Prevention of model stratum corneum lipid phase transitions in vitro by cosmetic additives : differential scanning calorimetry, optical microscopy, and water evaporation studies

DSC of the model lipid (32% water content) showed a broad endothermic transition at about 52oC, with an enthalpy of 15 J/g lipid. The enthalpy of the transition 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 changes in enthalpy and water loss were significantly reduced by 24 h, indicating maintenance of the liquid crystalline phase of the model lipid with very little crystallization. Incorporation of Glyceridacid (5-15 wt%) into the model lipid produced enthalpies between 17-21 J/g lipid after 24 h at 6% RH, with reduced water loss (20-26%). Therefore, Glyceridacid can function as a skin conditioner at low humidity by maintaining the liquid crystalline phase of the model lipid in addition to preventing water loss. Glycerol (10 wt%), a known skin conditioner in vivo, inhibited the crystallization of the model lipid, despite almost complete dehydration: enthalpy is 30 J/g lipid after 24 h with few solid crystals present. This suggests an alternative mechanism to humectancy for the moisturizing action of glycerol at low humidity, in agreement 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.