PREVENTION OF MODEL STRATUM-CORNEUM LIPID PHASE-TRANSITIONS INVITRO BY COSMETIC ADDITIVES - DIFFERENTIAL SCANNING CALORIMETRY, OPTICAL MICROSCOPY, AND WATER EVAPORATION STUDIES

Two potential moisturizers (maleated soybean oil, identified here as G lyceridacid, and glycerol) were compared for their effects on the liqu id crystalline phase of model stratum corneum lipids at 6% and 92% rel ative humidities (RH) using differential scanning calorimetry (DSC), w ater evaporation, and polarized light microscopy. DSC of the model lip id (32% water content) showed a broad endothermic transition at about 52-degrees-C, with an enthalpy of 15 J/g lipid. The enthalpy of the tr ansition and water loss from the model lipid were time-dependent; at 6 % RH, the enthalpy increased to 43 J/g lipid, with 29.5% water loss by 24 h. This suggests significant crystallization of the model lipid on dehydration, which was confirmed by microscopy. At 92% RH, both the c hanges in enthalpy and water loss were significantly reduced by 24 h, indicating maintenance of the liquid crystalline phase of the model li pid with very little crystallization. Incorporation of Glyceridacid (5 -15 wt%) into the model lipid produced enthalpies between 17-21 J/g li pid after 24 h at 6% PH, with reduced water loss (20-26%). Therefore, Glyceridacid can function as a skin conditioner at low humidity by mai ntaining the liquid crystalline phase of the model lipid in addition t o preventing water loss. Glycerol (10 wt%), a known skin conditioner i n vivo, inhibited the crystallization of the model lipid, despite almo st complete dehydration: enthalpy is 30 J/g lipid after 24 h with few solid crystals present. This suggests an alternative mechanism to hume ctancy for the moisturizing action of glycerol at low humidity, in agr eement with our earlier investigation (4) of the model lipid/glycerol system by microscopy. The results clearly show the potential of DSC as a quantitative tool to monitor the phase behavior of the model lipid and its alteration by potential moisturizers, under different humidity conditions.