Abstract
We report measurements of the ionic conductivity, shear viscosity, and structural dynamics in a
homologous series of quaternary ammonium ionic liquids (ILs) and a prototypical imidazolium-based
IL over a wide range of temperatures down to the glass transition. We find that the ionic conductivity
of these materials generally decreases, while the shear viscosity correspondingly increases, with
increasing volume fraction of aliphatic side groups. Upon crossing an aliphatic volume fraction of
∼0.40, we observe a sharp, order-of-magnitude decrease in ionic conductivity and enhancement
of viscosity, which coincides with the presence of long-lived, nanometer-sized alkyl aggregates.
These strong changes in dynamics are not mirrored in the ionicity of these ILs, which decreases
nearly linearly with aliphatic volume fraction. Our results demonstrate that nanophase segregation in
neat ILs strongly reduces ionic conductivity primarily due to an aggregation-induced suppression of
dynamics.
