AFM study of lipid monolayers: III. Phase behavior of ceramides, cholesterol and fatty acids

The outer part of the skin, stratum corneum, is essential to the skin's bar rier function. Monolayer and bulk phase behavior of stratum corneum model l ipids have thus been studied. Domain formation in Langmuir-Blodgett monolay ers of synthetic ceramides (C16CerIII and C24CerIII), cholesterol, and free fatty acids (Lignoceric acid, C24:0, and palmitic acid, C16:0) were invest igated by atomic force microscopy. Binary, ternary, and more complex lipid mixtures were examined. It was shown that small amounts of ceramide are mis cible in a cholesterol-rich phase with the miscibility dependent upon the c eramide chain length. Two phases are formed at low and intermediate cholest erol concentrations. In the ceramide-cholesterol monolayers, very small rec tangular-shaped ceramide domains thought to be two-dimensional single ceram ide crystals are formed. Small domains were also found in more complex mixt ures where the fatty acid is miscible in the ceramide phase, although these domains were not as regular in shape. Binary ceramide-cholesterol as well. as ternary ceramide-cholesterol-lignoceric acid bulk mixtures were also st udied by X-ray diffraction. The bulk Lipid miscibility is consistent with t he monolayer results.