Temperature Dependence of the Primary Relaxation in 1-Hexyl-3-methylimidazolium bis{(trifluoromethyl)sulfonyl}imide

Abstract

We present results from complementary characterizations of the primary relaxation rate of a room temperature ionic liquid (RTIL), 1-hexyl-3-methylimidazolium bis{(trifluoromethyl)sulfonyl} imide, [C6mim][Tf2N], over a wide temperature range. This extensive data set is successfully merged with existing literature data for conductivity, viscosity, and NMR diffusion coefficients thus providing, for the case of RTILs, a unique description of the primary process relaxation map over more than 12 decades in relaxation rate and between 185 and 430 K. This unique data set allows a detailed characterization of the VTF parameters for the primary process, that are: B=890 K, T0=155.2 K, leading to a fragility index m=71, corresponding to an intermediate fragility. For the first time neutron spin echo data from a fully deuteriated sample of RTIL at the two main interference peaks, Q=0.76 and 1.4 A(-1) are presented. At high temperature (T>250 K), the collective structural relaxation rate follows the viscosity behavior; however at lower temperatures it deviates from the viscosity behavior, indicating the existence of a faster process.

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

“…This suggests a mode with high symmetry such as a simple breathing mode of a cation-stacked or micelle-like cluster [50]. Recent X-ray scattering studies reported structure (nanoscale domains) in RTILs due to tail aggregation that increases with length of the alkyl substituents [15,44,[51][52][53][54][55]. We cannot presently study these larger cations due to bandwidth limitations, but in line with MD simulations, the present spectra suggest that structure exists even in RTILs with smaller (ethyl-substituted) cations.…”

Dynamics of Butyl- and Hexyl-Methylimidazolium Bromide Ionic Liquids

“…Results in this domain can be connected with important macroscopic parameters such as diffusion constants and viscosities and furthermore lend themselves to parallel numerical simulations with AMD, which turns out to be essential for interpreting experimental results on these complex systems. However, the number of QENS studies is still relatively small [18][19][20][21][22]. For C 2 mimBr, we have studied the dynamics of both crystal and liquid phases [23].…”

Intermolecular vibrations and fast relaxations in supercooled ionic liquids

“…[1][2][3][4][5][6] A common finding resulting from the thermophysical studies is that many ionic liquids are easily supercooled upon reduction of temperature below the melting temperature T m , and their glass transition temperature T g is typically around 190 K. [7][8][9] Transport coefficients change with temperature in such a way that prompts the classification of ionic liquids as fragile glass-forming liquids. [10][11][12] Fragility of a supercooled liquid measures the departure from an strict linear behavior of the viscosity at temperature close to T g in an Arrhenius plot, log(η) vs. 1/T. 13,14 Several spectroscopic techniques have been used to study ionic liquids aiming a microscopic understanding of macroscopic properties and eventual designing of new systems.…”

“…15,16 For instance, dielectric relaxation spectroscopy has been used to investigate ionic liquids dynamics in a broad frequency range, [17][18][19] and neutron scattering spectroscopy to unravel dynamics processes occurring within the nanosecond range. 11,20 Short-time intermolecular dynamics of ionic a) Electronic mail: mccribei@iq.usp.br.…”

“…The I QES is compared with excess entropy involved in the AdamGibbs theory, 42 mean hole volume involved in free volume theories, 43,44 and also mean square displacement 45 recently available for some ionic liquids. In the particular case of [hmIm][Tf 2 N], for which structural relaxation has been investigated within a wide temperature range, 185 < T < 430 K, 11 it will be shown that the I QES (T) dependence unravels the characteristic temperatures T o , T g , and T c , related to relaxation dynamics of glass-forming liquids.…”