Relationship between Translational and Rotational Dynamics of Alkyltriethylammonium-Based Ionic Liquids

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.

Relative Cation-Anion Diffusion in Alkyltriethylammonium-Based Ionic Liquids

“…It is of interest to compare the relative cation–anion translation diffusion coefficients with the relative cation–cation diffusion coefficients for [TEA–C6] [TFSI], [TEA–C8] [TFSI], [TEA–C10] [TFSI], [TEA–C12] [TFSI], [TEA–C14] [TFSI], and [TEA–C16] [TFSI] which were obtained in [ 6 ]. The comparison is shown in Figure 4 .…”

“…One should note the identical activation energies for [TEA–C4] [TFSI] [ 7 ], TEA–C6] [TFSI], and [TEA–C8] [TFSI] ((23.4 ± 0.6) kJ/mol, (23.2 ± 0.4) kJ/mol and (23.3 ± 0.5) kJ/mol for [TEA–C4] [TFSI], [TEA–C6] [TFSI] and [TEA–C8] [TFSI], respectively). It is worth noting at this stage that although the activation energy of the cation–cation translation motion for [TEA–C4] [TFSI], (23.4 ± 0.6) kJ/(mol × K) [ 4 ] is the same (within the uncertainty limits) as the cation–anion diffusion for [TEA–C6] [TFSI] and [TEA–C8] [TFSI], the activation energy for the cation–cation motion is lower ((20.8 ± 0.4) kJ/(mol × K) for [TEA–C6] [TFSI] and (19.0 ± 0.6) kJ/(mol × K) for [TEA–C8] [TFSI] [ 6 ]) than that of the cation–anion diffusion. Following this, with the increasing the length of the alkyl chain, the cation–anion translation diffusion for [TEA–C10] [TFSI] is slower than for the counterparts with shorter alkyl chains ( Figure 3 ).…”

“…Following this, with the increasing the length of the alkyl chain, the cation–anion translation diffusion for [TEA–C10] [TFSI] is slower than for the counterparts with shorter alkyl chains ( Figure 3 ). Moreover, the activation energy, (26.9 ± 0.7) kJ/mol, is much higher compared with that of the cation–cation motion, (19.0 ± 0.9) kJ/(mol × K) [ 6 ]. With the alkyl chain length increasing further ([TEA–C12] [TFSI]), the cation–anion diffusion becomes slower, and the activation energy for the cation–anion motion ((25.7 ± 0.6) kJ/mol) still remains larger than that of the cation–cation motion, (18.3 ± 0.7) kJ/(mol × K) [ 6 ].…”

“…This information is highly valuable, but it does not give insight into motional correlation. NMR relaxometry probes relative translational dynamics [ 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 ]. “Classical” NMR experiments are carried out in a single, high magnetic field.…”

“…This method has extensively been exploited for investigating the translational and rotational dynamics of molecular liquids; significantly fewer studies have been performed for ionic liquids. The applications of NMR relaxometry for ionic liquids in bulk [ 4 , 6 , 7 , 8 , 10 ] and confinement [ 8 , 12 , 13 ] have given a very valuable insight into the dynamics of cations and anions, especially for liquids composed of 1 H containing cations and 19 F containing anions.…”