.]. Soc. Cosmetic Chemists, 21, 193-204 (Mar. 4, 1970) Microemulsions LEON M. PRINCE, A.B., M.A. • Presented September 8-9, 1969, Seminar, St. Louis, Mo. Synopsis--Recent advances concerned with the theory of MICROEMULSIONS are revicwed and a theorctical interpretation of the H¾•)ROemum-LiPom•mm BALANCE (HLB) scheme for rating EMULSIFIERS is offered. It is submitted that HLB measures the partitioning of emulsifier be- tween the oil phase and interphase rather than bctween the oil phasc and the water phase of an emulsion. Under thcse circumstances, the interrelationship betwcen the flat film pressure, oil/water interfacial tension and HLB determines the character of the emulsion to be formed. INTRODUCTION Coarse (macro) emulsions of water and certain oils stabilized by soap may be titrated with alcohols to transparent, optically isotropic, fluid, water-in-oil (W/O) or oil-in-water (O/W) systems. Schulman coined the term "microemulsion" to identify such dispersions of spheri- cal droplets as emulsions and to distinguish them from systems of swollen micelles (1). The definition was subsequently expanded to include emulsions whose droplets exhibit faint light scattering (2). Such emul- sions, the droplet diameters of which are less than 1400 •., are thermo- dynamically stable, differentiating them from macroe•nulsions which achieve equilibrium by separating into their original two mutually in- soluble liquid phases. It is the purpose of this paper to point out some of the commercially important microemulsions and to describe the conditions which favor their formation. In the process, macroemulsions will be viewed from a Lever Brothers Co., Research Center, Edgewater, N..l. 07020. 193

194 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS new perspective and a theoretical basis for Griffin's HLB scheme (3) will be submitted. Many products and components of products in the marketplace today are microemulsions. What was probably the first microemulsion ap- peared in 1928 in the form .of a self-polishing carnauba wax dispersion. This product owed its outstanding performance to the very small size of its droplets which permitted them to coalesce on the floor to a shiny film. Most importantly, this stable emulsion continued to perform in this way despite its age. The machine tool industry uses stable micro.emul- sions of cutting oils because they are more efficient than coarse emulsions. Microemulsions of many gels and liquids of the pharmaceutical and cos- metic industries are not only fashionable because of their optical effects but are useful because of their stability. This same reasoning applies to transparent microemulsions of fragrances for mouthwash or shaving lo- tion. We have all consumed transparent microemulsions of flavor oils in cola or cream soda or other confections. There are stable microemul- sions of hexachlorophene and chlordane in antiseptic and insecticidal formulations. Water-in-oil microemulsions of cleaning fluids assure that dirt and stains will constantly be exposed to both liquids during the clean- ing operation. Possibly the largest single use of microemulsions is in the paint industry. The stability of the alkyd or polymer emulsion of the paint assures uniformity of application and, as in the case of floor polishes, the small particle size enables high gloss finishes to be formulated. OPTICAL PROPERTIES Microemulsions are easily recognized by their optical properties. Al- though droplets smaller than 2000 3, cannot be seen in the light micro- scope, their presence is plainly visible because they scatter light. This Tyndall Effect can be seen as a blue haze to reflected light or orange-red to transmitted light, very similar to the blue of the sky and the orange-red of the sunset. This permits ready measurement by the microphotometer. When droplets are smaller than 100 •_, the systems usually appear opti- cally void to the naked eye and their droplet sizes are difficult to measure with accuracy. They may be calculated indirectly by low angle X-ray measurements, the ultracentifuge, Rayleigh scattering, and, in certain special cases, may be seen directly in the electron microscope. For example, one of the alkyd paint emulsions was discovered to be quite photogenic in the electron microscope. Because of the alkyd's double bonds, it was easily stained by osmium tetroxide, a biological