Study of Nanostructural Organization of Ionic Liquids by Electron Paramagnetic Resonance Spectroscopy

Abstract

The X-band electron paramagnetic resonance spectroscopy (EPR) of a stable, spherical nitroxide spin probe, perdeuterated 2,2,6,6-tetramethyl-4-oxopiperidine-1-oxyl (pDTO) has been used to study the nanostructural organization of a series of 1-alkyl-3-methylimidazolium tetrafluoroborate ionic liquids (ILs) with alkyl chain lengths from two to eight carbons. By employing nonlinear least-squares fitting of the EPR spectra, we have obtained values of the rotational correlation time and hyperfine coupling splitting of pDTO to high precision. The rotational correlation time of pDTO in ILs and squalane, a viscous alkane, can be fit very well to a power law functionality with a singular temperature, which often describes a number of physical quantities measured in supercooled liquids. The viscosity of the ILs and squalane, taken from the literature, can also be fit to the same power law expression, which means that the rotational correlation times and the ionic liquid viscosities have similar functional dependence on temperature. The apparent activation energy of both the rotational correlation time of pDTO and the viscous flow of ILs and squalane increases with decreasing temperature; in other words, they exhibit strong non-Arrhenius behavior. The rotational correlation time of pDTO as a function of η/T, where η is the shear viscosity and T is the temperature, is well described by the Stokes-Einstein-Debye (SED) law, while the hydrodynamic probe radii are solvent dependent and are smaller than the geometric radius of the probe. The temperature dependence of hyperfine coupling splitting is the same in all four ionic liquids. The value of the hyperfine coupling splitting starts decreasing with increasing alkyl chain length in the ionic liquids in which the number of carbons in the alkyl chain is greater than four. This decrease together with the decrease in the hydrodynamic radius of the probe indicates a possible existence of nonpolar nanodomains.

An analysis of radical diffusion in ionic liquids in terms of free volume theory

“…The size of nanostructured soft apolar regions increases with the length of the cation alkyl chain. 24,42,43 PALS gives only the hole radius distribution in the bulk ionic liquid; 38 as far as we know, there are no such distributions for each domain. Therefore, if one assumes both homogeneous hole radius distribution and diffusion throughout the bulk ionic liquid, the growth of bicontinuous apolar nanodomains may not change much the overall diffusion of the spin probe.…”

“…In this case, the dispersion line component is not the conventional experimental dispersion signal, but it is the dispersion EPR line shape induced by HSE and DD interactions and is opposite for the low-and high-field EPR lines as described in Ref. 24. In a series of articles, [25][26][27][28][29][30] our group and Salikhov's group have continued to work on the separation of the HSE and DD interactions so that the concentration induced broadening of the EPR spectrum can be used to measure the translational diffusion of radicals in highly viscous liquids.…”

Structural Anomalies in Ionic Liquids near the Glass Transition Revealed by Pulse EPR

“…In this technique, the paramagnetic molecule (most often a stable nitroxide radical) is used as a probe, reflecting its interaction with surrounding media via the EPR spectrum. A number of papers have focused on the study of heterogeneities in ILs using continuous wave (CW) EPR. − Because the CW EPR spectrum is very sensitive to the motional regime of the nitroxide, one easily identifies whether the probe is localized in the liquid (or softened) medium or in the solid one . However, only relatively fast and large-amplitude motions of nitroxides, e.g., diffusive rotation with characteristic times of ∼0.1–10 ns, are accessible using the CW approach.…”

High-Pressure ESR Spectroscopy: On the Rotational Motion of Spin Probes in Pressurized Ionic Liquids

“…Peric et al reported in a detailed study about the nanostructured organization of RTILs using perdeuterated TEMPONE radical as a spin probe. 14 Only two papers deal with time-resolved (TR) ESR-spectroscopy. Kawai et al and recently Fedin et al report on the spin dynamics of triplet and doublet states in RTILs.…”