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j. Soc. Cosmet. Chem., 44, 89-100 (January/February 1993) Prevention of model stratum corneum lipid phase transitions in vitro by cosmetic additivesDifferential scanning calorirnetry, optical microscopy, and water evaporation studies JAIRAJH MATTAI, CLAUDIA L. FROEBE, LINDA D. RHEIN, F. ANTHONY SIMION, H. OHLMEYER, DEAN T. SU, and STIG E. FRIBERG, Colgate-Palmolive Technology Center, 909 River Road, Piscataway, NJ 08854 (J.M., C.L.F., L.D.R., F.A.S., H.Oo, D.T.S.), and Center for Advanced Materials Processing, Clarkson University, Potsdam, NY 13676 (S.E.F.). Received June 2 7, 1992. Synopsis Two potential moisturizers (maleated soybean oil, identified here as Glyceridacid, and glycerol) were compared for their effects on the liquid crystalline phase of model stratum corneum lipids at 6% and 92% relative humidities (RH) using differential scanning calorimetry (DSC), water evaporation, and polarized light microscopy. DSC of the model lipid (32% water content) showed a broad endothermic transition at about 52øC, 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. INTRODUCTION It is well established that the stratum corneum layer of the epidermis is the barrier to water loss from skin and that the decisive influence of intercellular lipids on this barrier 89