Elucidating effect of dicarboxylate anions on nanostructural organization of dicationic imidazolium-based ionic liquids

This work reports the influence of the dicarboxylate anion (oxalate, malonate succinate, glutarate, adipate, pimelate) on the aggregation process of ionic liquids derived from 1,8-bis(3-methylimidazolium-1-yl)-octane [C-8(MIM)(2)] and 1,10-bis(3-methylimidazolium-1-yl)-decano [C-10(MIM)(2)] in water solution. The aggregation behavior was investigated using electrical conductivity, nuclear magnetic resonance, transmission electron microscopy, atomic force microscopy, and zeta potential. Results showed that the aggregates have a wormlike morphology. The CAC values for [C-10(MIM)(2)](2+) increase in the following order: [Oxa] < [Suc] < [Mal] < [Glu] < [Adi] < [Pim], which indicates a direct relationship with the increase in the number of methylene groups of the dianion spacer. In the case of [C-8(MIM)(2)](2+), there was no relationship between the CAC value and the anion's alkyl spacer chain length, which was attributed to the flexibility and complexity of the dynamic behavior of the aggregates. Anions on the nanorganized structures of micelles are oriented in the way that carboxylate groups are at the aggregate surface (forming hydrogen bonds with cationic heads and water), and apolar alkyl chains are in the hydrophobic (downshift) region of the micelles making an arrangement where they also adopt the bend conformation of alkyl chains. The aggregation of dicationic ILs with dicarboxylate anions is a very innovative topic of investigation in the literature, and more research on different structures, system conditions, and techniques could be done to gain a better understanding of these systems.