Characterization of the phase behavior and elastic properties of surfactant bilayers

The formation of spontaneous surfactant bilayer vesicles is observed in a few chemical systems, most widely in mixtures of cationic and anionic surfactants. While PEG-lipids have been studied as stabilizers of rigid phospholipid vesicles (in applications including drug delivery), the effect of grafted polymers on the micro-mechanical properties of a thermally fluctuating vesicle membrane is not well understood either theoretically or experimentally. Here we use a variety of tethered polymers to tailor the bilayer properties of membranes, including the rigidity and spontaneous curvature, and are able to create extremely monodisperse vesicles. Using dynamic light scattering (DLS) we are able to obtain an average diameter and polydispersity and from cryogenic transmission electron microscopy (cryo-TEM) we can obtain membrane rigidity and spontaneous curvature from the exact size distribution of the vesicles. Thus, by direct experimental observation of the spontaneous vesicle size distribution as a function of PEG-lipids, we can better understand the effect of grafted polymers on fluctuating membranes, assisting in the development of new nano-structured materials.; Catanionic bilayers also form multilamellar vesicle (MLV) and lamellar (Lalpha) phases and higher concentration of surfactant. There is generally a transition from a clear lamellar phase to a cloudy MLV phase. The formation of these phases and their transition points was explored by small angle x-ray scattering (SAXS) and freeze-fracture transmission electron microscopy (FF-TEM). Similar phase behavior was found for the different catanionic systems with the transition occurring at different concentration depending on the particular surfactant mixture. A possible new phase of "lumpy" lamellar sheets was found near the lamellar sheet/MLV transition point for a mixture of cationic hydrocarbon and anionic fluorocarbon surfactant.; The release properties of phospholipid vesicles (liposomes) were also explored by cryo-TEM and diffusion nuclear magnetic resonance. The cryo-TEM technique was found to be inadequate to characterize the release of heavy salts from the liposomes. The diffusion NMR method was found to successfully characterize the release of an entrapped molecule from the liposomes.