THERMODYNAMIC AND STRUCTURAL ASPECTS OF THE SKIN BARRIER

The effect of water and propylene glycol (PG) on the physical-chemical properties of the human stratum corneum has been studied with small angle X-ray scattering (SAXS) and differential thermal analysis (DTA). Hydration (already published by Knutsen et al. [1] or pretreatment of the stratum corneum with PG resulted in a shift of two gel-liquid transitions of the lipid bilayers to a lower temperature region indicating that water and PG interacts with the stratum corneum lipids. From SAXS experiments it appeared that the repeat distance between the lamellae found in untreated human stratum corneum is only 6.5 nm. This distance remained unchanged upon water and PG pretreatment suggesting that no swelling of the lipid bilayers occurred. This indicates the neither water nor PG is intercalated into the bilayers. The apparent contradictory results obtained with DTA and SAXS can be explained as follows. It is possible that incorporation of water or PG in the head group regions of the lipids results in an increase in the mean interfacial area per lipid without changing the repeat distance. The interaction of N-alkyl-azocycloheptane-2-one (C(n) zone, azone derivatives in which n is the number of carbon atoms in the alkyl chain) in combination with PG with human stratum corneum has also been studied using the two techniques mentioned above. The results of DTA experiments indicate that azone derivatives possessing a longer alkyl chain have stronger interactions with the stratum corneum lipid bilayers than shorter alkyl chain azone derivatives do. The former ones probably induce pronounced changes in the bilayer arrangement of stratum corneum lipids, since SAXS experiments showed that C-12 azone in combination with PG resulted in a disappearance of all diffraction peaks.