94 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Table III Percent Water Loss From Model Lipid, Model Lipid plus 5%, 10%, and 15% Glyceridacid, and Model Lipid Plus 5% and 10% Glycerol, Incubated for Different Periods of Time at 6% and 92% Relative Humidities % Water loss Model lipid + Model lipid + Glyceridacid glycerol Time Model lipid 5% 10% 15% 5% 10% 6 h at 6% RH 18.2 + 0.1 11.3 10.7 11.7 17.9 19.0 24 h at 6% RH 28.6 + 0.5 25.8 19.9 20.8 29.0 27.8 48 h at 6% RH 29.5 + 1.1 27.7 25.7 23.8 30.5 27.9 6 h at 92% RH 13.3 + 0.5 6.3 6.0 6.1 8.9 1.3 24 h at 92% RH 21.6 + 0.2 13.9 10.7 11.3 11.8 + 1.8 48 h at 92% RH 24.9 + 0.4 17.8 15.0 15.4 11.2 + 1.5 loss due to water, the model lipid is maintained to a large degree in the liquid crystalline state at 92% RH, with minor changes in hydrocarbon chain-packing in the bilayer. Previous studies on the phase behavior of phospholipids show transition enthalpies and temperatures to increase rapidly with dehydration, attaining maximum values at very low hydration, as the lipid molecules become fully dehydrated (8). At 25% water loss, corresponding to 7% hydration, the model lipids are apparently sufficiently hydrated to prevent crystallization, with its concomitant large increase in enthalpy. These results are confirmed by polarized light microscopy. Figures 2a and 3a show photomicrographs of the model lipid at 100 x magnification at the initial time and after 24 h at 6% RH. At the initial time (Figure 2a), the pattern is characteristic of a liquid crystal. After 24 h (Figure 3a), significant crystal formation has occurred, with a solid-crystal network of large plate-like crystals after 48 h (data not shown). At 92% RH, crystallization of the model lipid is far less dramatic than that observed at 6% RH the photomicrographs reveal largely liquid crystalline structures even after 48 h incu- bation (data not shown). This is to be expected, as the water loss from the model lipid (a) (b) Figure 2. Model lipid with 0% (a), 5% (b), 10% (c), and 15% (d) Glyceridacid viewed under polarized light at 100X magnification, at the initial time.

STRATUM CORNEUM LIPIDS 95 Figure 3. Model lipid with 0% (a), 5% (b), 10% (c), and 15% (d) Glyceridacid after 24-h exposure to 6% RH, viewed under polarized light at 100 X magnification. is suppressed at high humidities. The liquid crystal state, which is highly dependent on water content, is therefore maintained. STUDIES WITH MODEL LIPID CONTAINING GLYCERIDACID Incorporation of 5-15% Glyceridacid into the hydrated model lipid resulted in a re- duction in intensity of the DSC transition, with a shift to lower transition temperature (Figure 4b) after equilibration for 6 h at 6% RH, the original shape of the transition was recovered [compare Figures 4a (model lipid) and 4c (model lipid plus Glycerid- acid)], with subsequent sharpening of the transition on further incubation for 24 and 48 h (Figures 4d and 4e). However, the transition enthalpies were considerably reduced compared to the model lipid (Table II) the values ranged from 8 J/g lipid at 0 h to 23 J/g (lipid + Glyceridacid) after 48 h at 6% RH for the 15% Glyceridacid sample. Clearly, at 6% RH, Glyceridacid prevented the hydrocarbon chains from crystallizing and thus maintained the liquid crystalline phase even at low hydration, a behavior similar to that observed with glycerol (4). In addition, the percent water loss at both low and high RH values (Table III) is significantly lower compared to the model lipid for all incubation times. Comparable enthalpy and water loss data were obtained for the 10% Glyceridacid sample. At 5% concentration of Glyceridacid, the enthalpy of the transition was 21 and 35 J/g (lipid + Glyceridacid) after 24 and 48 h, respectively, with some increase in water loss. There is some crystallization of the model lipid after 48 h at 6% RH, although the liquid crystalline state is largely maintained. Therefore, in addition to maintaining the liquid crystalline state of the model lipid, Glyceridacid is also able to reduce water loss. Thus Glyceridacid is able to maintain the lameliar liquid crystalline state by two mechanisms. Note the DSC curves of the model lipid at 0 h, prepared at different times, show minor variation in the shape of the transition (compare Figures la and 4a) however, the enthalpy of the transition is not affected. The above results are in good agreement with optical microscopy data. Figures 2b, 2c, and 2d and Figures 3b, 3c, and 3d show photomicrographs of the model lipid with 5 %, 10%, and 15% Glyceridacid after 0 and 24 h, respectively, at 6% RH. At 0 time, the