Role of the surfactant structure in the behavior of hydrophobic ionic liquids within aqueous micellar solutions.

The behavior of an ionic liquid (IL) within aqueous micellar solutions is governed by its unique property to act as both an electrolyte and a cosolvent. The influence of the surfactant structure on the properties of aqueous micellar solutions of zwitterionic SB-12, nonionic Brij-35 and TX-100, and anionic sodium dodecyl sulfate (SDS) in the presence of the "hydrophobic" IL 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF(6)]) is assessed along with the possibility of forming oil-in-water microemulsions in which the IL acts as the "oil" phase. The solubility of [bmim][PF(6)] within aqueous micellar solutions increases with increasing surfactant concentration. In contrast to anionic SDS, the zwitterionic and nonionic surfactant solutions solubilize more [bmim][PF(6)] at higher concentrations and the average aggregate size remains almost unchanged. The formation of IL-in-water microemulsions when the concentration of [bmim][PF(6)] is above its aqueous solubility is suggested for nonionic Brij-35 and TX-100 aqueous surfactant solutions.