Dynamics of RTILs: A comparative dielectric and OKE study

Abstract

The dynamics of room-temperature ionic liquids (RTILs) were studied by investigating their dielectric relaxation (DR) and time-resolved optical Kerr-effect (OKE) spectra in the frequency range of ~10MHz to ~20THz. For the studied RTILs the OKE and DR spectra are dominated by a relaxation in the GHz region and extend to a relatively sharp band at around 10THz. Whilst the first feature is mainly associated with the structural relaxation of the fluid through ion rotation (α relaxation), the second indicates the short-time limit of intermolecular dynamics. The rather featureless intermediate region is mainly associated with intermolecular vibrations that are strongly coupled to hindered rotations. In contrast to other RTILs, imidazolium salts show an additional sub-α relaxation which dominates the OKE signal and is indicative of the breathing motion of rather long-lived cages.Mixed with polar solvents RTILs were found to retain their ionic liquid-like character up to relatively high levels of dilution, but with the overall dynamics considerably speeded up. Below RTIL mole fractions of ~0.2–0.4 these systems behave like conventional electrolyte solutions with more or less pronounced ion pairing.