Polyhedral vesicles with crystalline bilayers formed from catanionic surfactant mixtures of fluorocarbon and hydrocarbon amphiphiles

摘要

Salt-free catanionic mixtures of perfluorononanoic acid (C_8F_(17)COOH) and tetradecyltrimethylammonium hydroxide (TTAOH) in aqueous solution show rich and interesting self-assembly properties. With the addition of TTAOH to a C_8F_(17)COOH solution, a structural transition from polyhedral vesicles with crystalline bilayers to spherical vesicles with fluid bilayers was observed at room temperature, which was demonstrated by cryo- and FF-TEM observations, DSC and WAXS measurements. It is interesting that both uni-lamellar and multi-lamellar polyhedral vesicles were formed in these fluoro- and hydrocarbon catanionic surfactant mixtures. SAXS measurements were performed to determine the interlamellar spacing of the multi-lamellar polyhedral vesicles. The driving force for the formation of the polyhedral vesicles was the crystallization of the C_8F_(17)COO~- - TTA~+ ion pairs in the bilayers, which induced the excess anionic surfactants to be segregated and concentrated to produce the vertices of the polyhedral vesicles. Besides, the rheological results show that the rheological properties of the C_8F_(17)COOH/TTAOH system are greatly affected by the fluorocarbon chains due to their high rigidity and Krafft point. The solutions consisting of polyhedral vesicles have high viscoelasticity and high yield stress values while spherical vesicle solutions are not viscoelastic.