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The structure and hydration of a stratum corneum (SC) lipid model membrane composed of N-(alpha-hydroxyoctadecanoyl)-phytosphingosine (CER6)/cholesterol (Ch)/palmitic acid (PA)/cholesterol sulfate (ChS) were characterized by neutron diffraction. The neutron scattering length density across the SC lipid model membrane was calculated from measured diffraction peak intensities. The internal membrane structure and water distribution function across the bilayer were determined. The low hydration of the intermembrane space is a major feature of the SC lipid model membrane. The thickness of the water layer in the SC lipid model membrane is about 1 A at full hydration. For the composition 55% CER6/25% Ch/15% PA/5% ChS, in a partly dehydrated state (60% humidity) and at 32 degrees C, the lamellar repeat distance and the membrane thickness have the same value of 45.6 A . The hydrophobic region of the membrane has a thickness of 31.2 A . A decrease of the Ch content increases the membrane thickness. The water diffusion through the SC lipid model multilamellar membrane is a considerably slow process relative to that through phospholipid membranes. In excess water, the membrane hydration follows an exponential law with two characteristic times of 93 and 44 min. At 81 degrees C and 97% humidity, the membrane separates into two phases with repeat distances of 45.8 and 40.5 A . Possible conformations of CER6 molecules in the dry and hydrated multilayers are discussed.

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The lipid matrix of the stratum corneum (SC) is the major diffusion-rate-limiting pathway by which most drugs intracellularly pass the SC. The major lipid classes extracted from the SC are ceramides, cholesterol and free fatty acids. Ceramides that comprise nine subclasses play a crucial role in maintaining the barrier function of the skin. A profound knowledge of the physical properties of ceramides is essential for a deeper understanding of the impact of each ceramide species on the barrier function. The review summarizes the thermotropic and/or lyotropic behaviour of sphingosine-type ceramides (CER AS, CER NS) and phytosphingosine-type ceramides (CER AP, CER NP) revealed by differential scanning calorimetry, X-ray diffraction, Fourier transform infrared spectroscopy and Fourier transform Raman spectroscopy in past decades. Polymorphism is a characteristic feature of ceramides. At physiological temperatures, all crystalline phases of ceramides exhibit lamellar structures with highly ordered hydrocarbon chains. The differential behaviour of the head groups of ceramides may be an important determinant for the skin barrier function.