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

Abstract

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.

Investigation of Dynamics in BMIM TFSA Ionic Liquid through Variable Temperature and Pressure NMR Relaxometry and Diffusometry

“…Using methods described in the literature, this dispersion data were used to calculate 1 H and 19 F time-dependent D data as shown in Figure 7. 55–58 …”

Translational dynamics of ionic liquid imidazolium cations at solid/liquid interface in gel polymer electrolyte

“…As was reported recently for ionic liquids containing [BMIm] cations, homonuclear ( 1 H-1 H) intermolecular (interionic) couplings were considered as a major source of the proton spin-lattice relaxation measured in low magnetic fields [26,27]. Other couplings between spins of the nuclear spin number 1/2, i.e., 1 H-19 F intermolecular ones (see Fig.…”

“…The pulsed and static field gradient NMR techniques can be classified as direct ones because the diffusion coefficients are determined directly from the NMR signal attenuation, recorded as a function of the magnetic field gradient strength and/or the time at which the diffusion is followed. Recently, the fast field-cycling (FFC) NMR relaxometry technique has been also successfully used for characterization of the translational diffusion, especially in liquid and soft systems [4,26,27,31,32]. This indirect and non-gradient method gives the possibility to determine the diffusion coefficient from measured frequency dependency (dispersion profile) of spin-lattice relaxation time by applying a suitable theoretical model describing the dynamics of the molecules (ions).…”

“…In this aspect, nuclear magnetic resonance (NMR) method is more sensitive and especially more selective one which allows to determine separately the microscopic ion dynamics. The advantage of this method over other ones is that it can be employed in a number of ways, including spectroscopy, diffusometry or relaxometry, to provide a variety of information, but first of all, it allows selective identification of the dynamic properties of each type of ions if only the anions and cations are differentiated by their own ''NMR-active" nuclei [7,[26][27][28]. Using commercially available NMR spectrometers the diffusion measurements are carried out routinely, particularly in liquids and soft matter, by applying the pulsed magnetic field gradient (up to 30-40 T/m) that encodes the spatial information about nuclear spins travelling with molecules [29].…”

Field-Cycling Relaxometry as a Molecular Rheology Technique: Common Analysis of NMR, Shear Modulus and Dielectric Loss Data of Polymers vs Dendrimers

“…5,9,10 In the case of 1 H, relaxation is caused by fluctuations of the magnetic dipole−dipole interactions and one has to distinguish intra-and intermolecular relaxation pathways. 1,4,8 While the intramolecular contribution R 1 intra (ω) originates from interactions between nuclei belonging to one molecule, the intermolecular relaxation R 1 inter (ω) stems from interactions between nuclei belonging to different molecules.…”