JOUI•NAI. OF THE SOCIETY particle size range for the disper•d droplets, although it is usual that dispersions of large droplets are relatively unstable while dispersions of very small• droplets assume 'more' and more the properties of the colloidal state. Most practical emul- sions have the great majority of dis- persed particles in the range of 0.5 to 5 microns- the particle sizes will depend on many factors such as the nature of the substances, the nature and amount of emulsifying agent, and the mechanical work done in forming the emulsion. When a cosmetic emulsion is de- scribed as "tinstable ", one or more types of change may be occurring. One such change is the actual separ- ation of the oil and water phases, involving the coalescence of the dis- persed droplets. Thermodynamically, emulsions are not stable in that they involve a larger free interfacial sur- face energy for the disperse phase than does the unemulsified state, and so there is a tendency to coalescence of the disperse droplets. In an emulsion containing an emulsifying agent there will be few cases of coalescence as compared with the number of contacts between drop- lets, and the percentage of contacts resulting in coalescence will be a measure of instability. The closeness of packing and frequency of drop- let contacts is the other important factor in phase separation. A second change undergone by emulsions is sedimentation or "creaming", this last term being derived from the behaviour of ordin- ary: milk• An emulsion may settle OF COSMETIC CHEMISTS in such a manner that the dispersed droplets fall slowly to the bottom of the container, but do not coalesce, thus producing a mixture consisting of a layer of concentrated emulsion underlying a more or less clear serum. This disperse phase will settle or rise, according to the rela- tive specific gravities of the two phases, and the process may be de- scribed as upward creaming or downward creaming. If the particles are not closely packed, and are spherical, their rate of sedimentation will obey Stokes' Law. Accordingly, a large difference in specific gravity between the phases will favour rapid creaming, as will large par- ticle sizes and low viscosity of the continuous phase. Creaming can usually be retarded by changing specific gravities to bring those of the two phases nearer each other, by increasing viscosity of the exter- nal phase, and by reducing particle size of the disperse droplets. Reduc- tion of particle size can be attained by mechanical work and may be the only practical way to increase stability where change in formula is tindesirable. A third unwanted effect sometimes observed in emulsions is an increase in viscosity on ageing. This is more common with concentrated emul- sions, or with products thickened with the aid of colloidal substances like starches or gums, than with dilute emulsions. This kind of change accounts for the lotion that can't be poured from the bottle after a period of standing, or for th e. •thin cream that thickens o n age- 144

STABILITY OF COSMETIC EMUI,SIONS ing. It is common among substances ir• the colloidal state for the material to gel or develop a kind of internal structure that interferes with free flow and produces a falsely high viscosity. The product returns to its fluid state if agitated sufficiently. The degree of gelation permissible will depend on the particular pro- duct. The final type of change which may be undergone by emulsions is phase inversion, wherein an oil-in- water emulsion may change over to the water-in-oil type, or vice ver.•a. While the technique of phase inver- sion used during the manufacture of an emulsion may be highly de- sirable in producing an emulsion of very fine particle size, an inversion occurring spontaneously after the product has been manufactured may be highly undesirable. It will com- monly have a p•'ofound effect on the properties of the emulsion. If the phase change is not immediately obvious through change in viscosity or some other property, it may be detected by one of the methods used to determine emulsion type. One may observe microscopically the distribution of a water-soluble dye in the emulsion, and thus determine whether it is of the O/W or W/O type or a drop of emulsion can be placed in a volume of water to see whether it disperses readily and is thus of the O/W type. A third method is the measurement of the electrical conductivity of the emul- sion, since W/O types are very poor conductors as compared with O/W emulsions. In testing the stability of an emul- sion the only procedure giving per- fect]y reliable results is an actual use test extended over a sufficiently longe time period. However, most manufacturers cannot afford the time involved, and so we chemists are called upon to devise accelerated te.•ts whose results are as reliable as possible for the degree of accelera- tion required. Some phases of the per[ormance of the emulsion are really matters o[ stability, and a laboratory test duplicating the use of the product also is a measure of the degree to which the emulsion has the neces- sary stability or instability. For ex- ample, •concentrates must be cap- able of dilution without breaking of the emulsion, while a cleansing cream must permit coalescence of oil droplets after a certain amount of water has evaporated from a thin layer of the cream. Before starting laboratory tests on emulsion stability it is well to con- sider the question o[ chemical sta- bility of the emulsion ingredients. If the emulsifying agent is an ester, is the alkalinity so high that slow saponification may occur ? Will there be a pH change due to evap- oration of a volatile amine ? Will traces of metals or irradiation by sunlight promote undesirable chemi- cal changes ? Knowing what to ex- pect in the way of chemical change enables one to carry out more intel- ligently other, sometimes lengthy, tests. Acceleration of an ageing process usually involves subjecting • the 195