Abstract
The monolayer structure of l-α-dipalmitoyl phosphatidylcholine (DPPC) at the air/water interface was examined using improved electron microscopic techniques. When the experimental conditions of the Langmuir-Blodgett monolayer transfer and deposited film stability are carefully optimized, a monolayer structure is observed which is homogeneous in the liquid-expanded (LE) state, nonhomogeneous and biphasic in the intermediate region and, once again, homogeneous in the liquid-condensed (LC) state. Our electron microscopic results are compared with those of Ries, Matsumoto, Uyeda, and Suito [ Adv. Chem. Ser. 144, 286 (1975)] and Fischer and Sackmann [ J. Physique 45, 517 (1984)], and we discuss why their experimental techniques yield film structures which are artifacts arising from nonquantitative film transfer or film stability problems after deposition or both. Our finding of two coexisting phases is consistent with the interpretation of a first-order phase transition occurring between the LE and LC states in DPPC monomolecular films.
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