JOURNAL OF THE SOCIETY OF COSMETI• CHEMISTS cleansing lotions, hair creams. (d) Pourable emulsions of the water-in-oil type, e.g. hair creams and certain massage preparations. (e) Creams of the oil-in-water type, e.g. foundation creams, "all purpose creams", hand creams, brushless shave creams, cold creams, etc. (f) Creams of the water-in-oil type, e.g. cold creams, emol- lient creams, "skin foods", etc. THE NATURE OF EMULSIONS Briefly, an emulsion is a system in which one liquid is suspended as globules in a second liquid. From the cosmetical aspect we may pre- sume one phase to be water and the other a water-immiscible substance such as mineral oil, vegetable oil, stearic acid, cetyl alcohol or possibly a compound perfume. The emul- sions to be discussed are those stabilised by a third substance, namely the emulsifier. Extremely dilute suspensions of oil in water which can be stabilised by an electric charge and not an emulsifier, are not normally employed in cosmetics and will not, therefore, be considered. Although cosmetical emulsions often contain water-immiscible ingredients which are not liquid at room tem- perature, it is more convenient to refer to. them as oil. When two liquids such as water and mineral oil are shaken together, one of them will disperse into 142 globules. The more vigorous the shaking the smaller the particle size of the globules, and the greater the degree of dispersion, but if left to stand undisturbed, the particles will re-unite or coalesce unless a stabiliser is introduced to the system. When two immiscible liquids are in con- tact, there is an urgent desire for the interface to take up its smallest possible area, for each phase to gather all its scattered offsprings into one united family. This is explained by the fact that each individual liquid possesses very strong family ties, internal forces which strain to keep the molecules together, reluct- ant to allow even one small member to migrate. The strength of these molecular cohesive forces are measurable and are responsible for the surface tension, each liquid hav- ing its own value. At the same time two liquids exert a certain attraction for one another and the forces tend- ing to draw them together are known as forces of adhesion. When the forces of cohesion of two liquids are much stronger than their forces of adhesion, then the particles will not mix that is, the liquids are immis- cible. In other words, there will exist a strong state of tension between the surfaces of the two liquids. The effect of reducing this state of tension is simply demonstrated by taking a vessel con- taining distilled water and a second vessel containing a solution of a sur- face active agent such as sodium lauryl sulphate in distilled water. If a coloured mineral oil is run into each vessel from a pipette, dipping

EMULSIONS AND THE COSMETIC CHEMIST below the surface, it will be clearly apparent that the droplets entering the sodium lauryl suphate 'solution are much smaller than those entering the pure water. In addition, the oil entering the latter spreads on the surface in a continuous. film, while the droplets in the other vessel remain separated from one another on the surface. The surface active agent, by lowering the interfacial tension, has reduced not only the tendency to coalesce but also the mechanical energy required to obtain small particle size. Unfortunately, conditions favouring ease of disper- sion of the globules are not in them- selves sufficient to guarantee emul- sion stability. The definition of stability must rest with the require- ments of the preparation the chemist is attempting to produce. .%dium lauryl sulphate by itself is a poor emulsifier while soaps of the alkali fatty acids are, under certain con- ditions, effective emulsifying agents. To explain why one surface active agent should be more effective than a second in promoting emulsion stability, it will be necessary to dis- cuss some of the more prominent theories of emulsification. THEORIES OF THE PART PLAYED BY THE EMULSIFIER THE VISCOSITY THEORY At one time it was thought that viscosity played an important part in emulsification and, remembering some of the old fashioned ointments and gum-containing preparations, this is understandable. Robinson's modified Stokes equation suggests that emulsion stability depends upon the attainment of a small particle size, the prevention of coalescence, the use of an oil phase having a density close to that of water, and lastly, the final emulsion being highly viscous. A study of modem emulsified products, especially thin milks, will make it plain that, while high viscosity may favour the stability of an emulsion, it is certainly not an essential condition. THE ADSORPTION THEORY This theory suggests that the emulsifier accumulates at the oil/ water interface and by positive adsorption, reduces the tension between the two phases, i.e. the dis- solved substance is said to be surface active. Positive adsorption refers to the accumulation of molecules at a surface. When a solute behaves in such a manner that the molecules concentrate at the surface of the sol- vent, the solute is said to be surface- active. The effect of this migration of the molecules from the interior of the solvent is to reduce the surface tension of the latter and if the sur- faces of two immiscible liquids are in contact, then by positive adsorp- tion, the inter/acial tension is lowered. Soaps are typical examples of substances which are positively adsorbed. On the other hand, most electrolytes and inorganic acids tend to accumulate at the interior of the solvent (water) and increase the sur- face tension. This phenomenen is kno•wn as negative adsorption. It is !43