The Stratum Corneum (SC) is the outer layer of the skin, and can be described as a mortarbrick structure, the bricks being the keratinized cells (corneocytes) and the mortar a complex assembly of lipids, with a dominantly lamellar structure. Ceramides (CERs) form the major two-tailed component of the SC lipid matrix. CER molecules contain a sphingosine tail (S-chain) linked to a fatty acid tail (N-chain) via an amide bond. The abundance of saturated long alkyl tails and the lack of polarizable head groups in SC lipids endow them with very different properties from those found in cell membranes. As the skin is renewed, the outermost surface becomes eventually that of corneocytes that bear a covalently bonded, single monolayer of CER, with possibly addition of fatty acids or cholesterol. It is this outermost layer that will support the hydrolipidic film (sweat+sebum), or will be exposed to cosmetic treatments.

The aim of this project is to gain a better understanding of the structure of this monolayer, in relation to its composition and surface density, and the evolution of this structure under exposition to various agents. The project combines soft matter techniques for sample preparation and characterization, neutron reflectometry techniques for analyzing in detail the structure, complemented by molecular modeling using coarse grained force fields. While the project will concentrate on model systems using a few CER, some aspects of these models will be confronted to realistic samples at the L’Oréal research facility.

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