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.

SURFACE-ACTIVE AGENTS It has long been known that some emulsifiers tend to create and sta- bilize oil-in-water (O/W) emulsions, and others water-in-oil (W/O) emul- ' o s•ons. Many attempts have been ß :. made to classify emulsifiers accord- : ing to this tendency. Probably the most general rule is that emulsifiers will tend to make that liquid which .-•: preferentially wets them or prefer- i.: phase. Thus cetyl alcohol is strongly overbalanced in the non- polar direction, and it shows a defi- nite tendency to stabilize W/O emulsions. Glyceryl monostearate has a more effective polar group and can be made to stabilize either O/W or W/O emulsions. Because it is not far• from the point of optimum balance, for this specific purpose, its effect can be influenced by rela- tively small proportions of decidedly unbalanced emulsifiers. Sodium :.. stearate, which has • still more ef- fective polar group, shows a strong tendency to form O/W emulsions. Solid .emulsifiers may be classified similarly, depending on which phase preferentially wets them. Too strong preferential wetting by either phase, however, prevents concentration at the interface, and is similar to great unbalance in the structure of a molec- ularly-dispersed emulsifier, whereby the agent becomes ineffective. The classic emulsifier f6r W/O emulsion is lanolin. This material is rich in the higher alcohols known as steroIs. It is not too difficult to separate the sterol fraction and in- corporate it in a mineral oil or petrolatum base. This gives the IN COSMETIC INDUSTRY 34• "absorption bases" which are useful as emulsifiers without the odor and tackiness of lanolin. Lanolin and absorption bases are widely used as emulsifiers and skin emollients in cosmetic creams. It is easy to for- mulate creams analogous to cold creams, but with lanolin or an ab- sorption base as the emulsifier. The currently popular emulsion hair dressings generally contain lanolin, both for its emulsifying value and its effect on the hair. EMULSIFIER FORMED IN SITU A freshly created interface in a three-component system may re- quire considerable time to reach its equilibrium interfacial tension. Let us suppose that in a specific system the equilibrium tension is very low- less than one dyne--but that several minutes or hours are required to reach that value, and that a freshly formed in terrace has a high tension-- say 15 or 20 dynes per cm. During agitation, as soon as any portion of the oil begins to be divided, fresh interface with a high interfacial ten- sion is formed. Therefore the oil re- sists subdivision. However, if by vigorous action, the oil is subdivided and maintained in very small drop- lets for an appreciable time, those droplets will achieve the low equilib- rium tension, and the emulsion will be comparatively stable. Here is an example from the field of metal cleaners. We found that a solution of 5 per cent of an oleate soap in kerosene shows high initial tension--12-1{ dynes--against water. Over a period of several