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

Abstract

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 (Λ NMR) 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][BF 4], [PyrMe][BF 4], [MorMe][BF 4], and [MorEt][BF 4], whereas the ratio is 32% for [PyrOMe][BF 4] at 303 K, this difference can be explained by taking the ionic association into consideration for [PyrOMe][BF 4].