Abstract
1H spin-lattice relaxation experiments have been performed for a series of ionic liquids including bis(trifluoromethanesulfonyl)imide anion and cations of a varying alkyl chain length: triethylhexylammonium, triethyloctylammonium, decyltriethylammonium, dodecyltriethylammonium, triethyltetradecylammonium, and hexadecyltriethylammonium. The relaxation studies were carried out in abroad frequency range covering three orders of magnitude, from 10 kHz to 10 MHz, versus temperature. On the basis of a thorough, quantitative analysis of this reach data set, parameters characterizing the relative, cation-cation, translation diffusion (relative diffusion coefficients and translational correlation times), and rotational motion of the cation (rotational correlation times) were determined. Relationships between these quantities and their dependence on the alkyl chain length were discussed in comparison to analogous properties of molecular liquids. It was shown, among other findings, that the ratio between the translational and rotational correlation times is smaller than for molecular liquids and considerably dependent on temperature. Moreover, a comparison of relative and self-diffusion coefficients indicate correlated translational dynamics of the cations.
Highlights
One of the fundamental questions of condensed matter science concerns dynamical properties of molecular and ionic liquids
The activation energies for the rotational dynamics span the range of (19.2 ± 0.4) kJ/(mol × K) for [TEA-C4] [TFSI] to (9.1 ± 0.5) kJ/(mol × K) for [TEA-C10] [TFSI] (the other values are: (15.3 ± 1.2) kJ/(mol × K) for [TEA-C6] [TFSI], (12.1 ± 0.8) kJ/(mol × K) for [TEA-C8] [TFSI], (9.4 ± 0.4) kJ/(mol × K) for [TEA-C12][TFSI], (12.5 ± 0.4) kJ/(mol × K) for [TEA-C14] [TFSI] and (10.3 ± 0.6) kJ/(mol × K) for [TEA-C16] [TFSI]).The range of the activation energies is considerably broader with no systematic dependence of the alkyl chain length
[TFSI], [TEA-C8] [TFSI], [TEA-C10] [TFSI], [TEA-C12] [TFSI], [TEA-C14] [TFSI], and [TEAC16] [TFSI] were thoroughly analyzed in terms of a relaxation model including inter-ionic and intra-ionic relaxation contributions associated with relative translation movement of the cations and their rotational dynamics, respectively
Summary
One of the fundamental questions of condensed matter science concerns dynamical properties of molecular and ionic liquids. As a result of the translation diffusion, one observes changes in the resonance frequency of the nuclei originating from changes in the magnetic field at different locations in space [1,2]. This description of NMR diffusometry is simplified, but it captures the essential concept. NMR relaxation studies give access to the molecular (ionic) dynamics It is, very important to point out that the relaxation rate reaches its maximum when the correlation time matches the reciprocal resonance frequency. One should point out that Electron Spin Resonance gives a highly valuable insight into dynamical properties of ionic liquids by investigating the performance of paramagnetic probes placed in ionic liquids [30,31,32,33,34]
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