Interconvertible Self-Assembly and Rheological Properties of Planar Bilayers and Vesicle Gels in Anionic/Nonionic (CF/CH) Surfactant Solutions

The surface and bulk properties of aqueous mixtures of nonionic tetraethylene glycol monododecyl ether (C12EO4) and anionic perfluorolauric acid (PFLA) were studied. These mixtures exhibited pronounced synergistic effects in terms of their mixed micellization and rheological properties. Only one type of mixed micelle is formed in dilute mixed solutions. At low PFLA content, with increasing the total surfactant concentration, short thread-like micelles grow to form larger discs. On increasing the amount of PFLA, owing to the fact that the fluctuant planar bilayers of nonionic C12EO4 become charged, vesicle gels that are comprised of densely packed unilamellar and multilamellar vesicles spontaneously form, in which the fluorocarbon chains in the bilayers are in a fluid state at room temperature. The elastic properties and the yield stress of the lamellar solutions largely increase upon the addition of small amounts of PFLA and they then pass through a maximum at a saturation of effective membrane charge density. Interestingly, increasing the pH value of the gels causes a backward transition from stiff vesicles to flexible planar bilayers. On increasing the amount of NaOH, the bilayers exhibit greater flexibility, owing to the decrease in membrane charges, along with a large decrease in the solution viscosity and their elastic properties. The combination of strong stability of the vesicle gels with the interconvertible transition of bilayers and vesicles is expected to be of practical use for controlled drug delivery and release.