452 JOURNAL OF COSMETIC SCIENCE lipophilic sunscreens moreover, stearyl methicone was introduced in the oil phase to reduce the tendency of the sunscreen to be washed off. The compositions of the micro- emulsions were obtained by means of pseudoternary diagrams that allowed a more systematic approach and an easier method of formulation, using many components with different characteristics. After a preliminary screening of the varying percentages ratios of the components, the systems with the largest amounts of lipid phase (that could be partially replaced by a sunscreen) and surfactant and cosurfactant percentages as low as possible were chosen. EXPERIMENTAL MATERIALS The materials employed and their sources are as follows: 4-methylbenzylidene camphor (MBC), octylmethoxycinnamate (Eusolex © 6300, Parsol © MCX (OMC), menthol, allan- toin and MgCI2 ß 6H20 were obtained from Merck C•2_•5-alkylbenzoate was from Prodotti Gianni cyclomethicone and stearyl methicone were gifts from Tego Cosmetics- Goldschmidt decylpolyglucose was a gift from Seppic cetearyl sulfate (CS) was obtained from Henkel ethanol, 1,2-hexanediol (HD), 2-methyl-2,4-pentanediol (MPD), sodium dodecyl sulfate, and 1-dodecanol were from Fluka-Chemika 1-decanol was from ICN- Biochemicals lecithin was obtained from Rh6ne-Poulenc Rorer Filter Millex©-GS 0.22 lam and Filter Millipore © 3.0 lam were from Millipore dialysis membrane Visking Type 36/32 and Neopelle were obtained from Roth and Silastic © brand sheeting was from Dow Corning Corporation. METHODS Use of pseudoternary diagrams in preparation of O/W microemulsions. Ternary diagrams were prepared as described in the literature (4-9), using an equilateral triangle (see Figure 1), whose corners, clockwise from the top, represent 100%, respectively, of lipids, a blend of two surfactants, and a mixture of water/cosurfactant (polar phase). All the percentages Lipids ?0/30 8O/20 Water/ Surfactants cosurfactants Figure 1. Ternary diagram of systems with lipids, surfactants, water, and cosurfactants.

OW MICROEMULSION IN SUNSCREENS 453 throughout are weight/weight. The combinations of the surfactant blend and lipid phase are located to the right side, and the dilutions of the surfactant blend/lipid with the polar phase are located to the lower side. From the intersections of the lines departing from 100% water/cosurfactant to the opposite side, dissecting the triangle and the lines parallel to the surfactant/lipid side, we can determine the proportion of ingredients that give O/W microemulsions or systems that separated. Simple systems were obtained by diluting the concentrated surfactant/lipid mixtures with different amounts of water/ cosurfactant solution. This method can be extended to very complex systems, locating several ingredients together at each corner of the pseudoternary diagram, on the basis of their similar chemical-physical behavior. For this purpose it is important to state the exact proportions of the mixture of ingredients. Ternary diagrams can also allow one to obtain oily solutions at different component ratios and to choose the best ones to be used to redraw ternary diagrams, whose corners, respectively, represent the mixture of lipid/ethanol/cyclomethicone/sunscreen, the sur- factants, and the water/cosurfactant solution. Preparation of microemulsions. Blends of the two surfactants were prepared with different proportions. Appropriate amounts of fluid lipids or mixtures of fluid lipids and lipo- philic actives were added. Some of these systems were diluted with amounts of polar phase corresponding to percentages individualized at preselected points of the ternary diagram. Among all the obtained mixtures, only the clear ones were microemulsions. Characterization of Microemulsion (a) Microscopy and laser light scattering Microemulsions were examined by optical microscopy and under polarized light (Mi- croscope Leitz Labovert). Microemulsions, filtered through Millex©GS (0.22 larn), were also analyzed by means of a laser light-scattering technique (Laser 90 Plus Particle Sizing Software 2.27, Brookhaven Instruments). Each determination was the mean of three runs. With these techniques, concentrated and diluted systems were examined. (b) Evaporation at constant humidity A thin layer of microemulsion was placed in a Petri vessel in a closed chamber at 37% relative humidity (MgC12 ß 6H20). At scheduled times, the weight loss and the rheo- logical flux of the microemulsions were determined. The systems were then analyzed by microscopy. (c) Rheology studies Flux rheograms were determined for microemulsions 24 hours after preparation or after evaporation, using a rotational viscometer (Brookfield RVTDVII, with a small adapter chamber and spindle SC 4-21/13R) at 25øC + 0.5. Apparent viscosity was determined at three shear rates (2.325 s -•, 4.65 s -•, and 18.6 s-•). (d) Water resistance Water resistance was tested on amounts of microemulsion spread on a collagen felt sheet and placed in water. Microemulsions with a poor water resistance gave an aqueous dispersion. The sheets used consisted of pure collagen (sequence alpha 1, first type).