196 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS staining agent. When an O/W microemulsion of the alkyd was exposed to this agent, the alkyd droplets were turned into little cannon balls. When these were exposed to the electron beam under vacuum, the organic matter burned off, leaving behind an osmium metal skeleton of the origi- nal spherical droplets. These are seen directly, without shadow casting, in Fig. 1.* The average diameter of the droplets is estimated at 300 •. One can clearly see the hexagonal packing characteristic of the theoretical closest packing ratio of internal to external phase of 74/26, for uniform spheres. This picture of the dispersed phase in microdroplet form con- stitutes strong evidence that these systems are, indeed, emulsions. THEORY The explanation of how such emulsions are formed stems from the concept of a transient, r•egative tension at the microemulsion interface (5). In order to visualize this concept, let us consider a mixed mono- molecular film of soap and fatty alcohol adsorbed at a fiat oil/water inter- [ace, as in Fig. 2. For reasons that will become obvious, this film shall be 'o%111 Ilto,oo, OIL Figure 2. Schematic diagram of mixed film of soap and alcohol at oil/water interface referred to as an ir•terphase or third phase. The soap and alcohol mole- cules in this interphase are oriented with their heads in the water and tails in the oil phase. As the number of these molecules per unit area is increased, they begin to crowd one another, thereby developing a lateral two dimensional pressure, •-. Early in the study of these monomolecular tilms it was observed that the surface tension, •q, of the interphase creased in proportion to this development of pressure among its tenants. This idea is expressed in the thermodynamic equation % = •o/w -- •r (1) In accordance with this relationship, if as a consequence of great physical Picture taken by Dr. Walther Stocckenius at the Rockefeller Institute in 1958 (4).

MICROEMULSIONS 197 repulsion among the film species ,• should exceed yo/w, yi will become negative, i.e., it will become a pressure. Now energy (--yidd, where d equals interfacial area) becomes available to spontaneously expand the interphase. The temporary existence of a film pressure, ,•, greater than the original tension, yo/w, is the driving force which reduces the droplet size of the fixed volume of oil until no more energy is available to increase the interfacial area. Equilibrium is reached when the negative tension returns to zero by virtue of the uncrowding of the molecules and loss of pressure in the interphase. In pursuing this approach, it soon became evident that a negative tension is the result not so much of a high initial film pressure as of a large depression of the original tension between oil and water. This comes about because an alcohol like cetyl, being soluble in both the oil phase and the interphase, may partition between these phases, making the fraction remaining in the oil phase available to depress the original tension from 7o/•v to (7o/w)•. This is depicted in Fig. 3. Due to this partitioning, the film pressure Oil Oil. Figure S. Schematic diagram of mixed film of soap and alcohol at oil/water interface when partitioning of alcohol occurs is now opposed by a much lower tension. This is illustrated by the new equation 70 = (?o/w)• - •ro (2) in which the transient tension of the fiat interphase becomes y½ and the pressure in the film before curvature becomes T 0. Values of (7o/w)• as low as 15 dynes/cm can easily be attained in this way (5). Since values of =c• in the 30-50 dyne/cm range are also easy to achieve, this comfortably accounts for transient negative tensions of from --15 to --35 dynes/cm in the fiat interphase. The development of film pressures of this magnitude, however, can- not be taken for granted. Certain special precautions have to be taken. As already indicated, mixed films of surfactant and alcohol are required.