Terahertz dynamics of ionic liquids from a combined dielectric relaxation, terahertz, and optical Kerr effect study: evidence for mesoscopic aggregation

Abstract

To exploit the great potential of room-temperature ionic liquids (RTILs) as solvents that offer both low environmental impact and product selectivity, an understanding of the liquid structure, the microscopic dynamics, and the way in which the pertinent macroscopic properties, such as viscosity, thermal conductivity, ionic diffusion, and solvation dynamics depend on these properties, is essential. We have measured the intermolecular dynamics of the 1,3-dialkylimidazolium- and [bmim][DCA], at 25 °C from below 1 GHz to 10 THz by ultrafast optical Kerr effect (OKE) spectroscopy and dielectric relaxation spectroscopy (DRS) augmented by time-domain terahertz and far-infrared FTIR spectroscopy. This concerted approach allows a more detailed analysis to be made of the relatively featureless terahertz region, where the higher frequency diffusional modes are strongly overlapped with librations and intermolecular vibrations. In the terahertz region, the signal-to-noise ratio of the OKE spectra is particularly high and the data show that there is a greater number of librational and intermolecular vibrational modes than previously detected. Of greatest interest though, is an intense low frequency (sub-alpha) relaxation that we show is in strong accordance with recent simulations that observe mesoscopic structure arising from aggregates or clusters; structure that explains the anomalous and inconveniently-high viscosities of these liquids.Terahertz spectroscopy, dielectric relaxation spectroscopy, Kerr spectroscopy, room-temperature ionic liquids

Dynamics of RTILs: A comparative dielectric and OKE study

“…relaxation times τ sub−α > τ α . Whilst the sub-α mode largely dominates the OKE signal it is hardly discernible in the dielectric spectrum (Figure 3) [12,37]. In marked contrast, the sub-α relaxation cannot be detected the spectra of pyrrolidinium-, pyridinium-and sulfonium-RTILs.…”

“…Already published material can be found in Refs. [12,33,34,35,36,37,38,39]. From a review of these results the general characteristics discussed below can be observed.…”

“…The sub-α relaxation is unique to imidazolium RTILs where it dominates the OKE spectra [12,37] (Figure 3). Sofar it has not been observed for other cations.…”

Influence of intramolecular dynamics on the relaxation spectra of simple liquids

“…Besides the above mentioned work, there exists a large body of dielectric as well as light scattering studies that analyze high frequency spectra in detail, [23][24][25][26][27][28][29] where most of the studies use a similar analysis approach combining several relaxation and damped oscillator models, often using more than 4 model functions. In particular when the liquids are cooled to the point where the main relaxation peak shifts out of the frequency window of the used methods, 13,14,19,20,[30][31][32] the full complexity of the high frequency dynamics is revealed and a minimum between the main relaxation peak and the microscopic peak is observed.…”

Understanding the Microscopic Behavior of the Mixture of Ionic Liquid/Ethylene Glycol/Lithium Salt through Time-Resolved Fluorescence, Nuclear Magnetic Resonance (NMR), and Electron Paramagnetic Resonance (EPR) Studies

“…Many experimental techniques such as small-angle X-ray scattering (SAXS), 8 small-angle neutron scattering (SANS), 9 dielectric spectroscopy, 10,11 NMR, 12,13 fluorescence, 14,15 optical Kerr effect spectroscopy, 10,16 and also molecular dynamics (MD) simulation studies 17 have been exploited to understand the structure and dynamics of RTILs. Among these, studies based on steady state and time-resolved fluorescence have been proved to be quite useful in probing the structural organization of RTILs.…”