COLL 529 |
| Marjorie L. Longo, Mark A. Borden, Gang Pu, and Gabe Runner. Department of Chemical Engineering and Materials Science, U. C. Davis, 1 Shields Ave, Davis, CA 95616 |
| Microbubbles stabilized by a lipid/emulsifier monolayer shell are important for a variety of reasons in fundamental and engineering science. The shell composition initially deduced from naturally occurring microbubbles and recently innovated by methods of rational design, can be engineered to serve an assortment of functions. Our results obtained from electrochemical, optical and fluorescence microscopy on lipid monolayer-coated microbubbles and model Langmuir monolayers demonstrate the relationship between composition, microstructure and transport properties of the lipid shell and provide insight into microbubble stability. We show that the monolayer shell reduces surface tension and impedes gas transport. The stabilization mechanism is determined by the phase state of the monolayer acyl chain region. Surface tension is maintained at the equilibrium value for expanded state lipids and completely diminished for condensed phase lipids. The shell resistance to gas permeation is only significant for rigid-monolayer forming lipids and increases monotonically with acyl chain length. Shedding of excess shell material during dissolution occurs in a quasi-continuous manner for soft-monolayer forming lipids. In contrast, shells composed of rigid-monolayer forming lipids exhibits crumpling and shedding in distinct cycles. We propose a qualitative mechanism involving zippering of apposing monolayer portions at a critical point. The emulsifier partitions into the fluid phase and, in the case of a single hydrophobic chain, squeezes out at surface pressures below that required for lipid shedding. The size and shape of the crystalline lipid domains are controlled by lipid hydrophobic chain length and quench rate; a rich shell microstructure is observed. |
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Symposium in Memory of Arthur W. Adamson
2:00 PM-4:40 PM, Thursday, April 1, 2004 Marriott -- Orange County 4, Oral
Division of Colloid and Surface Chemistry |