Room-Temperature Ionic Liquids: Slow Dynamics, Viscosity, and the Red Edge Effect

Abstract

Ionic liquids (ILs) have recently attracted significant attention from academic and industrial sources. This is because, while their vapor pressures are negligible, many of them are liquids at room temperature and can dissolve a wide range of polar and nonpolar organic and inorganic molecules. In this Account, we discuss the progress of our laboratory in understanding the dynamics, spectroscopy, and fluid dynamics of selected imidazolium-based ILs using computational and analytical tools that we have recently developed. Our results indicate that the red edge effect, the non-Newtonian behavior, and the existence of locally heterogeneous environments on a time scale relevant to chemical and photochemical reactivity are closely linked to the viscosity and highly structured character of these liquids.

Correlation between Dynamic Heterogeneity and Local Structure in a Room‐Temperature Ionic Liquid: A Molecular Dynamics Study of [bmim][PF6]

“…[38,48,49] Del Popolo and Voth demonstrated this kind of behaviour for the IL 1-ethyl-3-methylimidazolium nitrate ([emim][NO 3 ]). [37] Margulis and Hu examined the differential dynamics of cations in liquid [bmim] [PF 6 ] [50,51] and correlated the same to the existence of regions of the IL possessing varied relaxation times. Although on macroscopic timescales the IL is homogeneous (i.e.…”

Multiple Mechanisms for Electron Transfer at Metal/Self‐Assembled Monolayer/Room‐Temperature Ionic Liquid Junctions: Dynamical Arrest versus Frictional Control and Non‐Adiabaticity

“…, which spans the range of one to tens of ns or more, [17,[30][31][32][33][34] and the recent disclosure of ultra-slow collective relaxational motion in electrode-adjusting layers occurring within the time domain from 0.1 ms to a few hundreds of ms [36,37] (even slower motion has been recently detected by the method of dynamic light scattering), [48] we conclude that the observed effect of at least the first drop in l 0 is due to the onset of an ergodically broken regime. Meanwhile, the value of l = (0.45 AE 0.10) eV holds for the cases of c (n + 1) = 8 and 12 for which the dependence of k 0 ET on c (n + 1) appears to obey Equation (6) with a decay parameter b of approximately 1 per methylene unit.…”

“…[21,22,[25][26][27][28][29] In addition, as demonstrated previously, [25] alkanethiol SAMs (including thin SAMs, which are considered to be the most inconvenient type because of a higher probability of defect formation) immersed into [ that have exceptional physical properties. [30][31][32][33][34][35][36][37] A broad distribution of relaxational characteristics, involving both the fast and slow degrees of freedom of the solvent, should be mentioned. The results of recent experimental (dielectric and solvation spectroscopy, NMR spectroscopy, Kerr relaxations, etc.)…”

“…The results of recent experimental (dielectric and solvation spectroscopy, NMR spectroscopy, Kerr relaxations, etc.) [30][31][32][33][34][35] and molecular dynamics (MD) simulations [30,31] indicated essentially heterogeneous nanostructural organisation of RTILs in which relaxation dynamics occur over a broad time range of 100 fs to 10 ns and longer. The response functions were found to be biphasic, consisting of a sub-picosecond component (100 fs to 200 ps) of modest amplitude (10 to 20 %), and a dominant slower component extended down to tens and, presumably, hundreds of nanoseconds.…”

Theoretical Description of Ionic Liquids

“…While there are many properties that individual members of the ionic liquids family exhibit, the single ubiquitous property that we expect to find in all of them is ionic conductivity (Galinski et al, 2006;Garcia et al, 2004). Unfortunately while the ionic liquids are intrinsic ion conductors, their ion conductivity often falls short of that of solvent-based electrolytes because of the high viscosities (Hu & Margulis, 2007). Of course viscosity is the result of the electrostatic interactions that are intrinsic to the nature of these substances.…”