60 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 6h 24 96 hr. Figure 6. Model lipid with 0% glycerol (A) and 10% glycerol (B), after 6 hours, 24 hours, and 96 hours of exposure to 6% RH, viewed under polarized light at 400 X magnification (for the 6-hr timepoint) or 100 X magnification (for the 24-hr and 96-hr timepoints). environmentally induced changes in the skin. Because such responses are rapid, they probably involve physical changes in the stratum corneum structure as opposed to bio- chemical changes in cell growth and differentiation or lipid metabolism. One of our objectives was to understand the structural basis for the moisture barrier of the stratum corneum. Because the intercellular lipid of stratum corneum is strongly implicated in hydration of the skin, we have adopted an in vitro model of stratum corneum lipid to study water binding in skin and the concomitant structural changes. We have found that the mixture of partially neutralized fatty acid and neutral lipids, with 33% water, forms a structure with considerable liquid crystalline character. Al- though polarized light microscopy did not reveal solid crystals in freshly prepared

GLYCEROL EFFECT ON STRATUM CORNEUM LIPID 61 B 96 hr. Figure 7. Model lipid with 0% glycerol (A) and 10% glycerol (B) after 6 hours, 24 hours, and 96 hours of exposure to 92% RH, viewed under polarized light at 400 x magnification (for the 6-hr and 24-hr time- points) or 100 x magnification (for the 96-hr timepoint). model lipid, this does not prove the absence of a solid phase. Very find solid crystals dispersed in a liquid crystal are not detectable using this technique. Friberg (unpub- lished data) has demonstrated that the model lipid is a mixture of solid crystal and liquid crystalline phases. We have studied the effect of incorporating glycerol into the stratum corneum model lipid mixture. Glycerol is a well-known humectant. Incorporation of 5-15% glycerol (by weight) into the model promotes the uptake of water under conditions of high humidity. The extent of humectant action of glycerol within the lipid matrix in high humidity is consistent with the behavior of neat glycerol. Neat glycerol absorbed its own weight in water over three days. The model lipid containing 10% glycerol ab-