Non-Arrhenius behaviour of NMR self-diffusion of the neat room temperature ionic liquid 1-butyl-3-methyl imidazolium tetra- fluoroborate [bmim][BF4].
Abstract
Room temperature ionic liquid (RTIL) 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]) was investigated using 1H, 19F, 11B and 10B DOSY NMR spectroscopy in a temperature range of 288–338 K. The dynamics of the [BF4] anion and [bmim] cation have been found to be similar, indicating that both constituents behave as a rigid unit in the studied temperature range. The diffusion coefficients varied from 6.9 ∙ 10–11 m2 s–1 at the highest temperature to 6.1 ∙ 10–12 m2 s–1 at the lowest temperature. The dynamics of the diffusion coefficient was found not obeying the Arrhenius law and therefore was analyzed using the Vogel–Fulcher–Tammann (VFT) equation for the diffusivity. The glass transition temperature was found to be in a range of 172–181 K, which coincides with the values obtained using other techniques and found in the literature; the parameter B which is related to activation energy was found equal to 828–1115 K.
