Mechanical Properties and Microstructure of Polycrystalline Phospholipid Monolayer Shells:  Novel Solid Microparticles

Abstract

W This paper contains enhanced objects available on the Internet at http://pubs.acs.org/ journals/langd5.A polycrystalline phospholipid monolayer self-assembled at the surface of an air microbubble in aqueous solution represents a novel material structure: in essence, a solid shell of wax with micrometer-scale dimensions and a thickness of only a single molecule. Micropipet manipulation of these microparticles revealed the dependence of the mechanical properties of the lipid shells, specifically, yield shear and shear viscosity, on the composition, grain microstructure, and thermal processing of the material, in particular the cooling rate of the shells from the melt. Properties were measured as a function of the (1) lipid composition at a fixed cooling rate and (2) cooling rate at a fixed lipid composition. Epifluorescent microscopy and transmission electron microscopy revealed that the morphology of the 1,2-distearoyl-sn-glycero-3-phosphatidylcholine monolayer microstructure, which develops upon freezing from the melt, is dependent on the cooling rate through the lipid transition temperature Tm, with larger micrograins being formed at slower cooling rates. Mechanical properties of the lipid shell follow micrograin size, with the coarse grain structure exhibiting a higher resistance to shear deformation than the fine grain structure does, which is behavior consistent with that of more traditional bulk crystalline materials. IntroductionThe same principles of self-assembly that describe the formation and structure of a lipid monolayer on a flat trough of water also apply to the spontaneous organization of amphiphilic material at a curved gas-liquid interface such as that posed by the surface of an air microbubble in water. 1-3 It is well-known that at sufficient concentrations the surface contaminants on a "dirty" bubble in water can effectively immobilize the air-water interface and cause the bubble to behave as a rigid particle. A relatively recent addition to the family of experimental techniques for studying monolayer properties known as axisymmetric drop shape analysis (ADSA) was employed by Kwok and co-workers as a film balance, albeit on a curved droplet surface, to obtain surface pressure-surface area (Π-A) isotherms for monolayer films of 1-octadecanol. 4,5 ADSA experiments have demonstrated the equivalence of curved monolayers and flat monolayers with respect to surface phase states and transitions. Until recently, however, the direct measurement of the mechanical properties of monolayers spread at micrometer-scale curved interfaces has eluded investigators; furthermore, extant work on monolayers, whether on flat or curved interfaces, has been mostly confined to the liquid-expanded or liquid-condensed-expanded coexistence regimes of their phase diagrams. The current study presents the first mechanical property measurements made for solid lipid monolayer shells of well-defined composition and micrograin structure formed on the surfaces of gas microbubbles, thus essentially comprising gas microparticles s...

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