Self-Assembly of a Fluorocarbon−Hydrocarbon Hybrid Surfactant: Dependence of Morphology on Surfactant Concentration and Time

The morphologies of aggregates formed from a hybrid surfactant (F6H5OS) whose molecules have a hydrocarbon chain (pentyl group) and a fluorocarbon chain (perfluorohexyl group) were studied along with changes in the morphologies with time. F6H5OS exhibits a micelle-vesicle transition at a concentration of 5.5 mM in water. Cryo-TEM observation of 5 mM aqueous F6H5OS solution reveals the existence of only micelles with a diameter of ca. 6 nm, while the coexistence of micelles and vesicles is observed in 10 mM aqueous F6H5OS solution. When the 10 mM aqueous F6H5OS solution is aged, the micelles are transformed into vesicles. Further, the vesicles have different structures, namely, spherical and tubular structures. (1)H NMR, (1)H-(1)H NOESY, and FT-IR experiments suggest that upon aging, the conformation of hydrocarbon chains in F6H5OS changes, and the hydrocarbon chains are located close to the phenyl group in the surfactant molecules. At concentrations above 5.5 mM, small micelles are formed immediately after the preparation of the F6H5OS solutions; those micelles subsequently transform into vesicles. The hybridity of F6H5OS, i.e., having a fluorocarbon chain and a hydrocarbon chain in its molecule, plays an important role in the transformation of micelles into vesicles upon aging.