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

• Published in: The Journal of chemical physics Vol. 140; no. 24; p. 244509
• Main Authors: Kruk, D, Meier, R, Rachocki, A, Korpała, A, Singh, R K, Rössler, E A
• Format: Journal Article
• Language: English
• Published: United States 28.06.2014
• Subjects: Diffusion; Imidazoles - chemistry; Ionic Liquids - chemistry; Magnetic Resonance Imaging; Magnetic Resonance Spectroscopy; Thermodynamics
• ISSN: 1089-7690
• 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-3-methylimidazolium tetrafluoroborate (BMIM-BF4, 243-318 K), and 1-butyl-3-methylimidazolium hexafluorophosphate (BMIM-PF6, 258-323 K). The dispersion of (1)H spin-lattice relaxation rate R1(ω) is measured in the frequency range of 10 kHz-20 MHz, and the studies are complemented by (19)F spin-lattice relaxation measurements on BMIM-PF6 in the corresponding frequency range. From the (1)H 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 (1)H 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 (19)F 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.