The Effect of Ionic Liquid Hydrophobicity and Solvent Miscibility on Pluronic Amphiphile Self-Assembly

The phase behavior of the triblock copolymer, (EO)20(PO)70(EO)20 (P123), in the water-immiscible (hydrophobic) ionic liquids (ILs), 1-butyl-3-methylimidazolium hexafluorophosphate (bmimPF6), and tris(pentafluoroethyl)trifluorophosphate (bmimFAP), has been investigated, and its amphiphilic self-assembly examined using small-angle X-ray scattering. The results obtained are contrasted with those for P123 in water. Direct and water-swellable micellar, hexagonal, and lamellar phases of P123 are found in bmimPF6, which behaves like a polar solvent despite being water immiscible, but bmimFAP behaves as a truly hydrophobic solvent, forming only a lamellar phase over a narrow composition range. The miscibility of bmimPF6 and water is increased by P123 addition, and at sufficiently high P123 concentrations, a single lamellar phase forms in which bmimPF6 and water are miscible in all proportions. In contrast, the preferential solubilization of bmimPF6 by PEO chains and bmimFAP by PPO chains causes the nanosegregation of these miscible ILs in concentrated P123 solutions. This leads to the formation of a P123/bmimPF6/bmimFAP microemulsion where bmimPF6 is the polar solvent and bmimFAP is the non-polar solvent.