96 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS • I ' I • I • I J 0 2o 4o 6o 8o lOO Temperature (o½) Figure 4. Differential scanning calorimetry of model lipid plus 15% Glyceridacid incubated for 0 h (b), 6 h (c), 24 h (d), and 48 h (e) at 6% RH. Model lipid incubated for 0 h (a) included for comparison. liquid crystalline phase is clearly present. After 24 h, Glyceridacid samples (5% and 10%) exhibited co-existing liquid crystalline and crystalline phases, with a greater proportion of crystalline characteristics. This contrasts with much greater crystallization of the model lipid alone after 24 h (compare Figure 3a with Figures 3b and 3c). The 15% Glyceridacid sample showed more liquid crystalline character, along with finer crystals (Figure 3d). By 48 h, the samples containing 5 and 10% Glyceridacid exhibited many small crystals against a liquid crystal background, while the 15% Glyceridacid sample exhibited a roughly half-liquid/half-solid, crystal material (data not shown). Glyceridacid, at 5%, 10%, and 15% of the model lipid, is capable of preventing solid crystallization of the model lipid that normally accompanies water loss at low humidity. This is in excellent agreement with the quantitative results obtained by DSC above. Glyceridacid is able to maintain the liquid crystalline state of the model lipid at low humidities by incorporating into the bilayer structure this is as a result of its similarity to other lipids of the model lipid system in having a hydrophobic (fatty acyl chain) and a hydrophilic (glycerol headgroup) moiety. The bulky cyclohexene ring containing trans carboxylic acid of Glyceridacid will reduce lateral chain-packing and Van der Waals interaction within the bilayer, and this will increase lipid fluidity or reduce lipid crystallization. Therefore, lower transition enthalpies will be observed. Glyceridacid also reduced water loss from the model lipid. Water acts by hydrating the polar head groups of the lipids, thus increasing their size this affects the hydrocarbon portion of the bilayer by reducing their lateral interaction and thus increasing their fluidity (8).

STRATUM CORNEUM LIPIDS 97 Imokawa et al. (15,16) have shown that stratum corneum lipids are important in the water-holding function of the stratum corneum. In addition, a number of pseudocer- amides have been synthesized by Imokawa et al. (17) topical application of these pseudoceramides significantly improved dry skin conditions, and this was accompanied by recovery in the water content of the stratum corneum. Based on our in vitro studies, Glyceridacid is also predicted to behave like the pseudoceramides in improving dry skin and recovering the water content of the stratum corneum. Photomicrographs of the samples were also taken at 100X after 6, 24, and 48 h of exposure at 92% RH (Figures 5b, 5c, and 5d for the 24-h exposure). Crystallization of the lipid is far less dramatic than that observed at very low humidity. These photographs reveal largely liquid crystalline structures, even for the control material. One unusual feature occurred in both the 10% Glyceridacid and 15% Glyceridacid samples after 48 h at 92% RH: flat, plate-like crystals with sharp right-angle contours appear mingled with the liquid crystalline phase. These crystals are unlike the fine, needle-shaped solid crystals that precipitated from the model upon loss of water. Fur- thermore, the crystals are more numerous in the 15% Glyceridacid sample than in the 10% Glyceridacid sample. This dose-dependence would indicate that Glyceridacid, either alone or in combination with one or more components of the model, separates from the gross structure. This formation was not apparent in samples exposed to low humidity perhaps the simple crystallization of lipid upon dehydration is so rapid as to pre-empt the slow formation of this unidentified crystal form. These phenomena are under further investigation to define the compositions of the different solid crystals. STUDIES WITH MODEL LIPID CONTAINING GLYCEROL The DSC curve for the model lipid plus 5 % glycerol (Figure 6b) is similar to that of the model lipid (Figure 6a) and was slightly changed after 6 h at 6% RH (Figure 6c) however, further incubation for 24 and 48 h induced the formation of a higher- temperature phase at a maximum of 70-75øC (Figures 6d and 6e). While we do not know the nature of this phase, we speculate that it is due to lateral phase separation of lipids induced by glycerol. Because of the broadness of the transitions, deconvolution of Figure 5. Model lipid with 0% (a), 5% (b), 10% (c), and 15% (d) Glyceridacid RH, viewed under polarized light at 100X magnification. (d) after 24-h exposure at 92%