98 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS • (a) i I i I I I I I t 0 20 40 60 80 100 Temperature (øC) Figure 6. Differential scanning calorimetry of model lipid plus 5% glycerol 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. the separate phases was not possible. For these incubation times, the enthalpy of the main lipid transition was obtained by extrapolating the y-axis to intercept the temper- ature axis, cutting out the area for the transition, weighing this area, and comparing it with the weight of the total area, whose enthalpy is known. The enthalpy of the model lipid plus 5% glycerol at 6% RH after 6, 24, and 48 h is comparable to that of the model lipid without glycerol (Table II). Therefore, the liquid crystalline phase is not maintained at 5% glycerol concentration under low humidity conditions. Addition of 10% glycerol did not induce the higher temperature phase (at 70-75øC) even after 48 h at 6% RH (data not shown), but this phase was observed after 48 h at 92% RH. However, the liquid crystalline phase is partially maintained after 24 and 48 h at 6% RH (Table II), as indicated by the lower enthalpy of the model lipid plus glycerol compared to that of the control. At 92% RH, both 5% and 10% glycerol maintained the liquid crystalline phase of the model lipid, as observed from the enthalpy changes in Table II. Water loss from the model lipid was not affected by 5% or 10% glycerol at 6% RH (Table III) and was also noted previously (4). At 92% RH, 5% glycerol significantly reduced water evaporation from the model lipid, while humectancy of glycerol was observed at 10% concentration after 24 and 48 h (Table III) and was also observed previously (4). These quantitative results are in good agreement with our recent study of the model lipid/glycerol system by optical microscopy (4) glycerol prevented crystallization of the model lipid at low humidity without preventing water loss. Therefore, glycerol, at 10%

STRATUM CORNEUM LIPIDS 99 concentration or greater, functions as a skin conditioner at low humidities by this mechanism, while at high humidities, it acts as a humectant. CONCLUSIONS In the present investigation, we showed that DSC can be used as a quantitative tool to measure the changes in transition enthalpies of model stratum corneum lipids as the liquid crystalline phase is dehydrated under low-humidity conditions. DSC can also quantitatively measure the changes in enthalpy induced by dehydration or addition of a potential cosmetic ingredient. DSC results suggest that Glyceridacid can be considered a potential moisturizer because it maintained the liquid crystalline state of the model lipid and prevented water evap- oration the enthalpy values showed small changes with time at low RH, and water loss was reduced compared to the model lipid alone. The results are consistent with previous observation on the skin-conditioning benefit of Glyceridacid (10). Glycerol is also considered a potential moisturizer at 10% concentration and at low humidity, since it maintained the liquid crystalline phase of the model lipid (although water loss was not prevented). This suggests an alternative mechanism for the action of glycerol at low humidity. The results are in agreement with previous findings of the model lipid/ glycerol system, observed by polarized light microscopy (4). The ability of additives to maintain the liquid crystalline phase was dose-dependent. Glyceridacid was more ef- fective at lower doses than glycerol. The results also showed at least two mechanisms by which additives can prevent crystallization of lipids: preventing water loss and inter- acting with the bilayer structure, thus fluidizing the lipid chains. While Glyceridacid is proposed to act by both mechanisms, glycerol functions mainly via the latter mech- anism. In our future research, we will utilize the DSC technique, together with water evapo- ration and optical microscopy studies, to examine the effects of a range of moisturizers/ humectants on the phase behavior of model stratum corneum lipids. We will then determine the relevance of the findings to in vivo skin moisturization. REFERENCES (1) P.M. Elias, B. E. Brown, P. Fritsch, J. Goerke, G. M. Gray, and R. J. White, Localization and composition of lipids in neonatal mouse stratum granulosum and stratum corneum, J. Invest. Der- matol., 73, 339-348 (1979). (2) S. E. Friberg and D. W. Osborne, Small angle X-ray diffraction patterns of stratum corneum and a model structure for its lipids, J. Disp. Sci. Technol., 6(4), 485-495 (1985). (3) S. E. Friberg, I. Kayali, and L. D. Rhein, Direct role of linoleic acid in barrier function: Effect of linoleic acid on the crystalline structure of oleic acid/oleate model stratum corneum lipid, J. Disp. Sci. Technol., 11(1), 31-47 (1990). (4) C. L. Froebe, F. A. Simion, H. Ohlmeyer, L. D. Rhein, J. Mattai, R. H. Cagan, and S. E. Friberg, Prevention of stratum corneum lipid phase transitions in vitro by glycerol--An alternative mechanism for skin moisturization, J. Soc. Costnet. Chem. 41, 51-65 (1990). (5) B. F. Van Duzee, Thermal analysis of human stratum corneum, J. Invest. Dermatol., 65(4), 404-408 (1975). (6) G. M. Golden, D. B. Guzek, R. R. Harris, J. E. McKie, and R. O. Potts, Lipid thermotropic transitions in human stratum corneum, J. Invest. Dermatol., 86(3), 255-259 (1986).