AEROSOL EMULSION SYSTEMS 275 OIL-IN-WATER EMULSIONS Aerosol products formulated as oil-in-water emulsions have been on the market since about 1950. These products are the well-known and widely accepted shaving lathers. In these products, a relatively small amount of propellent, usually about 8 to 10 per cent, is emulsified in an aqueous soap solution. As the emulsion is discharged from the foam valve, the pro- pellent expands, forming countless small bubbles. These give the rich lather characteristic of this type of system. Attempts to obtain fine sprays from such systems by increasing the proportion of propellent and using a spray valve instead of a foam valve result in products which stream. Excessive foaming occurs when the stream impinges on a surface. A series of oil-in-water formulations (4) has been developed that give relatively wet sprays and are essentially non- foaming. These formulations have low concentrations of propellent, usually less than 5 per cent. By the use of auxiliary solvents, such as ethyl alcohol, almost transparent systems are obtained. These systems have the advantage that no creaming occurs during standing and shaking of the formulation before use is not necessary. Window cleaning formulations employing the low propellent oil-in- water system have been marketed for several years and have been well received. These formulations give a soft spray and foam slightly on glass. WATER-IN-OIL E•ULS•ONS During the past several years the Du Pont Company has carried out studies on aqueous-based aerosol systems in an attempt to extend the range of spray characteristics of the water-based aerosol products. This has been found to be possible with the water-in-oil emulsion systems. With such systems, spray properties varying from coarse to very fine may be achieved (5). It was considered initially that the water-in-oil emulsions should have a fairly good chance of producing nonfoaming sprays. In such systems the water is dispersed throughout the propellent. As the emulsion is forced through the spray actuator, the propellent evaporates, leaving the water droplets, the emulsifying agent, and any auxiliary solvents present. Since very little propellent is dissolved in the aqueous droplets, there is less tendency for the vaporizing propellent to cause foam formation with the droplets. Likewise, since the emulsifying agents suitable for water-in-oil emulsions are generally oil soluble, they are usually poor foaming agents for aqueous solutions. In choosing the series of emulsifying agents to study, considerable use was made of the list of synthetic detergents assembled by McCutcheon (6). A wide variety of agents were chosen from this list with the selection

276 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS confined generally to the oil-soluble or known water-in-oil emulsifying agents. The ultimate selection of the most satisfactory emulsifying agents was based upon the stability of the emulsions, the lack of foaming with the sprays, and the lack of corrosion resulting from the water-in-oil emul- sions in metal containers. Of all the agents that were investigated, the polyglycerol esters of the fatty acids were found to be the most satisfactory. Surface-active agents of this type are Eracol 14 (Emulsol Chemical Company) and Soleonic PGE (Sole Chemical Company). Era col 14, a water-in-oil emulsifying agent, was used exclusively for the preparation of the emulsions described in this paper. In addition to its emulsifying properties, it is reported to be an antifoaming agent (6). Another effective water-in-oil emulsifying agent for the aerosol systems was Span 20 (sorbitan monolaurate, Atlas Powder Company). This material also is reported to possess antifoaming properties (7). There are, without doubt, many other surface active agents that would also be effective for preparing aerosol water-in-oil emulsion systems. However, in view of the large variety and number of agents that are commercially available and the time required to evaluate a given agent, it was not possible in the present study to test all materials that conceivably might have been effective. EVALUATION OF EMULSION STABILITY AND SPRAY CHARACTERISTICS The true stability of the emulsion systems as determined by size-fre- quency analysis was not determined. However, visual observations of the creamed-aqueous phase at various intervals showed that no observable coalescence of the dispersed water drops had occurred with the emulsions that are discussed in the present paper. Although creaming in an emulsion is not a sign that the emulsion is unstable, it is commercially important because an emulsion that has creamed must be shaken before use. In the present case, rate of creaming is indicated by "separation times." This is the time interval after shaking of the emulsions before visible separation of the two phases occurs. Sepa- ration times were determined within one hour after preparation of the emulsions. In many cases it was observed that the separation times for a given series of emulsions changed considerably after the emulsions had aged. The emulsions were packaged in coated glass bottles for visual observa- tion. The spray characteristics of the emulsions as reported in the tables were determined with a standard glass-bottle valve. In practically all cases, changing from standard actuators to mechanical break-up type of actuators will produce a much finer spray. In many instances, emulsions