo (PGSE) NMR technique at various temperatures. The self-diffusion coefficients determined for the cations and anions of ionic liquids by PGSE NMR method exhibit tetrafluoroborate anion have higher value than cations over a wide temperature range. The temperature dependence of the self-diffusion coefficient obeys the Vogel-Tamman-Fulcher (VTF) equation, and the VTF parameters were presented. The self-diffusion coefficient is analyzed in terms of the Stokes-Einstein equation and the Nernst-Einstein equation, the hydrodynamic radius of ILs is calculated from Stokes-Einstein equation and the molar conductivity (ΛNMR) calculated from the Nernst-Einstein equation is derived without considering ionic association. The dissociation degree of cations and anions range from 72 to 86% for [PipMe][BF4], [PyrMe][BF4], [MorMe][BF4], and [MorEt][BF4], whereas the ratio is 32% for [PyrOMe][BF4] at 303 K, this difference can be explained by taking the ionic association into consideration for [PyrOMe][BF4].