Cation structure-dependence of the induced free charge density gradient in imidazolium and pyrrolidinium ionic liquids.

We report on the structure-dependence and magnitude of the induced free charge density gradient ( ρ f ) seen in room-temperature ionic liquids (RTILs) with imidazolium and pyrrolidinium cations. We characterize the spatially-resolved rotational diffusion dynamics of a trace-level cationic chromophore to characterize ρ f in three different pyrrolidinium RTILs and two imidazolium RTILs. Our data show that the magnitude of ρ f depends primarily on the alkyl chain length of RTIL cation and the persistence length of ρ f is independent of RTILs' cation structure. These findings collectively suggest that mesoscopic structure in RTILs plays a significant role in allowing charge density gradients to form.