Phase behaviors and self-assembly properties of two catanionic surfactant systems: C(8)F(17)COOH/TTAOH/H(2)O and C(8)H(17)COOH/TTAOH/H(2)O.

Two fatty acids, perfluorononanoic acid (C(8)F(17)COOH) and nonanoic acid (C(8)H(17)COOH), were mixed with a cationic hydrocarbon surfactant, tetradecyltrimethylammonium hydroxide (TTAOH), in aqueous solutions for comparative investigation. Phase behaviors of the two systems are quite different because of the special properties of the fluorocarbon chains. For the C(8)H(17)COOH/TTAOH/H(2)O system, a single L(α) phase region with phase transition from planar lamellar phase (L(αl) phase) to vesicle phase (L(αv) phase) was observed. For the C(8)F(17)COOH/TTAOH/H(2)O system, two single phases consisting of vesicles were obtained at room temperature. One is a high viscoelastic gel phase consisting of vesicles with crystalline state bialyers at the C(8)F(17)COOH-rich side, which was confirmed by freeze-fracture transmission electron microscope (FF-TEM) and differential scanning calorimetry (DSC) measurements. With the increase of TTAOH proportion, another vesicle phase consisting of liquid state bilayers was observed after the two-phase region. The fluorosurfactant systems prefer to form vesicle bilayers than the corresponding hydrocarbon ones because of the rigid structure, the stronger hydrophobicity, and the larger volume of fluorocarbon chains.