6533b7defe1ef96bd1276649

RESEARCH PRODUCT

An X-ray scattering study of lipid monolayers at the air-water interface and on solid supports

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Abstract Monolayers of the lipid arachidic acid (C20) and of the phospholid dimyristolyphosphatidic acid (PMDA) have been studied by X-ray reflection and diffraction technique, using a purpose-built Langmuir trough installed at the sample stage of our high-resolution X-ray diffractometer at the DORIS synchroton X-ray source in Hamburg. For comparison we also report data for monolayers of C20 on a solid support using a 10 kW rotating anode X-ray source. By the X-ray reflection method, the density profile across the interface is probe, while in-plane diffraction measurements gauge the two-dimensional crystalline properties of the monolayers. Flourescence microscopy experiments of DMPA monolayers idicates coexistence of a “fluid” and the denser “gel” phase between surface pressures πc and πc where the π-A isotherm changes slope. X-ray data show that the positional correlation length increase from less than 10 to about 100 lattice spacing on increasing the pressure beyond πs.X-ray reflectivity data indicate 40% increase in monolayer thickness when the surface pressure is increased from below πc to above πs. For C20 monolayers, the reflectivity measurements indicates similar monolyer thickness on water surfaces and on solid supports. On water, the correlation length was 22 lattice spacing while, on thermally oxidized silicon ξ≈ 8 lattice spacings.