Phospholipid foam films studied by contact angle measurements and fluorescence microscopy

Foam films drawn from suspensions of the phospholipid 1,2-dimyristoyl-sn-gl ycero-3-phosphorylcholine (DMPC) in water/ethanol mixtures were used for th e investigation of the relation between the properties of the monolayers an d the interaction between the film surfaces. The film thickness and the con tact angle between the film and the meniscus were measured as a function of the temperature in a range around the temperature of the main phase transi tion for the lipid. Additionally, fluorescence microscopy was applied to in vestigate the distribution of a fluorescent lipidlike dye in the surface of the film and the meniscus. From the contact angle the free energy of film formation was calculated. At the temperature of the chain-melting phase tra nsition the film thickness decreases by 0.7 nm. This can be related to a de crease in the thickness of the hydrocarbon layers of the lipid monolayers a t this temperature. The decrease in the film thickness leads to a reduction in the free energy by increasing the van der Waals attraction between the film surfaces. No structures were observed in the monolayers of the film in the fluorescence investigation. However, on formation of the very thin equ ilibrium film the dye was expelled from the film area, indicating an increa se in the packing density of the lipid, if the monolayers are in adhesive c ontact in the film.