AEROSOL EMULSIONS AND FOAMS 95 Figure 5. Photomicrographs of an emulsion (left) and foam (right) from a triethanolamine myristate/Freon 12/Freon 114 (40/60) system with an excess of triethanolamine Increase in Foam Bubble Size After Discharge After an aerosol emulsion has been discharged, the bubble size of the re- suiting foam continues to increase as the foam ages. This was reported previ- ously by Augsburger and Shangraw (2) and is shown in the present work by the data in Table II and the photomicrographs in Figs. 6 and 7 of the foam from the excess myristic acid system. The diameters of the smallest and lm-g- est bubbles measured on the photomicrographs of the foams from the two triethanolamine myfistate systems are listed for various aging periods. The data in Table II show that the triethanolamine myristate-myristic acid system with propellant droplets having smaller diameters produced foams that increased less rapidly in bubble size with age than those from the exccss triethanolamine system. This probably is due to the resistance to expansion by Table II Emulsion Droplet Size and Rate of Increase in Foam Bubble Size with Time (after 5% Discharge) Propellant Emulsion Droplet Size (/•) Excess Excess Myristic Acid Triethanolamine 2.0-30 2--100 Foam Bubble Size Excess Excess Time after Foara Discharge Myristic Acid Triethanolamine 10 sec 2--30 ... 20 sec 2--60 ... 40 sec 2--120 60 see 10--170 20' '•10 2 rain 30--200 40--200 5 rain 50--350 60--459 10 rain 50--440 120--700 30 min 90--450 Foam collapsed 60 rain 240-700

96 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Figure 6. Increase in bubble size with age of triethanolamine myristate/Freon propellant foam Top left. 10 seconds right, 20 seconds Bottom left. 40 seconds right, 60 seconds the strong interfacial film formed by the triethanolamine myristate-myristic acid complex. Augsburger and Shangraw (2) have listed four possible reasons why the bubble size of foams increases vith age. One, which the present author con- siders the most important, is that the initial discharge contains a mixture of the initial emulsified propellant droplets as well as foam bubbles formed by vaporized propellant. If some of the bubbles contained residual liquefied pro- pellant, continued vaporization of the entrapped residual liquefied propellant would increase the bubble size. The evidence for the existence of emulsified propellant droplets in the ini- tial discharge is somewhat circumstantial since it is difficult to distinguish be- tween a small bubble and an emulsified droplet in the photomicrographs. However, in Fig. 6, top right and bottom left, there are a number of small droplets around the interfacial areas of the bubbles which have a different ap- pearance and are much brighter than the foam bubbles. It is believed that these are emulsified propellant droplets. However, even stronger evidence for the existent presence of emulsified droplets in the initial discharge is suggested by the work of York and Veiner