Study of pH-responsive surface active ionic liquids: the formation of spherical and wormlike micelles

A noncovalent bonding method is utilized for forming pH-responsive surface active ionic liquids in mixed N -hexadecyl- N -methylpyrrolidinium bromide-based cationic surfactant (C 16 MPBr) and potassium phthalic acid (PPA). Rheology, cryogenic-transmission electron microscopy, and dynamic light scattering results revealed that the microstructure transition between spherical micelles and wormlike micelles was the fundamental cause of the pH-sensitive rheological properties. In addition, combined with nuclear magnetic resonance and UV–vis analysis, we found that the structure transition of micelles was attributed to different binding abilities of hydrotropes to C 16 MPBr as pH varies. It is confirmed that the binding ability of PPA to C 16 MPBr is strongest. This noncovalent bonding method is not only versatile but also economical for fabricating pH-responsive surface active ionic liquids.