Cationic, anionic, and global dynamics in 1-hexyl-3-methylimidazolium halide supercooled ionic liquids studied by 1H and 35Cl magnetic resonance and oscillatory shear rheology

Abstract

The dynamics of the different constituents of the ionic liquid 1-hexyl-3-methylimidazolium chloride (HmimCl) is investigated using nuclear magnetic resonance including chlorine relaxometry, line shape analysis, and proton-detected diffusometry, as well as frequency-dependent shear mechanical measurements. This combination of techniques is useful to probe the individual motions of the anions and the cations, and the sample's overall flow response. The 35Cl− dynamics appears to be close to the structural (or α-) relaxation as seen by rheology. To examine possible sub-α responses, we scrutinize different representations of the viscoelastic response, including the shear modulus G*, compliance J* = 1/G*, fluidity F* = iωJ*, and viscosity η* = 1/F*, with some of these quantities being more susceptible to low-frequency features than others. This way, we are able to detect supramolecular rheological signatures not only for HmimCl but also for 1-hexyl-3-methylimidazolium bromide and 1-hexyl-3-methylimidazolium iodide. These results call for caution in the course of choosing particular response functions when estimating the degree of decoupling between the mesoscale dynamics and the structural rearrangements in ionic liquids.