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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 form spontaneously, unlike traditional kinetically stabilized systems such as liposomes. 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.

The phase behavior of combinations of cationic and anionic surfactant mixtures of cetylpyridinium bromide (CPB), 1,1,2,2-tetrahydroperfluorododecylpyridinium chloride (HFDPC), sodium perfluorooctanoate (SPFO; FC7) and sodium perfluorohexanoate (SPFH; FC5) has been established using a combination of dynamic light scattering (DLS), visual inspection and transmission electron imaging (TEM).  The selection of mixed hydrocarbon/fluorocarbon catanionic pairs, CPB/SPFO andCPB/SPFH, and fully fluorinated bilayers, HFDPC/SPFO and HFDPC/SPFH provides a framework in which the impact of homogeneous and heterogeneous bilayer matrices of varying asymmetry can be assessed.  Regions of spontaneous vesicle formation in the anionic-rich surfactant mixtures have been identified for each of these four surfactant pairs.  The largest isotropic vesicle phase region is observed in the fully fluorinated bilayer, HFDPC/SPFH (total surfactant concentration range of 2 – 5% wt/wt; weight fraction of anionic surfactant, γ = 0.6 – 0.95), which also possesses the largest chain asymmetry. The smallest vesicle region was observed in the least asymmetric fluorinated pair, HFDPC/SPFO (0.6 – 1.5% wt/wt; γ = 0.7 - 0.9).  

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, HFDPC/SPFO, 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 morphology and phase behavior of spontaneously formed vesicles is used to interpret the mechanism of stabilization of the vesicles.  Rigid bilayers are enthalpically stabilized, with a narrow size distribution of vesicles due to the energetic penalty associated with deviation from this spontaneous curvature.   Factors that reduce membrane rigidity, such as branched chains and chain asymmetry, typically produce entropically stabilized vesicles and expand the vesicle phase region.

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.

Relative encapsulation and retention of a model cationic dye, rhodamine 6G (R6G) was determined for catanionic vesicle systems with mixed hydrocarbon /fluorocarbon bilayers (CPB/SPFO), and compared with the fully fluorinated analogue (HFDPC/SPFO).  Both vesicle systems were prepared with excess molar concentrations of the anionic surfactant, SPFO, resulting in negatively charged aggregate systems.  Size exclusion chromatography (SEC) was used to determine the efficiency of solute encapsulation and vesicle stability. Vesicle aggregate size was monitored with dynamic light scattering.  Significant encapsulation of R6G is achieved.  This is consistent with specific ionic association of the oppositely charged species (anionic vesicles and cationic R6G) as a principal driving force for encapsulation.  Greater encapsulation of RG6 is achieved with CPB/SPFO (85% and 65% for 1 mM and 0.05 mM) relative to HFDPC/SPFO (14% for 0.05 mM).  The HFDPC/SPFO vesicles demonstrated higher retention of the captured R6G (11%) while more than half the captured dye was released in CPB/SPFO vesicles. The increased hydrophobicity and lipophobicity of the fully fluorinated bilayer may provide a greater barrier effect to permeation than in the mixed hydrocarbon/fluorocarbon vesicles.

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.