DSC STUDIES OF SEBUM MODELS 215 was kept constant and variation was done in the unsaturated and saturated portions. As an example for the fatty acids, which made up 17% of the total, the ratio of unsaturated to saturated fatty acids was varied, keeping the total to 17% of the mixture, as shown in Table IV. EFFECT OF CHANGE IN TRIGLYCERIDE AND FATTY ACID RATIOS Triglycerides are hydrolyzed to fatty acids by P. aches. The total amount of triglycerides and fatty acids, however, is 60% (10). For these experiments, the carbon chain length was 16 and the ratio of unsaturation to saturation was 1:2, as given by Nordstorm et al. (11). In these experiments only the saturated wax ester (palmityl myristate) was used. The rest of the ingredients were in the same quantities as in Table IV. RESULTS AND DISCUSSION A typical DSC thermogram of model sebum has four distinct transitions that are assigned to different components in the sebum, as shown in Figure 1. Their melting temperature, referred to as Mp-1 through Mp-4, characterizes each of these transitions. While Mp-1 and Mp-2 occur below 0øC and represent the unsaturated portion, Mp-3 and Mp-4 occur above 0øC and represent the saturated portion. From Figure 1, as the temperature is increased from -50øC, a fraction of the lipid mixture melts at approxi- mately -20øC, which is seen as transition Mp-1. As the temperature is increased further, another transition occurs at approximately -15øC, which is designated as Mp-2. As the temperature is further increased, another fraction of the model sebum melts (at about 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 Mp-1 -2 Mp-4 -50 -25 0 25 50 Temperature (øC) Figure 1. Typical rhermogram of a model sebum. 75 100

216 JOURNAL OF COSMETIC SCIENCE 40øC) and is designated as Mp-3, and then finally the last solid fraction of the model sebum melts at approximately 55øC and is designated as Mp-4. At -50øC the lipid mixture is a solid, and by 100øC all components have completely melted and the mixture is a liquid. From the DSC profiles and melting endotherms, we can conclude that the model sebum is crystalline in nature. Generally, pure lipids or homogeneous systems yield single sharp peaks whereas heterogeneous systems produce broad multiple peaks (12). Ifsebum was a homogeneous mixture, there would be one melting transition, but the presence of multiple melting transitions indicates that there are multiple phases. From the temperatures of these melting transitions, it appears that at skin temperature (32øC), some of the components of sebum are solids and some are liquids, which are not completely miscible with each other. In order to identify the component associated with each transition, singular components were run through the DSC under the same conditions as the model sebum. The com- ponents used for carbon chain length 16 are given in Figure 2. Adding other substances -50 -30 -10 10 30 50 70 90 Transition Temperature(øC) a. Tripalmitolein m.p. = -21.184øC b ct-Palmitoleic acid m.p. = -18.7øC c. Oleyl Oleate m.p. = -4.234øC d ¾-Palmitoleic acid = 2.1 øC e. Palmityl Palmitate = 55.166øC f. Palmitic acid = 63.866øC g. Tripalmitin = 67.326øC Figure 2. Thermograms showing transition temperatures of individual components