J. Soc. Cosmetic Chemists, 19, 683-697 (Sept. 16, 1968) Effect of Initial Surfactant Locations on the Viscosity of Emulsions TONG JOE LIN, Ph.D.* Presented December 6, 1967, New York City Synopsis--Viscosities of emulsions immediately following homogenization were studied as a function of HLB and the initial surfactant locations. Keeping the total surfactant concen- tration constant, the ratios of the initial concentration of the hydrophilic surfactant to that of the lipophilic surfactant in each phase were varied. The experimental results indicate that the initial locations of the surfactants not only affect the initial viscosity of the emulsions but also the emulsion stability, particle size distribution, and emulsion type as well. INTRODUCTION There are literally unlimited ways by which a given emulsion can be prepared (1, 2). Unquestionably, the method of preparation has a great influence on the physical properties of the finished emulsion. Stanko et al. (3) conducted a series of expehments with a mineral oil emulsion stabilized with nonionic surfactants. They discovered that the method of addition, the rate of addition, and the temperature of each phase at the time of emulsification all had some effects on the droplet size distribution, viscosity, and the emulsion stability. Using a two-level fractional factohal design, Benson et al. (4) con- ducted an extensive investigation of six preparative variables including: order of addition, emulsifier location, emulsifier concentration, propor- tion of water, emulsification temperature, and type of agitation. Four oils were used and thirteen different surfactants were employed in their investigation. They discovered that the chemical and physical nature * Max Factor & Co., 1655 N. McCadden Place, Hollywood, Calif. 90028. 68.3

684 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS of the emulsifiers and oils had most significant effect on the stability of the emulsions, and less than half of the systems studied showed statisti- cally significant dependence on the preparative variables. Most of the emulsions in cosmetic preparations are stabilized with a mixture of hydrophilic and lipophilic surfactants. These surfactants may be incorporated into the emulsion in many different ways. Becher (1) describes four different ways of preparing an emulsion depending on the methods of incorporating emulsifying agents: 1. Agent-in-water method. The emulsifying agent is dissolved in the water. 2. Agent-in-oil method. The emulsifying agent is dissolved in the oil phase. The mixture is then added to the water or water is added to the mixture. 3. Nascent soap (in situ) method. The fatty acid is predissolved in the oil and the alkaline part in the water so that the soap is sponta- neously formed during emulsification. 4. Alternate addition method. The water and oil are added alter- nately, in small portions, to the emulsifying agent. Since most emulsifying agents have some solubility in both oil and water, if a mixture of a hydrophilic and a lipophilic surfactant is dis- solved in the oil phase prior to emulsification, a great part of the hydro- philic surfactant and a small part of the lipophilic surfactant will un- doubtedly migrate to the aqueous phase after emulsification until an equilibrium is established. A similar situation will occur when the sur- factant mixture is dissolved or dispersed in the water prior to emulsifica- hon. It can be reasoned that if the rate of surfactant migration from one phase to another is finite, the initial location of the surfactant may have significant effect on the physical properties of the freshly prepared emul- sion. Since many surfactants have marked effects on the viscosity of the phase in which they are dissolved or dispersed, the initial surfactant location can, therefore, influence the viscosity of a fresh emulsion before it reaches equilibrium. Undoubtedly, the rate of surfactant migration in an emulsified sys- tem would be a complex function of the viscosities of both phases, sur- factant concentration, surfactant solubility, surfactant diffusivity, inter- facial area, the property of the interfacial film, etc. It is expected that the migration rate of a surfactant from the internal phase to ex- ternal phase would be relatively low if the viscosity of the internal phase were sufficiently high or if the resistance of the interfacial film to mass