Orientational dynamics of the ionic organic liquid 1-ethyl-3-methylimidazolium nitrate

Abstract

Optical heterodyne-detected optical Kerr effect ͑OHD-OKE͒ experiments are used to study the orientational dynamics of the ionic organic liquid 1-ethyl-3-methylimidazolium nitrate (EMIM ϩ NO 3 Ϫ) over time scales from ϳ1 ps to ϳ2 ns, and the temperatures range from 410 to 295 K. The temperatures cover the normal liquid state and the weakly supercooled state. The orientational dynamics exhibit characteristics typical of normal organic glass-forming liquids. The longest time scale portion of the data decays as a single exponential and obeys the Debye-Stokes-Einstein relation. The decay of the OHD-OKE signal begins (ϳ1 ps) with a temperature independent power law, t Ϫz , zϭ1.02Ϯ0.05, the ''intermediate power law.'' The power law decay is followed by a crossover region, modeled as a second power law, the von Schweidler power law. The longest time scale decay is the exponential ␣ relaxation. The intermediate power law decay has been observed in van der Waals supercooled liquids previously. These are the first such observations on an ionic organic liquid. The observation of the dynamical signatures observed in other liquids demonstrates that the orientational dynamics of ionic organic liquids are fundamentally the same as van der Waals liquids and supports the universality of the intermediate power law decay in the dynamics of complex liquids. Within the mode-coupling theory ͑MCT͒ framework, the MCT critical temperature T C is estimated to be T C Х255 K.

Dynamics in Supercooled Ionic Organic Liquids and Mode Coupling Theory Analysis

“…The b value for the imidazolium-based ionic liquid was 0.87 ( 0.06. 30 The b values from the ionic liquid studied here falls outside the range 0.73-0.85 reported 37 …”

“…All these features observed for these RTILs are typical of fragile van der Waals glass-forming liquids, which is consistent with previous OKE studies on a different organic ionic liquid. 30 Recently, heat capacity measurements on an imidazoliumbased ionic liquid suggest that it is a fragile and disordered system with thermodynamic properties that are similar to molecular glass-forming liquids. 54 Dynamics of a room-temperature ionic liquid were also compared to its analogous isoelectronic binary solution at room temperature.…”

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.…”

Low-frequency Raman spectra and fragility of imidazolium ionic liquids

“…The main issue in these OKE spectroscopy studies was the investigation of librational modes usually assigned to the imidazolium ring. [13][14][15][16][17][18] This work is not concerned with such librational dynamics, instead we focus here on the relative contribution of fast relaxational and vibrational motions in low-frequency Raman spectra of ionic liquids. In order to probe the slow relaxational dynamics, ionic liquids have been investigated by dielectric relaxation spectroscopy, 19,20 which covers a frequency range several orders of magnitude below the terahertz range that is available by low-frequency Raman spectroscopy.…”

“…2 The short-time intermolecular dynamics of ionic liquids has been the subject of many investigations by optical Kerr effect ͑OKE͒ spectroscopy. [13][14][15][16][17][18] In OKE spectroscopy, the original data in time domain are Fourier transformed to frequency domain covering a frequency range similar to Raman spectroscopy. The main issue in these OKE spectroscopy studies was the investigation of librational modes usually assigned to the imidazolium ring.…”