98 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Other processes causing an increase in bubble size occurred at the same time. Continual coalescence of the bubbles was observed. This caused an in- crease in bubble size, but a decrease in the number of bubbles. This is partial- ly a function of the strength of the interfacial film around the bubbles. The triethanolamine myristate-myristic acid complex forms a strong interfacial film that retards bubble coalescence. This is probably why the bubbles from the excess myristic acid system increase in size less rapidly with age than those from the system with excess triethanolmnine, in which the interfacial films are weak. Another reason that has been postulated for growth of bubbles in a foam is that because of the pressure differential in small and large bubbles, the large bubbles grow larger and the small bubbles become smaller as the foam ages (21). The pressure inside a small bubble is higher than that inside a larger bubble. This difference causes the gas molecules in the smaller bubbles to dif- fuse into the liquid films and enter the larger bubbles. The total pressure, P•, in a gas bubble in a foam is equal to atmospheric pressure, P•, plus a factor, Ap. P• = P• + AP The factor, AP, is a function of surface tension T, and the radius of the gas bubble, R. AP- 2y R Therefore, the total pressure inside the gas bubble is: 2y P• = P• + • The change in bubble size due to this factor is also affected by the strength of the interfacial film. It has been shown that the diffusion of gas bubbles through a strong interfacial film of the type formed by the triethanolamine myristate-myristic acid complex is much slower than that through the weaker interfacial films of systems without complexes (1, 21). This may be an addi- tional reason why the growth of bubbles in the excess myristic acid system is slower than that in the excess triethanolamine system. Effect of Discharge on Emulsion Droplet and Foam Bubble Size The properties of aerosol foam products change with discharge (2, 7, 15). As the product is discharged, the volume of the vapor phase in the aerosol container increases with respect to that of the liquid phase. Propellant then migrates from the liquid to the vapor phase to maintain equilibrium. This de- creases the concentration of propellant in the liquid phase. Since foam proper-

AEROSOL EMULSIONS AND FOAMS 99 Table III Effect of Discharge on Emulsion Droplet and Foam Bubble Size Emulsion Droplet Size Range (t•) a Excess Excess % Discharge Myristic Acid Triethanolamine 5 2.0--30 0.5--100 50 2.0--10 0.5--70 75 2.0--10 0.5--40 Foam Bubble Size Range 5 10-105 10--180 50 10--105 10-180 75 10-160 15-250 Average Bubble Size (t•) 5 52 •63 50 41 61 75 41 55 aAverage of 5 determinations. øAverage of 3 determinations, 1 minute after discharge. CAverage of diameters of 50 bubbles on a photomicrograph, one measurement. ties are a function of the concentration of propellant in the liquid phase, the properties therefore change with discharge. Photomicrographs of the emulsions m•d foams from the system with excess myristic acid and triethanolamine were taken after product discharges of 5, 50, and 75%. The same container was used for all three discharges. The ranges of emulsion droplet and foam bubble diameters, and the average bubble size, are listed in Table III. Photomicrographs of the emulsified propellant droplets in the excess myristic acid emulsion are shown in Fig. 8. The emulsified propellant droplets decrease both in size and concentration, as would be predicted. As the product is discharged, the remaining emulsified droplets lose propellant molecules to the vapor phase and the smaller droplets probably disappear completely. Augsburger and Shangraw reported previously that as an aerosol foam is discharged, the average bubble size decreased (2). This would follow as a consequence in the decrease in the size of the emulsified droplets with dis- charge. This relationship is more noticeable in Table III in the system with excess triethm•olamine, where the change in emulsion droplet size is larger than that with the aerosol with excess myristic acid. One surprising phenomenon is that the foam bubble size range increases with discharge. This occurs with both the excess myristic acid and excess tri- ethanolamine systems. This indicates a lack of relationship between the aver- age bubble size of the foams and the range of bubble sizes. CONCLUSIONS A study of two triethanolamine myristate/Freon 12/Freon 114 (40/60) propellant emulsions (one with excess myristic acid and the other with excess