BUBBLES IN GELS 797 25 F, 2.01 o _ o x _ 0 25 50 I00 (100%0) % A IN AO SOLUTION (0%B) Figure 1. Solubility curve of gas X in AB solution tion consisting of liquids A and B. The solubility curve of this gas, rep- resented by the solid line, is curved upward since the system is not an ideal solution. According to Fig. 1, if one takes 100 g of liquid A satu- rated with gas X (solubility = 2 g/100 g of A) and mixes it with 100 g of liquid B saturated with gas X (solubility = 1 g/100 g of B), he would have a total of 2 -+- 1 or 3 g of gas X in the system. However, this mix- ture, which is approximately a 50% solution of A (49.7%, to be exact), can dissolve only 1.2 g of the gas per 100 g of the liquid or 2.4 g per 200 g of the mixture. Therefore, approximately 0.6 g of the gas must escape from the mixture in order to establish an equilibrium under an isothermal condition. The above illustration implies that if the solubility of a gas in a mix- ture of two liquids is less than the value expected from the linear inter- polation of the solubilities in pure components, the gas can be liberated by mixing two saturated solutions together. While making clear Carbopol ©* gels containing water and alcohol, it was found that no matter how much care was exercised to prevent the externally entrained air, depending on the method of preparation, the finished gel could contain a considerable amount of air. This mecha- nism was considered a possible cause for the bubble formation and ex- periments were conducted to explore this theory using hydroalcoholic Carbopol gels. * Carbopol resins, B. F. Goodrich Chemical Co., Cleveland, Ohio.

798 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS EXPERIMENTAL In most experiments, 0.5% Carbopol 940 was used as the thickener to form hydroalcoholic gels. An equal amount of either Ethomeen C/2• ©* or triethanolamine was used as the neutralizer for Carbopol 940. De- natured, anhydrous ethyl alcohol (S.D.A. 40) or pure ethanol and deionized water were used in various proportions. In the experimental investigation, it was first assumed that the Car- bopol resin used had nothing to do with bubble formation except that its thickening action would trap the bubbles and prevent them from escap- ing into the atmosphere. If this were true, and if the mechanism involving a solubility de- crease as illustrated by Fig. 1 were indeed responsible for air bubble for- mation, one would observe generation of air bubbles when alcohol satu- rated with air was mixed with water saturated with air. Furthermore, the amount of air generated by such a mixture should be roughly equal to the amount trapped in the gel made by presaturating the water and alcohol with air. To test this hypothesis, two types of experiments were conducted. The first involved collection and measurement of the amount of air liberated by mixing various proportions of ethanol with water without a gelling agent under an isothermal condition. The second involved preparation of a series of Carbopol gels with varying water/ethanol proportions and determination of the bubble content after the systems had reached an equilibrium. A lcohol-Water Mixing Experiments The simple mixing of ethanol with water produced tiny gas bubbles which were visually observed. After the generated gas was collected and analyzed by a gas chromatographic method, it was confirmed that the liberated gas was air. However, since mixing of ethanol and water involved generation of heat, it was necessary to remove this heat to maintain the system under an isothermal condition. Figure 2 illustrates the setup used in the alcohol-water mixing ex- periments. The mixing chamber consisted of a 508-ml flask equipped with a precision thermometer, cooling coil, and a magnetic stirrer. The cooling coil was connected to a refrigerated constant-temperature bath through a pump, and the flow rate of the cooling water controlled by * Ethomeen C/25 (polyoxyethylene coco amine), Armour Industrial Chemical Co., Chi- cago, Ill.