Aggregate structures in a dilute aqueous dispersion of a fluorinated/hydrogenated surfactant system. A cryo-transmission electron microscopy study

The structural features of vesicles, micelles, and other aggregates spontan eously formed from a fluorinated surfactant (the ammonium salt of perfluoro polyether, PFPE) and a hydrogenated surfactant (n-dodecylbetaine) in dilute water solution were characterized by means of cryo-transmission electron m icroscopy (cryo-TEM) at different betaine mole fractions. The size distribu tion of the aggregates was found to depend critically on surfactant composi tion. In a narrow range of betaine molar fractions (x(bet) = 0.76-0.79), tw o different populations of unilamellar and spheroidal vesicles with mean ra dii of 70-120 and 20-30 nm, respectively, coexisted with globular and, in s ome cases, threadlike micelles. In the same region, vesicles with openings in the bilayer and disk-shape fragments began to appear. Further increase i n betaine molar fraction resulted in an increase of the number of globular micelles and discoid aggregates and, finally, to the complete PFPE solubili zation into mixed micelles. The large vesicles disappeared at x(bet) = 0.81 , whereas the small vesicles (mostly with open bilayers), occurred up to x( bet) = 0.86 At x(bet) < 0.75 no micelles were detected, and the sample cons isted of uni- and multilamellar vesicles with high polydispersity. Increasi ng the total surfactant concentration gave rise to a significant increase o f the vesicle size without modification of the size distribution of the agg regates themselves.