Monolayers of the phospholipids 1,2-dipalmitoylphosphatidylethanolamine (DPPE) and 1,2-dipalmitoylphosphatidylcholine (DPPC) are studied at the water/air interface in contact with n-hexadecane, n-dodecane, and bicyclohexyl by null ellipsometry and X-ray reflectivity along the isotherms. From recent diffraction data we derive refractive indices nx and nz to calculate the thickness of the mixed films in the condensed phase by ellipsometry. The thickness is compared with the results from X-ray reflectivity measurements. Both methods reveal that the mixed lipid/hydrocarbon film is not covered by a thin hydrocarbon film as known from short chain alcanes like hexane. The incorporation of the hydrocarbon affects the film thickness by reducing the tilt angle of the lipids. This way DPPC, which due to the large head group does not exhibit a non tilted condensed phase at the water/air interface at all, enlarges its value for nz at the expense of nx , yielding a condensed mixed phase with zero tilt angle. The resulting value for the thickness of the tail region from both methods is always smaller than 21 Å, showing that penetration is strictly limited to the monolayer region.

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Insoluble monolayers of hydrophobic polymers with low glass transition temperature (perfluoropolyethers, polyisoprenes) and a single ionic head group (carboxylic acid, sulfonate) have been characterized at the air/water interface via X-ray reflectivity measurements. The films are considerably thicker (30−420 Å) than conventional Langmuir monolayers of low molecular weight substances or polymers with surface active repeat units. The thickness is inversely proportional to the area per head group and is in accordance with a model assuming a solvent-free hydrophobic layer of the same density as the bulk material.

We report on X-ray reflectivity and diffraction studies using monolayers of phospholipids and of the protein streptavidin specifically bound to monolayers. For phospholipids with the phosphocholine head group attached to the glycerol backbone via (flexible) ethylene oxide spacers, we demonstrate that the lateral interactions can be reduced by increasing of the spacer length. This is reflected in a reduction of the tilt angle of aliphatic tails. Diffraction of the protein layer can be observed in situ. The data reveal that positional order is merely short-ranged and that the structure can be changed reversibly via film compression and expansion.