Comparison of vesicle formation in zwitanionic and catanionic mixtures of hydrocarbon and fluorocarbon surfactants: phase behavior and structural progression.
The phase behavior of zwitanionic and catanionic mixtures of perfluoro and hydrocarbon surfactants has been studied for the case of lithium perfluorooctanoate (LiPFO) as anionic surfactant and tetradecyldimethylamine oxide (TDMAO) as zwitterionic or tetradecyltrimethyl ammonium bromide (TTAB) as cationic surfactant. Samples as a function of the mixing ratio were characterized by means of electric conductivity, light scattering, cryo-TEM, and rheology. Despite the equal chain length of the zwitterionic and cationic surfactant we observe largely different phase behavior in these systems with the formation of precipitates around equimolar mixing for the catanionic system, while no such precipitation is observed for the zwitanionic system. This can be rationalized in terms of the much lower interaction parameter beta (from cmc measurements) for the case of the zwitanionic system. Accordingly, in the zwitanionic mixture a larger range of stable unilamellar vesicles is observed, while for the catanionic system larger vesicles are present, with a much stronger tendency for formation of multilamellar vesicles. Another interesting effect is that only for the case of the zwitanionic mixture a substantial increase of viscosity is observed prior to the formation of vesicles, which means that here the transition from spherical micelles to vesicles takes place via strongly anisometric micellar aggregates, which are not observed for the case of the catanionic mixtures. From these results it can be concluded that the structure and stability of vesicles formed in these mixtures are largely controlled by the extent of the electrostatic interactions which allows modification accordingly.
Wolf
Christian
C
Lehrstuhl for Physikalische Chemie I, Universität Bayreuth, D-95440 Bayreuth, Germany.
Bressel
Katharina
K
Drechsler
Markus
M
Gradzielski
Michael
M