Phospholipids will spontaneously aggregate and form extended monolayer at the interface between oil and a saline solution. They will behave similarly at the air surface of any aqueous solution; here the hydrocarbon tails form a very thin oil film and the polar heads project into the solution. Given this behavior it's not hard to imagine how a stable phospholipid bilayer might be formed within an aqueous solution from two monolayers laid "back-to-back". The hydrocarbon tails from each monolayer would touch, forming the hydrophobic core of the bilayer. The polar heads would interact with each other and with the adjacent solution and their interactions would exclude the tails and help stabilize the core. Do such bilayers exist?
If an aqueous suspension of amphipathic lipids in high concentration is aggitated, small vesicles called liposomes are formed. If these vesicles are then dried and examined by high resolution x-ray crystallography, a highly symmetrical, repeated pattern is obtained consistent with the presence of many lipid bilayers. The deduced cystalline structure of a segment of one bilayer is depicted on the opposite page, and it conforms to our expectations about how two monolayers might form a stable association. The picture certainly resembles many text book figures of a lipid bilayer, but its significance largely depends on how similar dried liposomes are to an aqueous suspension.
Study the crystalline structure and identify the components of the bilayer using the scrolling legend beneath the picture. Then turn the page to view a more reasonable bilayer structure.