Cloud point phenomena for POE-type nonionic surfactants in imidazolium-based ionic liquids: Effect of anion species of ionic liquids on the cloud point

Abstract

Cloud point temperatures, T c, of polyoxyethylene (POE)-type nonionic surfactants in a room temperature ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate (bmimPF 6), were measured and compared with those previously obtained for the surfactant solution in bmimBF 4. The T cs for bmimPF 6 solution are higher than those for bmimBF 4 solution by approx. 40 °C. This means that the surfactant molecules are more solvophilic in bmimPF 6 compared to bmimBF 4. The analysis of 1H NMR chemical shift measurements proved that the higher solvophilicity of the surfactants in bmimPF 6 is attributed to weaker hydrogen-bond interaction between bmim cation and PF 6 - anion than that between bmim cation and BF 4 - anion. This interpretation is consistent with the interaction energy parameters derived from the thermodynamic analysis of cloud point curve applying the Flory–Huggins model for phase separation in polymer solution. The present work demonstrates that the property of imidazolium-based ionic liquids as a solvent is determined by a balance of interactions among imidazolium cation, counter anion, and solute molecule.