MICROEMULSIONS OF TRIGLYCERIDE-BASED OILS Water side of the interface Surfactant SDHS Sl.JNa SO 3 N a a � -;:,,-s '.:.��:�;:,, --;:::-0:,�;:,,;.0;._;;., � � � f Hydrophilic linker SMDNS Oil side of the interface Lipophilic linker Dodecanol SDHS = sodium dihexylsulfosuccinate Combined linker SMDNS = sodium mono- and dimcthylnaphalene sulfonate 313 Figure 2. Schematic of the linker concept, showing the surfactant (e.g., sodium dihexyl sulfosuccinate or SDHS), lipophilic linker (e.g., dodecanol), and hydrophilic linker (e.g., sodium mono- and di­ methylnaphthalene sulfonate or SMDNS). There are only limited choices of surfactants/ingredients that can be used in cosmetic formulations and that form microemulsions with triglycerides successfully. The main benefit of alcohol-free formulations stems from a consumer perception that alcohols are drying and potentially irritating, and so alcohol-free formulations would be useful for sensitive-skin consumers. This paper describes our successful use of linker concepts to formulate alcohol-free microemulsions with artificial sebum (human oil) using biocom­ patible and cosmetically friendly ingredients at ambient temperature and low salt con­ centrations. We have investigated the opportunity to tune the curvature of surfactant film by adding co-oil to the triglyceride in order to help triglyceride microemulsification as well as to minimize the surfactant required to form single-phase microemulsions without adding alcohol. EXPERIMENT AL PROCEDURES MATERIALS The following materials were obtained from Aldrich (Milwaukee, WI) at the concen­ trations shown and were used without further purification: sorbitan monooleate (Span 80, 99%), squalene (98%), squalane (99%), isopropyl myristate (IPM, 98%), ethyl laurate (99% ), and sodium chloride (99% ). Sodium dioctyl sulfosuccinate (AOT, -100%) was purchased from Fisher Scientific (Fair Lawn, NJ). Hexylpolyglucocide AG 6206™, donated by Akzo Nobel (Chicago, IL), was received as a 75 wt% aqueous

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.