Phase behavior and Lα-phase of a new catanionic system formed by cationic hydrocarbon and anionic fluorocarbon surfactants

Cationic hydrocarbon surfactant, N-cetylpyridinium chloride (CPC), was mixed with an anionic fluorocarbon surfactant, 1,1,11-trihydroperfluorohendecylsulfate ammonium (THFHS). The phase behavior of CPC/THFHS mixtures in aqueous solutions was studied at the total surfactant concentration of 100 mmol L−1. From CPC-richer side, with THFHS increasing, one observes the phase behavior sequence of L1-phase, L1/Lα-phase, single slightly turbid Lα-phase, then gel-like phase and finally a two-phase region with clear L1-phase at the top and precipitates at the bottom. The Lα-phase solutions consisting of polydispersed uni-lamellar vesicles with diameters from about 30 nm to more than 500 nm and multi-lamellar vesicles with diameters of more than 1.5 μm were observed by Freeze-fracture Transmission Electron Microscopy (FF-TEM). The 1H and 19F NMR measurements have been taken to detect the interaction between the hydrocarbon surfactant and fluorocarbon surfactant of L1-phase, Lα-phase, and the gel-like phase. In this system, it is interesting to find that no precipitates form when the stoichiometry between the cationic and anionic surfactants being exactly 1, which is very different from the general catanionic hydrocarbon surfactant systems with excess salts.