Influence of the lipid composition on the organization of skin lipid model mixtures: an infrared spectroscopy investigation.

The polymorphism of the lipids of the stratum corneum (SC), the top layer of the epidermis, has a fundamental impact on the permeability properties of the skin barrier. In this work, we have examined by infrared spectroscopy the thermal behavior of model mixtures involving ceramide, palmitic acid and cholesterol, the three main components of the SC lipids, to gain a refined description of the participation of the various lipid species in the different phases observed as a function of temperature. The results show that below 40 degrees C ceramide, cholesterol and palmitic acid exist mainly in crystalline domains and the lipidic species show very limited miscibility. Between 40 and 50 degrees C, a transition from the crystalline to a liquid ordered (lo) phase occurs and it involves ceramides, cholesterol and palmitic acid. When the mixture has a high cholesterol content, this lo phase is stable up to 75 degrees C. For low cholesterol content, the mixtures undergo a second transition toward a more disordered phase which is likely not lamellar. The formation of these phases is critically dependent on the lipid composition and, therefore, it is likely that composition changes of SC lipids affect the phase behavior and, consequently, the skin barrier properties.