Unraveling the influence of mesoscale solvophobic structure on the transport properties of ionic liquids—Decoupling ionic conductivity and viscosity

Ion dynamics and transport properties of the ionic liquid 1-methyl-3-decylimidazolium chloride are investigated by broadband dielectric spectroscopy, shear rheology, and differential scanning calorimetry. This ionic liquid is known to undergo a low-temperature, liquid-liquid crystal (L-LC) phase transition previously associated with a transition or growth in the extent of mesoscale solvophobic aggregates. We find that the L-LC transition is accompanied by a significant change in both the timescale and the strength of dielectric and mechanical relaxation processes associated with the motions of the mesoscale solvophobic aggregates. However, no change is observed in faster, more-localized ion motions associated with ion diffusion. Due exclusively to the changes in the slow, supramolecular dynamics, the zero-shear viscosity of the ionic liquid is increased fivefold, while the ionic conductivity is unaffected. These results provide unique insight into the role of mesoscale solvophobic structure on the transport properties of ionic liquids, indicating that the supramolecular mesoscale solvophobic aggregates may, at times, have a sizable influence on the zero-shear viscosities while having a negligible influence on ion conduction, which is determined exclusively by the faster diffusive ion motions occurring at shorter length-scales.