Cationic–anionic vesicle templating from fluorocarbon/fluorocarbon and hydrocarbon/fluorocarbon surfactants

Abstract

Spontaneous catanionic vesicle formation is studied in systems comprising fluorinated surfactants, the cationic/anionic fluorinated surfactant system of 1,1,2,2-tetrahydroperfluorododecylpyridinium chloride (HFDPC)/sodium perfluorooctanoate (SPFO) and the analogous mixed hydrocarbon/fluorocarbon surfactant system of cetylpyridinium bromide (CPB)/SPFO. Aggregate formation is explored in the anionic-rich surfactant system (weight fraction of anionic surfactant, γ = 0.66–0.85) and a total surfactant concentration range of 0.1–2% wt/wt for the fluorinated system and 0.4–2.2% wt/wt for the mixed hydrocarbon/fluorocarbon system. Vesicle sizes range from approximately 40 to 200 nm for CPB/SPFO, as determined by negative staining transmission electron microscopy (TEM) and confirmed by dynamic light scattering. The primary vesicle diameter observed by TEM in the catanionic fluorinated/fluorinated surfactant system is smaller (20–50 nm). However, the relatively few larger vesicles (⩾100 nm) in the HFDPC/SPFO system dominate the dynamic light scattering measurements. Successful templating of silica hollow spheres is demonstrated in both HFDPC/SPFO and CPB/SPFO vesicle systems, using tetramethoxysilane (TMOS) as the silica precursor for the acid-catalyzed synthesis. The size of the resulting hollow silica particles is consistent with the templating of vesicles of the size range observed by TEM. Changes in zeta potential are used to monitor colloidal stability. At the conditions investigated (TMOS/surfactant weight ratios of 0.25–1.0, pH 3), the colloidal silica particles templated from fluorinated HFDPC/SPFO vesicles are more stable than the particles templated from the corresponding mixed fluorinated CPB/SPFO system. Further improvement of the stability of the colloidal particles is achieved in the CPB/SPFO systems by titrating the acid synthesis solution with base over the course of the particle synthesis.