Tuning the dynamics of imidazolium-based ionic liquids via hydrogen bonding. I. The viscous regime

Abstract

Combining results from impedance spectroscopy and oscillatory shear rheology, the present work focuses on the relation between the mass and charge flows and on how these are affected by the H-bonding in viscous ionic liquids (ILs). In particular, we compare the relaxational behaviors of the paradigmatic IL 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIM-TFSI) and its OH-functionalized counterpart 1-(2-hydroxyethyl)-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (OHEMIM-TFSI). Our results and their analysis demonstrate that the presence of cationic OH-groups bears a strong impact on the overall dynamics of OHEMIM-TFSI, although no signatures of suprastructural relaxation modes could be identified in their dielectric and mechanical responses. To check whether at the origin of this strong variation is the H-bonding or merely the difference between the corresponding cation sizes (controlling both the hydrodynamic volume and the inter-charge distance), the present study includes 1-propyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (PMIM-TFSI), mixtures of EMIM-TFSI and PMIM-TFSI with lithium bis(trifluoromethylsulfonyl)imide (Li-TFSI), and mixtures of OHEMIM-TFSI with PMIM-TFSI. Their investigation clearly reveals that the dynamical changes induced by H-bonding are significantly larger than those that can be attributed to the change in the ion size. Moreover, in the mixtures of OHEMIM-TFSI with PMIM-TFSI, a dilution of the OH-groups leads to strong deviations from ideal mixing behavior, thus highlighting the common phenomenological ground of hydroxy-functionalized ILs and other H-bonded liquids.

Translational and reorientational dynamics in deep eutectic solvents

“…48 More recently, it was demonstrated how the singleparticle mean-square displacement can be derived from complex viscosity spectra. 49 Within this approach, the rheological response should essentially reflect translational dynamics. In the same vein, the spectral behavior predicted by the RBM to arise from hopping mass transport should be un-obscured by reorientational dynamics in these experiments.…”

“…As discussed in Sec. I, this different behavior of the two spectroscopies can be rationalized if one considers that the shear mechanical data exclusively reflect translational dynamics 49 as modeled by the RBM, 48 which is not the case for the dielectric spectra. For the characteristic time parameter of the RBM, the fits lead to τ RBM ≈ 53 ms of comparable magnitude as ⟨τJ⟩.…”

“…We ascribe these to spurious contributions in J ′ (ν) due to the low-torque limitations of the rheometer. 48,49,55,61 To a lesser extent, they also affect the glyceline spectra [cf. the two highest temperatures in Fig.…”

“…42 Nevertheless, one should be aware that in some cases the RBM alone was successfully applied to systems featuring dipolar reorientations, including several ionic liquids and a few DESs. 11,40,47,48,49,50,51 It is clear that in such materials, ionic hoppingas treated by the RBMand dipolar reorientations in principle could occur simultaneously and it may well be possible that in some cases the hopping contribution dominates (see also discussion of Fig. 5 in Ref.…”

“…2), for the mechanical spectra obviously the RBM alone (i.e., ) without adding other contributions) is able to provide fits of comparable quality as the CD approach. As discussed in section I, this different behavior for the two spectroscopies can be rationalized if one considers that the shear mechanical data exclusively reflect translational dynamics 40 as modeled by the RBM, 39 which is not the case for the dielectric spectra. For the characteristic time parameter of the RBM, the fits lead to  RBM  53 ms, of comparable magnitude as  J .…”