Determining diffusion coefficients of ionic liquids by means of field cycling nuclear magnetic resonance relaxometry

作者：Kruk D^*; Meier R; Rachocki A; Korpala A; Singh R K; Roessler E A

来源：Journal of Chemical Physics, 2014, 140(24): 244509.

DOI：10.1063/1.4882064

Field Cycling Nuclear Magnetic Resonance (FC NMR) relaxation studies are reported for three ionic liquids: 1-ethyl-3-methylimidazolium thiocyanate (EMIM-SCN, 220-258 K), 1-butyl-3methylimidazolium tetrafluoroborate (BMIM-BF4, 243-318 K), and 1-butyl-3-methylimidazolium hexafluorophosphate (BMIM-PF6, 258-323 K). The dispersion of H-1 spin-lattice relaxation rate R-1(omega) is measured in the frequency range of 10 kHz-20 MHz, and the studies are complemented by F-19 spin-lattice relaxation measurements on BMIM-PF6 in the corresponding frequency range. From the H-1 relaxation results self-diffusion coefficients for the cation in EMIM-SCN, BMIM-BF4, and BMIM-PF6 are determined. This is done by performing an analysis considering all relevant intra-and intermolecular relaxation contributions to the H-1 spin-lattice relaxation as well as by benefiting from the universal low-frequency dispersion law characteristic of Fickian diffusion which yields, at low frequencies, a linear dependence of R-1 on square root of frequency. From the F-19 relaxation both anion and cation diffusion coefficients are determined for BMIM-PF6. The diffusion coefficients obtained from FC NMR relaxometry are in good agreement with results reported from pulsed-field-gradient NMR. This shows that NMR relaxometry can be considered as an alternative route of determining diffusion coefficients of both cations and anions in ionic liquids.

• 出版日期2014-6-28