Transition from fibrous structures to micelles of sodium 2-ketooctanoate with increasing concentration in water

Recently, we have reported the vesicle formation and vesicle-to-micelle transition (VMT) of sodium 2-ketooctanoate, a single-tailed keto-acid salt, in water with increasing concentration. Herein, we demonstrate that, upon prolonged standing at the vesicle-forming concentration, macroscopic fibrous structures spontaneously formed. Cryogenic transmission electron microscopy, atomic force microscopy, and laser scanning confocal microscopy revealed the microscopic pathway: vesicles aggregated and evolved through elongated intermediate structures (threadlike vesicles and chains of vesicles) into long strips, which ultimately assembled into floating fibers. As concentration was increased around the critical micelle concentration (CMC), these fibers transformed into bundles of nanofibers and threadlike micelles, and finally evolved to globular micelles above the CMC. The observed multi-step aggregation behavior is driven by the dynamic coexistence and rearrangement of multiple species, notably “acid-soap” dimers. This constitutes the first report detailing the microscopic transition pathway from fibrous structures to micelles for a single-tailed keto-acid salt amphiphile in water.