MICROEMULSIONS OF TRIGLYCERIDE-BASED OILS 323 50 � 40 0 30 ::I � 20 10 0 0 0.2 0.4 0.6 0.8 1 Sebum fraction in oil Figure 8. Fish diagram with squalane (solid line) and ethyl laurate (EL) (dashed line) at 1.5% NaCl as a function of surfactant concentration and sebum fraction in oil (a value of O is 100% co-oil and 1 is 100% sebum oil). Surfactant/linkers studied here are AOT (4%), hexylglucoside (5.06%), and sorbitan monooleate (5 .13% ). The concentration ratio is kept constant as the total surfactant/linker concentration is varied (25 ° C). Refer to Figure 4 for the phase behavior at 1.5% NaCl. vature with pure squalene at 0.5% NaCl, as shown in Figure 3. The addition of 1.5% NaCl makes the system become more hydrophobic and helps decrease the curvature of the surfactant membrane. Using co-oil that has a lower EACN, such as ethyl laurate, further decreases the curvature. As seen in Figure 8, a Type III microemulsion is observed in the absence of the sebum oil (the sebum fraction is zero). As the hydro­ philicity of the oil increases by increasing the sebum fraction in the mixed oil, the interaction between surfactant and oil is enhanced. This leads to a negative curvature, and the Type II microemulsion is formed. For the system with squalane, which is a more hydrophobic oil compared to ethyl laurate, a higher fraction of sebum oil is required to obtain the same curvature obtained from the system with ethyl laurate. CONCLUSIONS We have demonstrated that biocompatible and cosmetically friendly surfactants/linkers can be used to form single-phase triglyceride microemulsions (Winsor Type IV) at room temperature and low salt concentrations. The fish diagram observed in this study follows the classical patterns of phase behavior established for nonionic oligoethylene glycol­ based surfactants. We have also found that using co-oil helps the solubilization of triglycerides at lower surfactant concentrations and reduces the amount of surfactant necessary to form a single-phase microemulsion, which is desirable in cosmetic appli-

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