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.

TRANSPARENT EMULSIONS 279 If a long-chain fatty acid or fatty alcohol is spread on water, the surface tension 3? can be defined as where •'w= is the surface tension of pure water and •r is the surface pres- sure of the long-chain polar compound. If the surface area is maintained constant, and an anionic surfactant is injected into the aqueous phase, the surface pressure will be found to increase. If a high ratio of nonpolar oil molecules to emulsifier molecules is now added to the system, it will form a mixed film. Upon compression of the mixed film on a Langmuir trough, some of the oil molecules will be squeezed out and will spread on the mixed monolayer in the form of a thin oil film, about I50 to 500 •. thick. The tension at the newly formed oil-air interface holds the interface together and permits measurement of the surface pressure, from which the interfacial tension can be calculated. LiquidInterphase Required: In addition to a negative interfacial tension, the mixed interfacial film must not be too highly condensed, or a micro- emulsion will not form. Aqueous solutions used in the preparation of microemulsions generally contain from 10 to 40% of emulsifying agents. The micelles present in these concentrated aqueous solutions have a lamellar structure, and this structure can be considered as the interphase between oil and water. In the case of emulsions of the type stabilized by choles- terol and a straight-chain alkyl sulfate, the interphase is strongly con- densed. It cannot assume a large enough curvature to form droplets that are smaller than about one micron in diameter. One method of converting a highly condensed interphase into a liquid interphase is to add an alcohol of medium chain length to the system. The addition of an alcohol containing five to eight carbon atoms to an aqueous solution containing lamellar ionic micelles will cause the micelles to swell almost without limit in both water and oil. Interpenetration of the mi- celies by the medium chain-length alcohol results in a liquid interphase. If the chain length of the alcohol is greater than ten carbon atoms, inter- penetration of the micelles by the alcohol also occurs. However, the interphase is strongly condensed, and the micelles do not swell. The interphase can also be made less strongly condensed by raising the tem- perature or by using an ionic surfactant with a large counter ion, as in the case of an alkanolamine soap. Interpenetration of Nonpolar Oil: The third and final requirement, according to Schulman, is that the nonpolar oil must interpenetrate and associate with the interfacial film. The oils that can be used to form microemulsions with a given emulsifier combination must be structurally similar to the emulsifiers and of equal or smaller hydrocarbon chain length. For example, if the emulsifier consists of the combination of alkanolamine oleate and oleyl alcohol, it will not form a microemulsion with benzene.