Dilution-induced changes in room temperature ionic liquids. Persistent compositional heterogeneity and the importance of dipolar interactions

Abstract

Room temperature ionic liquids (RTILs) exhibit several unusual properties, including the ability to support an induced free charge density gradient, ρf. Upon dilution with water, RTILs form micelle-like structures, and compositional non-uniformity has been reported in other solvents, such as methanol and acetonitrile. The dependence of ρf on dilution is not consistent with uniform dissolution of the RTIL in the diluent. We report here our examination of two imidazolium RTILs diluted with polar aprotic solvents dimethyl sulfoxide (DMSO) and propylene carbonate (PC). Using the cationic fluorophore cresyl violet (CV+), we evaluate the dependence of ρf magnitude and persistence length on dilution with these two solvents. These properties are found to depend on the identity of the RTIL anion (BF4- and TFSI-). In regions of the RTILs not affected by the induced charge density gradient, we find evidence for the formation of aggregated domains of RTIL in the binary systems. The characteristic domain size is seen to depend on the identity of the diluent and RTIL anion, and for all systems examined, domain size increases with increasing dilution. We discuss these findings in the context of the magnitude of the RTIL ion pair dipole moments.