STRUCTURE OF STRATUM-CORNEUM LIPIDS CHARACTERIZED BY FT-RAMAN SPECTROSCOPY AND DSC - IV - MIXTURES OF CERAMIDES AND OLEIC-ACID

The thermotropic and lyotropic phase behaviour of mixtures of ceramide s type IV (CER) and oleic acid (OA) was investigated using Fourier tra nsform (FT) Raman spectroscopy and differential scanning calorimetry ( DSC). The Raman spectra of the mixtures in the solid state exhibit sha rp bands associated With trans sequences of the alkyl chain residues o f both lipids, indicating highly ordered chains. The DSC heating curve s for the dry and the fully hydrated mixtures show three endothermal t ransitions. The events at T = -5.5 and 12.3 degrees C do not depend on the composition of the mixtures. As in the case of pure OA, they are associated with the solid-solid transition of OA from gamma to alpha m odification and the melting of the alpha phase, respectively. The temp erature of the third event at higher temperatures decreases continuous ly with increasing OA content. This transition involves the melting of CER. Both DSC and Raman spectroscopic studies reveal that CER and OA are immiscible in the solid state. The phase diagram is a monotectic t ype one. Calculations of the melting curve of CER support this interpr etation. Full hydration of the system causes a decrease of the tempera ture of the upper transition, but, the general feature of the phase di agram remains. (C) 1998 Elsevier Science Ireland Ltd. All rights reser ved.