(50i) Composition–Driven Structural Transitions from Vesicles to Bicelles to Micelles Using Phospholipid and Nonionic Surfactant Mixtures

1. Tsengam, M. Omarova, S. Raghavan, G. Bothun, A. Mccormick, V. John

The self-assembly of environmentally benign, food grade lipid and surfactant systems is of utmost importance in designing highly efficient systems for oil-spill dispersion. We focus on a system containing L-α-phosphatidylcholine (PC) and Polyoxyethylene (20) sorbitan monooleate (tween 80) that spontaneously self-assembles upon transfer from a solvent such as a propylene glycol to water. Through small angle x-ray scattering (SAXS) and cryogenic transmission electron microscopy (Cryo TEM), we present a nanoscale understanding of the phase behavior of these structures. We observe a vesicle-to-micelle transition with an intermediate bicelle phase as the concentration of tween 80 increases. These structural transitions can be attributed to the solubilization of tween 80 into PC lipid bilayer structures which eventually leads to a system composed of mixed micelle aggregates. The packing limitation in the bilayers induced by the penetration of curvature inducing tween 80 (packing parameter ≈ 0.07 and large spontaneous curvature) leads to a breakdown of the bilayer structure and eventual micellization. We propose that the planar interfaces of the intermediate discoid bicelle phase is made up of the long double tailed molecules of PC which favor minimal curvature while the single tailed molecules of tween 80 which preferentially assume high curvature occupy the rim of the discoid structure. The translation of these fundamental concepts to membrane mimetics, drug delivery and oil spill dispersion will be briefly described.