Accelerated decarboxylation of 6-nitrobenzisoxazole-3-carboxylate in imidazolium-based ionic liquids and surfactant ionic liquids

We report the use of the unimolecular, spontaneous decarboxylation of 6-nitrobenzisoxazole-3-carboxylate, 6-NBIC, as kinetic probe to investigate the properties of aqueous solutions of a series of ILs, 1-alkyl-3-methyl imidazolium derivatives. The ILs are denoted as [Cnmim][X], where n indicates the number of carbon atoms in 1-alkyl chain. We studied [C4mim][X], with X = Cl-, Br-, and BF4- , and the surface-active ILs, SAILs, [C12mim][Cl], [C12mim][Br], and [C16mim][Br]. For comparison purposes we also studied nonmicellizing tetralkylammonium chloride and bromide, denoted as TRAX, where R is alkyl group and X the anion. We observed a steep increase of values of kobs after a certain salt concentration for all the systems used. Electrical conductivity of various aqueous systems was measured, in an attempt to rationalize the kinetic effects. Data from conductivity and kinetic are consistent with the idea that after a certain, high, concentration aggregates of ILs form, and data from the kinetics suggest that water is someway ‘‘squeezed out” from these aggregates. As can be deduced from kinetics, properties of the aggregates formed by [C4mim][X] ILs correlate well with bulk water structure affecting properties of the salts, and seem to have no relation to surface effects.