MICROEMULSIONS 341 of water that can be solubilized in microemulsions with 10 ml of n-decane oil and 4 ml of isopropanol and various amounts of surfactants as indicated. It is seen that with an increase in the concentration of the oil soluble surfactant Arlacel © 186, the water solubilization capacity of the microemulsion is increased (Figure 3). Of course, the 9 4 SYSTEM: n-Decone oil" I0 ml Isoprop,•ono I = •r ir• Arlace 1ø186 + 5= 6gm Temperature = 20 øC Isotropic Sing le Phase Region i i i i 0 I 2 3 4 5 6 7 ARLACE• 186 IN gm. Figure 3. The effect of Arlacel © 186/Brij © 35 ratio on the solubilization of water in microemulsions. water solubilization capacity of the microemulsions depends on the amount of alcohol as well as the chain length of oil employed. This is illustrated in Figure 4. With 3 ml of isopropanol and 10 ml of oil, the water solubilization capacity of the microemulsions increased from 5.0 ml to 5.9 ml when the oil chain length was increased from C8 to C•6. An increase in concentration of isopropanol at fixed concentrations of the surfactant and oil results in an increase in water solubilization followed by a decrease (Figure 5). The decrease in the water solubilization capacity at higher isopropanol concentrations

342 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS z SYSTEM: Bri?35.f 2 gm Ar lecele186 = 4grn Isopropenol = 3 ml Oil = I0 ml Tempereture = 20eC I I i C8 CIO CI2 614 CI6 OIL CHAIN LENGTH I I I. iJ Figure 4. The effect of oil chain length on solubilization capacity of water and saline in microemul- sions. might be due to increased solubilization of the surfactant at the interface in the alcohol and its partitioning into the oil phase since isopropanol is miscible with the hydrocarbon oils. Replacing water by 0.9% NaC1 solution showed interesting results. With 10 ml of various oils and 3 ml of isopropanol, microemulsions with the same amount of surfactant solubilized various amounts of saline (Figure 4). With octane and decane oils, the maximum brine solubilization was followed by the appearance of turbidity and phase separation similar to that in the presence of water. However, with dodecane and tetradecane, the initial isotropic region was followed by a viscous and birefringent region which was not observed with water. With hexadecane oil, the behavior was totally different from the rest of the oils so that even after the addition of 9 ml of 0.9% NaC1 solution, though the system became more viscous and slightly hazy, no