Ethanolamine-induced self-assembly of perfluorinated fatty acid: Structure control and phase transition mechanism from lamellar hydrogels to polymorphic vesicles

The self-assembly behavior study of surfactants in solution is traditional but full of fascination and novelties. Here, one anionic fluorinated surfactant, perfluorinated undecanoic acid (PFUDA), was mixed with an organic amine, monoethanolamine (MEA) in aqueous solution by changing the mixing ratio of the two components at a fixed total concentration and the self-assembly behavior was investigated in detail. Notably, increasing the MEA mole fraction ( r MEA ) progressively evolved the self-assembled structures from lamellar hydrogels to vesicles exhibiting polymorphic morphologies such as spherical, polyhedral, and rod-like shapes. This structural transition was corroborated by cryo-TEM and combined SAXS/WAXS measurements. Specifically, nearly all observed spherical and polyhedral vesicles were bilayers, seldom reported in catanionic systems. Based on the results of FT-IR, 19 F NMR, and XRD measurements, the mechanism of the phase transition of the different aggregates was proposed. The phase transition was induced by the formation of the C 10 F 21 COO - - + NH 3 C 2 H 4 OH ion pairs by expanding the interfacial area occupied by the headgroups, driven by the balance of electrostatic interactions, hydrogen bonds, van der Waals forces, and hydrophobic interactions. This work will enrich the fundamental study of the self-assembly behaviors of catanionic surfactant mixtures obtained by mixing fatty acids with short chain alkyl amines.