If lecithin is suspended in hot water, it forms a monolayer on
the surface that is similar to that found with soaps and long-chain
alcohols. If a loop of wire or a thin metal plate with a hole is
pushed down through this film, a lipid bilayer will be formed within
the loop or hole, as diagrammed on the right. In the original surface
film, the polar heads of the molecules were underneath, in contact
with water. The hydrocarbon tails were above, exposed to the air.
The lecithin bilayer formed at the opening in the plate will have
the structure shown right, with polar heads exposed on either face
of the bilayer and hydrophobic chains meeting at the center. Such
a bilayer made with chains the length of stearic acid will be 70
Å thick, similar to biological membranes. Lecithin bilayers
are like soap films turned inside out. Both are bilayers of half
hydrophobic and half polar molecules.
Soap films, in contact with air on either side, have their hydrophobic
chains exposed and their polar heads immersed in a layer of water
in the center of the film. Lipid bilayers in aqueous solution have
their molecules in the reversed position-hydrophobic portion in
the center and polar heads out.