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 spontaneously formed from a fluorinated surfactant (the ammonium salt of perfluoropolyether, PFPE) and a hydrogenated surfactant (n-dodecylbetaine) in dilute water solution were characterized by means of cryo-transmission electron microscopy (cryo-TEM) at different betaine mole fractions. The size distribution of the aggregates was found to depend critically on surfactant composition. In a narrow range of betaine molar fractions (x(bet) = 0.76-0.79), two different populations of unilamellar and spheroidal vesicles with mean radii of 70-120 and 20-30 nm, respectively, coexisted with globular and, in some cases, threadlike micelles. In the same region, vesicles with openings in the bilayer and disk-shape fragments began to appear. Further increase in betaine molar fraction resulted in an increase of the number of globular micelles and discoid aggregates and, finally, to the complete PFPE solubilization 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 consisted of uni- and multilamellar vesicles with high polydispersity. Increasing the total surfactant concentration gave rise to a significant increase of the vesicle size without modification of the size distribution of the aggregates themselves.