SURFACE-ACTIVE AGENTS IN COSMETIC INDUSTRY 339 but this is the condition required for solution, and two such liquids would therefore be miscible. However, an emulsion may be stable enough for practical purposes. A chemical re- action may involve a decrease in free energy and thus be thermodynam- ically possible. However a mole- cule, before reacting, may have to acquire a large amount of energy in order to form an activated complex. This energy is surrendered when the reaction occurs and hence does not figure in the net energy of the reac- tion. Nevertheless if this "barrier" energy is high, the rate of reaction will be very small. Similarly two droplets, to coalesce, may have-to overcome an energy barrier imposed by the droplets having similar elec- trical charges. The charges may not lower the free energy of coalescence substantially, and thus not alter the thermodynamic tendency toward coalescence. However, they may have a great effect on rate of coales- cence by forming a high energy barrier. So an emulsion can be stabilized in the sense of greatly re- ducing the rate of coalescence, with- out affecting fundamental insta- bility as determined by interfacial tension. Ordinarily when a fresh interface is generated, the final equilibrium interfacial tension is reached ap- proximately in a small fraction of a second. Further, electrical • barrier forces are often not great. Con- sequently, interfacial tension is a rough guide to ease of emulsification and rate of breaking of the emulsion formed, but with numerous excep- tions. All emulsions are inherently unstable, since coalescence of emul- sified phase will, by reducing area of interface, reduce interfacial free energy. ROLE OF EMULSIFYING AGEST Emulsions of practical interest have at least three components: Two liquidsmone of which is nearly always water or an aqueous solu- tionmand an emulsifying agent. The emulsifying agent must con- centrate at the oil-water interface. There it forms films which stabilize the emulsion. Like all surface- active compounds, emulsifying agents combine polar and non-polar groups in the molecule in proper balance to give the desired effect. Various mechanisms are possible: 1. The emulsifying agent, if a soluble surface-active agent, forins an oriented molecular layer at the interface, and thus reduces inter- facial tension. This facilitates rup- ture of droplets during agitation and permits formation of a finer emul- sion by agitation forces. The very fact that an emulsion is fine reduces its rate of breaking. According to Stokes' Law, small droplets will rise to the top of the liquid more slowly than large droplets. As was noted earlier, forcing together of droplets at the surface of the mix- ture is an important factor in caus- ing separation. Further, low interfacial tension decreases the rate of separation by decreasing the proportion of colli- sions which are effective in causing

340 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS coalescence of droplets. Thus inter- facial tension will often roughly pre- dict rate of separation. It follows that many widely used emulsifiers lower interfacial tension markedly. This is true of the classic emulsifier, soap. Soaps are the emulsifying agents in cold creams, conventional vanishing creams, and hand lotions, as well as many other cosmetic creams and lotions.' An example of a product which can be formulated with a synthetic agent but not with soap is antiperspirant cream. The usual formulation is a type of vanishing cream containing alu- minum salts. A water-soluble sur- face-active agent is required to form the cream emulsion, but soap would be precipitated completely by the aluminum ions. Many synthetic agents are not sensitive to aluminum ions and can be used. 2. The emulsifying agent may consist of microscopic or even macroscopic particles which con- centrate at the interface. Such particles would have to have suit- able wetting characteristics and not tend to be strongly wet by, and hence pass completely into, either the oil or the aqueous phase. As in the previous case, the essen- tial point is that the free energy of the disperse liquid-liquid system, normally much higher than that of the separated liquids, is reduced by the presence of emulsifying agent at the interface. Here, however, the free energies of the solid-aqueous and solid-oil interfaces must be con- sidered. Thus many powdered substances can have an appreciable emulsifying value. Insoluble inorganic pigments aid in stabilizing liquid make-up preparations. Bentonire is an es- pecially effective emulsifier of this type, owing to its colloidal particle size. 3. The emulsifying agent may create a high energy barrier and thus, without necessarily lowering the thermodynamic "tendency" of droplets to coalesce, greatly reduce the rate of breaking. The barrier may consist of a charge imparted to the droplets by ionic emulsifiers. It may consist of a continuous hydrated film which has measurable rigidity and which prevents the oil portions of two colliding droplets from actually making contact. If the film is highly hydrated, so that there is an increase in viscosity at some distance out from the droplet, considerable clumping of droplets together or fiocculation may occur without actual coalescence and breaking of the emulsion. An emul- sion of this type may cream rapidly but break very slowly. Classical gums such as traga- canth, karaya, and acacia, as well as modern synthetic gums such as carboxymethylcellulose and poly- acrylates, function as emulsifiers of this sort. Also, by increasing the bulk viscosity, they slow collisions between emulsified drops and de- crease the rate of creaming. Thus they may be valuable auxiliaries in emulsions stabilized primarily by surface-active agents. This is par- ticularly true in liquid emulsions such as hand lotions.