Mesoscale Aggregates and Dynamic Asymmetry in Ionic Liquids: Evidence from Depolarized Dynamic Light Scattering

Nanoscale structures in ionic liquids (ILs) are usually identified by X-ray or neutron scattering techniques and occur when the alkyl chains of the cations are long enough to show the tendency to segregate into apolar domains. In search of dynamic evidence for these nanostructures, different experimental techniques recently reported bimodal dynamic susceptibility spectra. In all cases, the faster process observed was ascribed to the structural alpha-relaxation and the slower one to the relaxation of long-lived aggregates. By contrast, we show by depolarized dynamic light scattering (DDLS) experiments on a systematic series of imidazolium-based ILs that the dynamics of the cation and anion are clearly separated for long alkyl chains. Therefore, the observation of a bimodal behavior is not related to any nanostructure but reflects the two-component nature of ILs. Thus, a consistent picture is obtained across different experimental methods, like dielectric and shear mechanical relaxation. Finally, the actual dynamic signature of nanostructures is identified for the first time as a weak feature in some of the DDLS spectra at even lower frequencies.