Dielectric Response of Imidazolium-Based Room-Temperature Ionic Liquids

Abstract

We have used microwave dielectric relaxation spectroscopy to study the picosecond dynamics of five low-viscosity, highly conductive room temperature ionic liquids based on 1-alkyl-3-methylimidazolium cations paired with the bis((trifluoromethyl)sulfonyl)imide anion. Up to 20 GHz the dielectric response is bimodal. The longest relaxation component at the time scale of several 100 ps reveals strongly nonexponential dynamics and correlates with the viscosity in a manner consistent with hydrodynamic predictions for the diffusive reorientation of dipolar ions. Methyl substitution at the C2 position destroys this correlation. The time constants of the weak second process at the 20 ps time scale are practically the same for each salt. This intermediate process seems to correlate with similar modes in optical Kerr effect spectra, but its physical origin is unclear. The missing high-frequency portion of the spectra indicates relaxation beyond the upper cutoff frequency of 20 GHz, presumably due to subpicosecond translational and librational displacements of ions in the cage of their counterions. There is no evidence for orientational relaxation of long-lived ion pairs.

Communication: Are hydrodynamic models suited for describing the reorientational dynamics of ions in ionic liquids? A case study of methylimidazolium tetra(hexafluoroisopropoxy)aluminates

“…In addition, there lacks evidence for spectral contributions from ion aggregates such as dipolar ion pairs. 3,4 The global picture thus resembles the interpretation of dielectric spectra of nonconducting dipolar solvents in terms of the collective reorientation of dipolar species.…”

“…17 There are several scenarios for the origin of the lowfrequency mode which can involve frequency-dependent contributions due to charge transport and orientational dynamics of dipolar species. The gross picture provided by experiments 3,4 and molecular dynamics ͑MD͒ simulations 18,19 for similar ILs attributes the low-frequency mode mainly to collective orientational relaxation of the dipolar cations. Given the high symmetry of ͓Al͑hfip͒ 4 ͔ − , the anion contribution may be neglected in the present case as well.…”

“…2 reveals an extraordinarily broad dispersion regime ˜1͑ ͒, signaling a broadly stretched dynamics from hundred picoseconds to nanoseconds. In the preceding work 3 we described this stretched dynamics by the asymmetrical Cole-Davidson relaxation time distribution, 10,11,13 but in more recent work by Buchner, Hefter, and co-workers 4,14 presented evidence that the symmetrical Cole-Cole ͑CC͒ distribution is more suited to parametrize the primary relaxation mode of ILs. The CC distribution reads 15…”

“…2 Due to their success for molecular liquids such models have also intuitive appeal for describing dynamical processes in ILs. 3,4 While hydrodynamic models have been addressed in some detail in studies of translational ion diffusion, 5 hydrodynamic models for ion reorientation have never been validated systematically and the dielectric response is still little understood. 3,4 Here, we address these questions by exploiting the extraordinary properties of ILs based on the tetra͑hexafluoro-isopropoxy͒aluminate ͓͑Al͑hfip͒ 4 ͔ − ͒ anion 6 ͑Fig.…”

“…Details are given elsewhere. 9 We used a coaxial reflection technique described recently 3 to determine the microwave dielectric response up to 20 GHz of the ILs at ͑343.1Ϯ 0.2͒ K, which is well above their melting temperatures ͑Table I͒. The dielectric response to an oscillating electric field of frequency can be described by the complex dielectric function 10,11 …”