46138-AC10
Synthesis, Tuning, and Application of Fluorinated Mixed Cationic/Anionic Surfactant Vesicles
Spontaneous vesicles formed by mixing single tailed anionic and cationic surfactants are potential microreactors and materials synthesis templates for catalysis, photochemistry, biochemistry, pharmaceutical, microelectronics, and petrochemical applications. Mixed cationic/anionic surfactant vesicles are easier to form and typically less expensive than traditional kinetically stabilized vesicles. Unique self-assembly behavior and bilayer solvent properties are expected when charged fluorinated surfactants are mixed to produce cationic/anionic surfactant vesicles. Fluorinated surfactants, whose tails are characterized as lipophobic and hydrophobic, self assemble more readily than their hydrocarbon analogues and favor low curvature aggregates. This research investigates the formation of spontaneous vesicles of cationic/anionic surfactants comprising fluorinated and mixed fluorinated/hydrocarbon surfactants and explores the potential to tailor the properties of the fluorinated vesicle bilayers.
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 its mixed hydrocarbon analogue/fluorocarbon surfactant system (cetylpyridinium bromide (CPB)/SPFO). Aggregate formation is explored in the anionic-rich surfactant system (weight ratio, γ = 0.66 – 0.85) and a total surfactant concentration range of 0.1 – 2 wt% for the fluorinated system and 0.4 – 2.2 wt% for the mixed hydrocarbon/fluorocarbon system. Vesicle sizes range from approximately 40 - 200 nm for CPB/SPFO, as determined by negative staining transmission electron microscopy (TEM) and confirmed by dynamic light scattering. The primary vesicle size 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.
The outer monolayer, bilayer and aqueous core of vesicles represent distinct environments for nano-scale applications. In the synthesis of silica hollow spheres, the surface/outer monolayer of the vesicle can serve as a viable templating site. This transcriptive templating mechanism is dependent on the stability of the colloidal system and proceeds via the hydrolysis, condensation and polymerization of silicon alkoxides. With the vesicle surface acting as the site for the synthesis reactions, the polymerized silica network assumes the morphology and dimensions of the vesicles while encapsulating the aqueous core.
Successful transcriptive templating was demonstrated in the fluorinated/hydrocarbon and fluorocarbon/fluorocarbon 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. The titration of the acidic synthesis solution with base during the synthesis reactions produced stable sols of CPB/SPFO templated particles. The robustness of fluorinated/fluorinated bilayers for transcriptive templating is suggested by the ability to template at higher precursor to surfactant ratios than in catanionic fluorinated/hydrocarbon vesicles while maintaining the original aggregate shape.
This research project has supported the training of two Ph.D. students and one high school student in a broad range of thermodynamic and materials synthesis techniques such as transmission electron microscopy (TEM), dynamic light scattering (DLS), differential scanning calorimetry (DSC), fluorescence spectroscopy and size exclusion chromatography (SEC). The project bridges the PI’s expertise in solubilization in bilayers (liposomes) and the synthesis of functionalized materials. Enabling technologies are suggested by the ability to tune the size, bilayer permeability, and solubilization in the bilayer of cationic/anionic vesicles. These include microparticle and materials synthesis, encapsulation, targeted delivery, vesicles as microreactors and separators, and vesicles as model cell membranes to advance the knowledge of biological function.
