(286d) Phase Behavior and Permeability of Hydrocarbon/Fluorinated and Fully Fluorinated Catanionic Vesicles

Spontaneously formed vesicles in dilute aqueous mixed solutions of cationic and anionic surfactants serve as attractive alternatives to conventional meta-stable liposomes in material synthesis, target drug delivery and molecular separation or transport. Although the electrostatic attraction between the oppositely charged headgroups dominate most applications in these systems, the nature of the vesicle bilayer cannot be completely ignored. To this end, we compare the bilayer characteristics of two similar cationic-anionic surfactant systems: mixed hydrocarbon/fluorinated, cetylpyridinium bromide(CPB)/sodium perfluorooctanoate(SPFO) and the analogous fluorinated/fluorinated, of 1,1,2,2-tetrahydroperfluorododecylpyridinium chloride (HFDPC)/sodium perfluorooctanoate(SPFO). Previously, stable unilamellar vesicles were established at lower surfactant concentrations in the anionic-rich phase region for HFDPC/SPFO (< 1% wt/wt) than in CPB/SPFO (> 1% wt/wt) with transmission electron microscopy images, dynamic light scattering and zeta potential measurements. Subsequent templating of silica hollow spheres revealed greater colloidal stability in HFDPC/SPFO than in CPB/SPFO.

Utilizing the model cationic dye, rhodamine 6G, the relative encapsulation and retention efficiencies were determined for both CPB/SPFO and HFDPC/SPFO with size exclusion chromatographic, fluorescence and UV spectroscopic techniques. The nature of the bilayer was further analyzed from the fluorescence spectra of pyrene and pyrene derivatized carboxylic acid probes. The pyrene derivatized probes possess varying lengths of alkyl chains, which facilitated probing of various depths of the bilayer with assessment of the microenvironment fluidity.

Results reveal that due to increased hydrophobicity and lipophobicity of fluorinated compounds, the fully fluorinated bilayer in HFDPC/SPFO, provides a greater barrier effect to permeation than in the mixed CPB/SPFO vesicles. This demonstrates that properties of catanionic surfactant mixtures can be effectively tuned, according to the desired application requirements.