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

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-3-methylimidazolium tetrafluoroborate (BMIM-BF₄, 243–318 K), and 1-butyl-3-methylimidazolium hexafluorophosphate (BMIM-PF₆, 258–323 K). The dispersion of ¹H spin-lattice relaxation rate R₁(ω) is measured in the frequency range of 10 kHz–20 MHz, and the studies are complemented by ¹⁹F spin-lattice relaxation measurements on BMIM-PF₆ in the corresponding frequency range. From the ¹H 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 ¹H 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 R₁ on square root of frequency. From the ¹⁹F 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.