TRANSPARENT EMULSIONS BY LLOYD I. OSIPO•V, B.S.* Presented March 6, !963, New York City ABSTRACT The formulation of clear cosmetic "lotions" and "gels" is discussed in terms of Schulman's theory of microemulsions. Conditions required for the preparation of these microemulsions include a metastable negative interfacial tension and a liquid condensed interphase. It is shown that conventional emulsifying agents of both nonionic and ionic types can be used to prepare these systems. Agents are selected to condense and to expand the interfacial film. When these materials are present in the proper proportions, as determined by a "titration" procedure, a transparent emulsion results. I. INTI•ODt:e•r•ON There appears to be a growing interest on the part of cosmetic chem- ists in transparent fluid and gel cosmetic products containing appreciable amounts of both oil and water. There are several reasons for this interest. First, the industry is constantly searching for new products that are both usefial and have cosmetic appeal. Certainly, a crystal-clear liquid or gel is elegant and has a different appearance from an opaque product. Second, the transparent products may have increased utility, because of the likeli- hood that absorption into the skin will be promoted by the extremely small size of the dispersed droplets. Third, for the formulator, there is the advantage that these systems are easily prepared, and they are thermo- dynamically stable. Further, they can be opacified, so that they look like conventional creams and lotions. Lanolin derivatives are commonly used to prepare transparent cosmetic products, with excellent results. The intent of this paper is to show that other emulsifying agents may also be used and to present a theoretical basis for the formulation of these products. Microemulsions The transparent fluid and visco-elastic systems under discussion are treated here as microemulsions. The nature of microemulsions and the conditions under which they will form have been discussed by Schulman and co-workers (1-3). However, it should not be assumed that this is * Foster D. Snell, Inc., New York ll, N.Y. 277

278 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS the only valid description of these transparent compositions. Solubiliza- tion in concentrated surfactant solutions as described by Lawrence (4) and by Winsor (5-9) may be equally applicable because no valid method is known for distinguishing between microemulsions and solubilized sys- tems. These different approaches have been reviewed recently (10). The microemulsions of Schulman are said to have a droplet diameter in the range of 100 to 600 A. In comparison, SPoherical anionic micelies in aqueous solution have a diameter of about 50 A., while nonionic micelies are double this size. Solubilization swells the micelies and their diameter can easily fall in the range of microemulsion droplets. Both solubilization and microemulsion formation occur spontaneously, without the necessity for homogenizing or colloid milling. The resulting systems are thermo- dynamically stable. Since the diameter of the droplets is less than 1/4 the wavelength of light, the systems are transparent. The advantage in using microemulsion theory to describe these systems is that the theory has been developed to the point of providing a satis- factory basis for the formulation of transparent cosmetic products con- taining high proportions of both oil and water. The essential aspects of microemulsion theory are as follows. (1) The combination of emulsifying agents and their concentration must be such as to produce a metastable negative interfacial tension. (2) The emulsi- fier interphase must not be too highly condensed. (3) Molecules of the oil phase must be able to interpenetrate or associate with the mixed inter- facial film constituting the interphase. These three features are discussed in more detail below. Negative Interfacial Tension: In order to form microemulsions, the con- centration of emulsifiers must be greater than that required to reduce the oil-water interfacial tension to zero. Under these conditions the inter- facial tension must have a metastable negative value. This would cause droplets to break up spontaneously and would also stabilize the dispersed phase and prevent phase separation. As the emulsified droplets become smaller, the interfacial area increases and the emulsifier would become de- pleted by adsorption until the interfacial tension increases to zero, consti- tuting the equilibrium condition. An appropriate combination of emulsifying agents will give a lower interfacial tension than either component used alone. For example, a suitable combination consists of an anionic surfactant with a long-chain fatty acid or fatty alcohol. A similar effect is obtained with a combination of water-soluble and water-insoluble nonionic emulsifying agents. The metastable negative interfacial tension cannot be measured di- rectly, since the interface emulsifies spontaneously. However, it can be calculated directly if a counter tension is placed on the interfacial measur- ing device to prevent surface breakup.