DSC STUDIES OF SEBUM MODELS 213 tion of skin surface lipids in Table II adapted from Downing (8) was used as a starting point for our experiments. It is apparent from the table that there is great variation in the composition of the fatty acids and triglycerides. It could be because the free fatty acids are formed from the triglycerides through the action of Propionibacteriz/m aches (P. aches). Bacterial lipases convert triglycerides to mono- and diglycerides as well as free fatty acids in the sebum, many of which are unique to the sebaceous glands (9). The extent of hydrolysis might vary in different persons, possibly due to variability in the P. aches content on their skin. For the DSC analysis, variations of the above composition were used. Diglycerides, cholesterol, and cholesterol esters were not used in our sebum models, as they made up a very small percent of the total. The percent of squalene was kept constant in all samples. Water was not part of the sebum model, as it is polar and would probably not be miscible with the non-polar lipid mixture. The aim of the study was to determine the effect of component characteristics on the phase behavior of the model sebum. In particular, we investigated the effects of the following on the melting behavior of a model sebum: (a) the carbon chain length of the components, (b) the ratio of unsaturated to saturated components, and (c) the ratio of triglyceride to fatty acid content. EXPERIMENTAL MATERIALS The materials listed in Table III were obtained fkom Sigma Chemical Co., St. Louis, MO. They were at least 99% pure, according to the supplier. Chloroform and methanol were also obtained from Sigma. PREPARATION OF LIPID SAMPLES AND DSC PROCEDURE The ingredients for a particular model were weighed out and dissolved in a mixture of chloroform:methanol (3:1). Small portions of the above model sebum were withdrawn and put onto a pre-weighed DSC pan. The solvent was evaporated and the weight of the pan taken again. The difference gave us the weight of the lipid mixture, and this weight was entered on the DSC run by computer. In the absence of chloroform-methanol co-solvent, the samples withdrawn were not uniform, since there was a separation of the different phases. The DSC pan was then covered with an aluminum lid and run at a scan Table II Average Composition of Human Skin Surface Lipid for 17 Subjects (ref. 8) Lipid class Mean (%) Range Triglycerides 41.0 19.5-49.4 Diglycerides 2.2 2.3-4.3 Fatty acids 16.4 7.9-39.0 Wax esters 25.0 22.6-29.5 Squalene 12.0 10.1-13.9 Cholesterol 1.4 1.2-2.3 Cholesterol esters 2.1 1.5-2.6

214 JOURNAL OF COSMETIC SCIENCE Table III Ingredients Used to Examine the Effect of Carbon Chain Length Carbon chain length Components 14 16 18 Wax ester (unsaturated) Oleyl oleate Oleyl oleate Oleyl oleate Triglyceride (unsaturated) Trimyristolein Tripalmitolein Triolein Fatty acid (unsaturated) Myristoleic acid Palmitoleic acid Oleic acid Wax ester (saturated) Myristyl myristate Palmityl palmirate Stearyl stearate Triglyceride (saturated) Trimyristin Tripalmitin Tristearin Fatty acid (saturated) Myristic acid Palmitic acid Stearic acid rate of 5øC/min from -50øC to 100øC, using a Perkin Elmer DSC-7. Each of the compositions was run in triplicate. For all the individual components, DSC runs were done in pure form and after dissolving in chloroform-methanol and evaporating the solvent. This was done to facilitate identification of the peaks. Polymorphic changes might occur when the individual components are dissolved in the co-solvent mixture, which is consequently evaporated. The above step identifies if any such change occurs. EFFECT OF CARBON CHAIN LENGTH The effect of three different carbon chain lengths (namely 14, 16, and 18) was examined. Major free fatty acids (FFA) of sebum fall in the carbon chain length of 14, 16, and 18 (C-14, C-16, and C-18, respectively) category, with negligible amounts of other carbon chain lengths (10). To investigate the effect of one carbon chain length (14, for example), the triglycerides, fatty acids, and wax esters were all of carbon chain length 14 (see Table III). The same was done for carbon chain lengths 16 and 18 also. According to Table II, the amount of triglycerides is 41%, wax esters are 25%, and fatty acids are 16.4% of the total lipid, and we used approximately that composition for each carbon chain length. The total triglycerides are, however, a mixture of unsaturated and saturated components, and so those were varied also for our second objective. In Table III, we have listed the ingredients used in each experiment. The quantities used are shown in Table IV. EFFECT OF PERCENT SATURATED To study the effect of percent saturated, the carbon chain length of all the components Table IV Sample Compositions With Varying Ratios of Unsaturation to Saturation of Each Lipid Class Components % Weight Unsaturated:saturated 0:1 1:1 2:1 3:1 1:2 1:3 1:0 Squalene 13 13 13 13 13 13 13 Wax ester (unsaturated) 0 13.5 18 20.25 9 6.75 27 Wax ester (saturated) 27 13.5 9 6.75 18 20.25 0 Triglycerides (unsaturated) 0 21.5 28.66 32.25 14.33 10.75 43 Triglycerides (saturated) 43 21.5 14.33 10.75 28.66 32.25 0 Fatty acids (unsaturated) 0 8.5 11.33 12.75 5.66 4.25 17 Fatty acids (saturated) 17 8.5 5.66 4.25 11.33 12.75 0 Total 100 100 100 100 100 100 100