314 JOURNAL OF COSMETIC SCIENCE Table I Composition of Artificial Liquid Sebum (25°C) Used in This Study Component Laurie acid Oleic acid lsostearic acid Tricaprin Triolein Glycerol triisostearate Oleyl oleate Myristyl myriscate Isostearyl isostearate Squalene Cholesterol Cholesterol oleate % 11.73 11.73 5.86 11.73 11.73 5.86 10.6 10.6 4.13 12.23 1.53 2.27 solution and used without further purification. Artificial sebum was prepared by Mary Kay Inc., with the composition shown in Table I the sebum was liquid at the experi­ mental temperature of 25 ° C. Properties of the co-oils are shown in Table II. METHODS Phase behavior studies were performed using equal volumes of water and oil (or sebum/ co-oil mixtures), giving a water/oil ratio (WOR) equal to one. Preliminary studies were conducted to determine an optimum formulation and optimum salinity. The optimum formulation of the aqueous phase was found to be a surfactant mixture of 4% AOT + 5.13% sorbitan monooleate + 5.06% hexylglucocide by weight the optimum salinity Table II Selected Properties of the Co-Oils Used in This Study Co-oil EACN MW (g/mole) Molecular formula Squalene 410 Squalane -24 422 Isopropylmyristate (1PM) 13 6 270 ft fHs CH 3 (CH 2 ) 1 1 CH 2 -C-O CHCH 3 Ethyllaurate (EL) 13 c 224 a From ref. 39. 6 From ref. 12. c From this work.

MICROEMULSIONS OF TRIGLYCERIDE-BASED OILS 315 (S*) for this composition is 0.5% NaCl. Stock solutions of AOT, hexylglucocide, and sorbitan monooleate at the selected weight ratios were prepared at different total sur­ factant/linker concentrations ranging from 14.19 to 56.06 wt %. The phase studies were carried out in 16 x 125 mm flat-bottomed tubes 2.5 ml of surfactant solution was added, followed by the addition of co-oil, then sebum oil. The total volume of co-oil and sebum oil required is equivalent to the amount of water present in the 2. 5 ml of surfactant solution this is to keep the WOR equal to one. The fraction of sebum oil in the oil mixtures is varied from zero (100% vol. co-oil) to one (100% vol. sebum oil). The prepared samples were gently shaken once a day for three days and left to equilibrate at room temperature (25°C) for two weeks. Phase diagrams (fish diagrams) were con­ structed by plotting the total surfactant/linker concentration as a function of the sebum fraction in oil. The microemulsion phases (Types I, II, III, and IV) were obtained by visual observation. The effect of salinity (0.5 wt %, 1.5 wt%, and 3 wt % NaCl) and co-oil on the phase behavior was investigated. RESULTS AND DISCUSSION EACN OF CO-OIL AND OPTIMUM SALINITY Formulating microemulsions requires the right combination of variables that will pro­ vide an optimum middle-phase microemulsion. Salager et al. (42) proposed a semiem­ pirical equation that relates the different formulation variables: ln(S*) = k(EACN) + /(A) - er+ arD..T (1) where S* is the optimum salinity, or electrolyte concentration k is a constant, normally between 0.1 to 0.17 and EACN is the equivalent alkane carbon number for nonlinear hydrocarbon (e.g., triglycerides). For linear alkane hydrocarbons, the alkane carbon number (ACN) is applied. The EACN is estimated based on the optimum salinity obtained in our formulation studies the higher the optimum salinity required, the higher the hydrophobicity or EACN of the oil. The effect of alcohol or additives is noted by f(A)! er is a function of the type of the surfactant, a is a constant, Tis the temperature of the system, and D..T is the difference in temperature between the temperature of the system and an arbitrary reference temperature. However, in this study, alcohol is not included and the temperature of the system is constant. Acosta et al. (12,39) determined the EACN of squalene and isopropyl myristate (IPM) as shown in Table II (24 for squalene and 13 for IPM). The EACN for squalane is expected to be close to the value for squalene (-24). Table III shows the optimum salinity of oil mixtures (co-oil and sebum mixtures). The optimum salinity of pure isopropyl myristate is 3.5% NaCl, whereas the optimum salinity is lower when the amount of sebum oil is increased (e.g., the optimum salinity for the oil mixture of 20% vol. IPM and 80% vol. sebum is less than 0.5%). This suggests that IPM has a higher EACN or is more hydrophobic than sebum oil. The optimum salinity of pure ethyl laurate (EL) is 1-1.5% NaCl, which is closer to the optimum salinity of the 20% vol. EL and 80% vol. sebum mixture, indicating that EL has an EACN closer to sebum oil than IPM does (EACNsebum EACNEL EACN1pM). This is a very important finding in formulating cleansing products because the amount of sebum in human skin can be different depending on skin types. The ideal objective is to be able to formulate a