Aggregation Behavior of Imidazolium-Based Ionic Liquids in Water

The UV absorption spectra of aqueous solutions of [C 4 mim][Br], [C 4 mim][BF 4 ], and [C 8 mim][Br], along with the differential UV spectra of aqueous [C 8 mim][Br] solutions, were measured at room temperature in this work. It is found that three transformations, denoted respectively as α , β and γ , can occur over the interested concentration range of solutions due to ionic liquid aggregation. Further, the content of ionic liquids and the wavelength of absorption maximum at these transformations are determined, and the implications at each transformation are analyzed. As a result, this paper discusses the effects of imidazolium cation, alkyl chain length and counterion type on the aggregation behavior and the contribution of interactions (such as hydrogen bonds, Coulombic, aliphatic and π – π stacking interactions) to the aggregation of 1-alkyl-3-methylimidazolium salts in water. Moreover, the possible structures of both aggregate and bulk phase in the aqueous solutions of ionic liquids are predicted also.