Dynamics in Supercooled Ionic Organic Liquids and Mode Coupling Theory Analysis

Abstract

Optically heterodyne-detected optical Kerr effect experiments are applied to study the orientational dynamics of the supercooled ionic organic liquids N-propyl-3-methylpyridinium bis(trifluoromethylsulfonyl)imide (PMPIm) and 1-ethyl-3-methylimidazolium tosylate (EMImTOS). The orientational dynamics are complex with relaxation involving several power law decays followed by a final exponential decay. A mode coupling theory (MCT) schematic model, the Sjögren model, was able to reproduce the PMPIm data very successfully over a wide range of times from 1 ps to hundreds of ns for all temperatures studied. Over the temperature range from room temperature down to the critical temperature T c of 231 K, the OHD-OKE signal of PMPIm is characterized by the intermediate power law t -1.00(0.04 at short times, a von Schweidler power law t -0.51(0.03 at intermediate times, and a highly temperature-dependent exponential (R relaxation) at long times. This form of the decay is identical to the form observed previously for a large number of organic van der Waals liquids. MCT analysis indicates that the theory can explain the experimental data very well for a range of temperatures above T c , but as might be expected, there are some deviations from the theoretical modeling at temperatures close to T c . For EMImTOS, the orientational dynamics were studied on the ps time scale in the deeply supercooled region near its glass transition temperature. The orientational relaxation of EMImTOS clearly displays the feature associated with the boson peak at ∼2 ps, which is the first time domain evidence of the boson peak in ionic organic liquids. Overall, all the dynamical features observed earlier for organic van der Waals liquids using the same experimental technique are also observed for organic ionic liquids.

Spotlight on ionic liquids

“…They observed multiple power-law relaxations that occur prior to the onset of the ␣-relaxation, in accordance with predictions from mode coupling theory. 95,99 How the nanostructural organization affects the dynamical environment in ILs has been probed by Xiao et al 100 They used the CS 2 probe at low concentrations to probe the intermolecular line shapes in ILs. The fs OHD-RIKES results of Xiao et al 100 for solutions of CS 2 in the IL 1-pentyl-3-methylimidazolium ͓NTf 2 ͔ − provide convincing evidence that the non-dipolar CS 2 localizes in the nanostructured domains of the self-aggregated pentyl chains.…”

Intermolecular vibrations and fast relaxations in supercooled ionic liquids

“…liquids has been investigated mainly by optical Kerr effect (OKE) spectroscopy 16,[21][22][23][24][25][26] covering a frequency range similar to low-frequency Raman spectroscopy. Raman spectra of glass-forming liquids exhibit a quasi-elastic scattering (QES) centered at zero frequency with few wavenumbers of bandwidth due to fast (picosecond) relaxational motions.…”

“…The OKE spectroscopy has given important insights on intermolecular dynamics of ionic liquids, for instance, the librational motions (hindered rotations) of the aromatic ring in imidazolium and pyridinium ionic liquids. 16,[21][22][23][24][25][26] In principle, OKE and Raman spectra give the same information because both techniques probe polarizability fluctuations. In OKE spectroscopy, the time domain data are Fourier transformed resulting in an intermolecular vibrational spectrum without long time relaxation.…”

“…In OKE spectroscopy, the time domain data are Fourier transformed resulting in an intermolecular vibrational spectrum without long time relaxation. [21][22][23][24][25][26]40 In case of ionic liquids, much more attention has been given to these vibrational components revealed by OKE spectroscopy, but in the context of glass-forming liquids, the temperature dependence of fast relaxations observed as the QES intensity in Raman spectra is also an important issue. [27][28][29][30][31][32][33][34] In this work, low-frequency Raman spectra are investigated for five different ionic liquids containing a common anion, bis(trifluoromethylsulfonyl)imide, [Tf 2 N], and different cations, including aromatic, non-aromatic cyclic, and tetraalkylammonium cations.…”

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

“…Raman and optical Kerr-effect (OKE) spectroscopy have been able to access the relatively high frequency region from ~0.1 to 10 THz. [26][27][28] The sub-THz region, corresponding to intermolecular modes, has been investigated by terahertz time-domain spectroscopy [29,30], dielectric relaxation spectroscopy (DRS) [31][32][33][34][35][36][37], and OKE spectroscopy [3,5,26,[38][39][40][41][42][43][44][45]. In general, DRS is limited to a maximum frequency of about 100 GHz whereas the spectra of most RTILs are continuous up to several THz.…”