Spectroscopic studies of the aggregation of imidazolium-based ionic liquids

Methyl orange (MO) exhibits a sensitive spectral signature to the polarity of medium environment. Using MO as the spectroscopic probe, this paper reports the results of studies on the aggregation of 1-alkyl-3-methylimidazolium-based ionic liquids (ILs, [Cnmim]Br, n = 8, 10, 12, and 16) in aqueous solutions. Along with the results obtained from the measurements of electrical conductivity and resonance light scattering (RLS), this investigation shows the characteristics of the IL aggregation and the properties of the bulk phase prior to the aggregation, and hence reveals the aggregation differences between surface active [Cnmim]Br and traditional surfactants. It is shown that hydrogen bonds play a dominant role in the aggregation of ILs in addition to Coulombic and hydrophobic interactions. Compared with traditional cationic surfactant, IL displays the stronger aggregation due to the formation of the hydrogen bonded network. Also, such hydrogen bonded network results in the loose arrangements of imidazolium rings in the subaggregates of [Cnmim]Br (n < 10). When the mid-wavelength absorption band is just beginning to appear as the band of absorption maximum, the corresponding concentration is indicative of the formation of the perfect aggregates, and is in agreement with the critical aggregation concentration (CAC) of the ILs except [C8mim]Br. Moreover, it was found that the polarity in the core of the perfect IL aggregates is similar to the polarity within micelles of traditional cationic surfactant with the same alkyl chain length.