EMULSIONS AND FOAMS 383 TABLE II Stability of Aqueous Aerosol Systems" by Polyoxyethylene Fatty Ethers Composition Wt % in Polyoxyethylene Water Aqueous HLB Fatty Ether Solubility ø Phase Value POE (4) lauryl ether D POE (23) lauryl ether S POE (2) cetyl ether I POE (10) cetyl ether D POE (20) cetyl ether S POE (2) stearyl ether I POE (10) stearyl ether D POE (20) stearyl ether D 1 8 60 1 6 33 54 1 8 34 55 Stability (min) Emulsion c Foam a 9.7 1 5 16.9 1 5 5.3 30 120 12.9 1 15 15.7 1 5 4.9 3O 120 12.4 1 15 15.3 5 5 • Aerosol formulation: 90% aqueous phase-10% Freon © 12/Freon © 114 (40/60) propel- ]ant. Freon is du Pont's registered trademark for its fluorocarbons. 0 S, soluble D, dispersible I, insoluble. c Time to initial phase separation after shaking. a Time to first indication of foam collapse. and foams (26). None of the ethers is soluble in the liquefied gas pro- pellant at the concentrations used. The concentrations, shown in weight per cent, were approximately 0.05M in the aqueous phase. Only two polyoxyethylene fatty ethers, POE (2) cetyl ether and POE (2) stearyl ether, were effective emulsifying and foaming agents for the aerosol systems. These two agents also were the only surfactants insoluble in water. They could be dispersed to some extent in the pro- pellant which indicated that they were wetted to some degTee. According to their HLB values (27), these two sur[actants should be useful for the preparation of water-in-oil emulsions but in the present case they stabilize oil-in-water emulsions. These data suggest strongly that these surfactants function as solid stabilizers in aqueous aerosol emulsions and foams. The other surfactants, all o[ which are either water-soluble or dispersible, are probably ineffective because they do not have the proper wettability characteristics by the two phases. They are wetted too much by the aqueous phase and not enough by the propellant phase so that they re- main dispersed or dissolved in the aqueous phase instead of concentrating at the interface. The addition of certain fatty alcohols to some o[ the unstable poly- oxyethylene fatty ether systems increases emulsion and foam stability, as shown in Table III (26).

384 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Table III Effect of Alcohols in Polyoxyethylene (4) Lauryl Ether Systems- Stability (min) Alcohol Emulsion Foam None 1 5 Myristyl 30 120 Cetyl 30 120 Stearyl 30 120 Aerosol formulation: 90% aqueous phase and 10% Freon 12/Freon 114 (40/60) pro- pellant. The tool ratio of surfactant and alcohol is 1:1. The stabilizing effect of the alcohols is attributed to complex formation with the polyoxyethylene fatty ethers and subsequent forma- tion of a strong interfacial film. It seems likely that part of the reason the complexes are effective is that they are more hydrophobic than the surfactants alone and possess the solubility and wettability characteristics which cause them to collect at the propellant-water interface where they can h•nction as solid stabilizers. A q •teous Alcohol and Nonaqueous Aerosol Systems In addition to the aqueous foams, there are two other foam systems of importance in the aerosol field: aqueous ethyl alcohol (28, 29) and nonaqueous (30). As in the aqueous systems, the stability of the foam oerom the latter two systems is directly related to the solubility of the sur[actant in the concentrate. This is additional evidence that aerosol foams are solid stabilized foams. Aqueous ethyl alcohol foams are formulated with a mixture of water, ethyl alcohol, a surfactant (such as "Polawax"*), and propellant. The sta- bility of foams from this system is almost a direct function of the solu- bility of the sur[actant in the aqueous ethyl alcohol phase. Increasing the concentration of ethyl alcohol increases the solubility of the sur- factant Polawax in the concentrate and decreases foam stability cor- respondingly, as shown in Table IV (28). Polawax is a mixture of ethoxylated fatty alcohols, free fatty alcohols, and other additives, so that it probably is a complexed surfactant mix- ture with the right wettability and solubility properties for this foam sys- * Croda, Inc., New York, N.Y.