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

Cloud point temperatures, Tc, of polyoxyethylene (POE)-type nonionic surfactants in a room temperature ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate (bmimPF6), were measured and compared with those previously obtained for the surfactant solution in bmimBF4. The Tcs for bmimPF6 solution are higher than those for bmimBF4 solution by approx. 40 °C. This means that the surfactant molecules are more solvophilic in bmimPF6 compared to bmimBF4. The analysis of 1H NMR chemical shift measurements proved that the higher solvophilicity of the surfactants in bmimPF6 is attributed to weaker hydrogen-bond interaction between bmim cation and PF6- anion than that between bmim cation and BF4- 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.

Graphical abstractCloud points of CnEm in bmimPF6 are higher than those in bmimBF4, because the interaction of bmim cation with PF6 anion is weaker than that with BF4 anion.Figure optionsDownload full-size imageDownload high-quality image (116 K)Download as PowerPoint slide