Dynamic-Mechanical and Dielectric Evidence of Long-Lived Mesoscale Organization in Ionic Liquids*

Experimental evidence of the dynamics of mesoscopic structure in room temperature ionic liquids - a feature expected to correlate with many physicochemical properties of these materials - remains limited. Here, we report the observation of slow, sub-α relaxations corresponding to dynamics of nanoscale hydrophobic aggregates in a systematic series of 1-alkyl-3-methylimidazolium-based ionic liquids from detailed analysis of dynamic-mechanical and broadband dielectric spectra. The emergence of the sub-α relaxations correlates with increases in the zero-shear viscosity and static dielectric permittivity, constituting direct evidence of the influence of mesoscale aggregation on the physicochemical properties of ionic liquids.