AEROSOL EMULSION SYSTEMS 283 oil-in-water type. The separation time of the former was generally greater than an hour with creaming occurring from tile bottom while tile separation time of the latter was less than a mh•utc with creaming occurring from the top. This was probably a case of the formation of dual emulsions (emulsions with the same liquids and emulsifying agents but havir•g opposite phase types) (10), where the type of emulsio• that was obtained depended upon how the bottle was shaken. This phenomenon, where the phase type of the emulsion is determined by the type of agitation, was first observed by Ostwald (11) and later confirmed by Cheesman and King (12). The electrical conductivity of the emulsions was found generally to be a reliable means for determining the phase of the propellent/water emulsions. In most of the cases investigated the water-in-oil emulsions were essentially nonconducting as the inversion point was approached and became con- ducting after inversion had occurred. However, with "12"/"11" (30/ 70)/water emulsions, it was observed that the emulsion containing 40 per cent propellent and 60 per cent water exhibited an electrical conductance intermediate between that of the nonconducting emulsions with 60 per cent propellent/40 per cent water and the conducting 20/80 emulsion. It is possible that in cases of this type, application of the electric current caused partial inversion of the water-in-oil emulsion to take place. This effect with water-in-oil systems has been reported by Dixon and Bennet- Clark (13). Particle Size of Emulsion Sprays At the present time, very little information on the particle size of the emulsion sprays is available. Attempts to determine the particle size of the emulsions at the "Freon" Products Laboratory by the standard tech- nique involving the rotating slide and wind tunnel (14) have not been successful as yet. In the cases studied, the water evaporated before the droplets reached the slide. In the water-in-oil propellent/water emulsions, the particle size of the sprays is probably controlled to a considerable extent by the degree of dispersion of the water droplets in the propellent. If this is true, the particle size from a given formulation will depend upon the method and efficiency of emulsification. An additional factor probably should be considered and that is the effect upon the emulsion stability resulting from the passage of the emul- sion through a valve. In the emulsions containing a high proportion of water, the droplets are close together. It appears likely that during the violent agitation occurring in the emulsions during the passage through the valve and immediately following their exit from the outer orifice, considerably more coalescence of the water droplets would occur with the concentrated emulsions than with the dilute emulsions. If this happens,

284 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS then the particle size of the sprays from the concentrated emulsions will be much larger than those from the dilute emulsions regardless of the fact that the size of the dispersed water droplets was the same in both emulsions before spraying. The picture becomes even more complicated in emulsions formulated with an auxiliary solvent, such as odorless mineral spirits. In this case the propellent is dissolved in the solvent and the flashing of the propellent during spraying breaks the solvent into fine particles. The particle size of the sprayed solvent will depend therefore upon the ratio of propellent to solvent, as in any homogeneous system. What effect the break-up of the solvent has upon the dispersed water droplets is not known. It is possible that the auxiliary solvent and water are sprayed as separate droplets. However, if this occurs, then re-emulsification occurs when the two phases impinge upon a surface. Experiments show that an emulsion is obtained when a propellent/solvent/water emulsion is sprayed into a container. Corrosion Studies with lgf ater-in-Oil Emulsions The combination of water and surface-active agents in aerosol systems always introduces the possibility of corrosion of metal containers. The difficulties 'hat have been encountered in attempts to package aerosol shampoos based on fatty alcohol sulfates are well known. On the other hand, aerosol shave lathers based on fatty acid soaps have an excellent record with respect to corrosion. The basic water-in-oil emulsion systems discussed in the present report appear generally to have remarkable stability as far as corrosion of metal containers is concerned. The emulsions were packaged in the following containers for the specified storage temperatures and times: 1. Crown lacquer-lined tin-plate cans equipped with Precision nylon- brass valves Aging periods: 2 months at 130øF. 1 year at room temperature 2. Continental tin-plate cans equipped with Precision nylon-brass valves Aging periods: 6 months at 100øF. 1 year at room temperature The particular emulsion systems that were evaluated were as follows: 1. Propellent"12"/deodorized kerosene/water--60/20/20ratio 2. Propellents "12"/"114" (15/85)/"Nujol"/water--60/20/20 ratio 3. Propellents "12"/"11" (30/70)/"Nujol"/water--60/20/20 ratio 4. Propellents "12"/"11" (30/70)/water--80/20 and 60/40 ratios The first three emulsions systems produced no observable corrosion in any of the containers under any of the storage conditions. The fourth