Microemulsions with hydrophobic ionic liquids: Influence of the structure of the anion

The high polarity of ionic liquids (ILs) often prevents the formulation of efficient and strongly structured aqueous microemulsions in which the IL is supposed to replace the hydrophobic oil phase. One strategy to overcome this problem is to reduce the polarity of the IL by introducing aliphatic moieties into the molecular structure. In a previous publication we demonstrated that increasing the number of octyl chains in the cation of the IL reduces the polarity: Decreasing the polarity of the IL, i.e. of the hydrophobic compound, one observes that the phase behavior of the microemulsion follows the same trends as the one known for conventional microemulsions. In the study at hand we demonstrate that the same holds true when alkyl chains are introduced in the anion of the IL. For this purpose we synthesized a set of three ILs with different numbers of octyl chains in the anion. We investigated the phase behavior of the pure compounds as well as of binary and ternary systems. Among these systems the first example of a fluorine-free IL microemulsion is presented. Additionally, the composition of the three coexisting phases of a representative ternary system was determined by an NMR method. It was found that, in analogy to conventional microemulsions, the reduction of the polarity increases the phase transition temperature and broadens the three-phase area of the system. The strategy of increasing the amount of aliphatic chains in the IL is limited by the increase in phase transition temperature, which, in turn, reduces the efficiency. It is shown that the remarkably low efficiency of these microemulsions can be attributed to the high solubility of the surfactant in the IL, which is most likely caused by the surfactant's ethylene oxide units.