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 (C16MPBr) 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 C16MPBr as pH varies. It is confirmed that the binding ability of PPA to C16MPBr is strongest. This noncovalent bonding method is not only versatile but also economical for fabricating pH-responsive surface active ionic liquids.