STRUCTURE OF STRATUM-CORNEUM LIPIDS CHARACTERIZED BY FT-RAMAN SPECTROSCOPY AND DSC .2. MIXTURES OF CERAMIDES AND SATURATED FATTY-ACIDS

Fourier transform (FT) Raman spectroscopy and differential scanning ca lorimetry (DSC) were used to study the thermotropic phase behaviour of mixtures of ceramides type IV (CER) and stearic acid (SA). For compar ison the melting behaviour of SA was re-examined. The Raman spectra of all mixtures in the solid state show sharp bands associated with tr a ns sequencies of the alkyl chain residues of both lipids. These featur es demonstrate that the hydrocarbon chains are highly ordered in the m ixtures, too. The temperature dependence of the conformationally sensi tive bands is used to estimate the degree of order in terms of the rel ative population of trans and gauche conformations. The DSC heating cu rves for the mixtures show two endothermic transitions which are typic al for eutectic melting. The factor group splitting of the CH2 scissor ing mode: arising from the orthorhombic subcell packing of SA, disappe ars in the course of the eutectic melting of samples with a SA content lower than 90 mol%. Both DSC and Raman spectroscopic studies reveal t hat CER and SA are immiscible in the solid state. The phase diagram of the system is a simple eutectic type one. The addition of SA to CER s hifts the melting temperature of ceramides to lower values. However, t hough SA is a major component of stratum corneum (SC) it is not effici ent enough to increase the fluidity of ceramides. (C) 1997 Elsevier Sc ience Ireland Ltd.