Tunable Aggregate Morphologies from Perfluorodecanoic Acid and Hydroxyalkylamines: Role of Amine Architecture in Fluorocarbon-Hydrocarbon Self-Assembly

The study of the self-assembly of surfactants in aqueous solutions, though a traditional field, remains fascinating and full of novelty. In this article, the anionic perfluorodecanoic acid surfactant (PFA) is separately complexed with three hydroxyalkylamines (monoethanolamine (MEA), diethylamine (DEA), and triethanolamine (TEA)) in aqueous solutions. The transformation of aggregate morphologies from spherical unilamellar to nanotubes and then to spherical bilamellar is observed at room temperature, which is confirmed by cryo-transmission electron microscopy (cryo-TEM). Small-angle X-ray scattering (SAXS) measurement is used to determine the wall thickness of the three types of aggregates. The main driving force behind the formation of spherical unilamellar vesicles is the hydrogen bonding between the ionized C₉F₁₉COO^- and the neutral C₉F₁₉COOH, as well as the electrostatic interaction between the HO-C₂H₄-NH₃⁺-^-OOCF₁₉C₉ ion pair. As the number of -CH₂CH₂OH groups increases, the synergistic interaction between steric hindrance, hydrogen bonding, and electrostatic interactions induces a transformation in aggregate morphologies from spherical unilamellar vesicles to nanotubes and then to spherical bilamellar vesicles. Additionally, rheological results indicate that the viscoelastic moduli and viscosities of the MEA/PFA/H₂O, DEA/PFA/H₂O, and TEA/PFA/H₂O systems are affected by steric hindrance.