336 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Such a solubility decrease can occur not only as a result of temperature increase or pressure decrease, but can also occur in the mixing ot5 two or more liquids. If the mixing results in a decrease of the solubility ot5 the dissolved gas, a liberation of the gas will take place until a new equilibrium is established. According to the experiments conducted by the author, this mech- anism is believed to be responsible for air bubble formation in the preparation of hydroalcoholic Carbopol©* gels (6). Mixing of ethanol and water, both saturated with air, will result in a decrease of the air solubility and liberation of air bubbles. The liberated air can escape from an ethanol-water solution readily. However, if an alcohol dis- persion of a Carbopol resin is neutralized with an aqueous solution of triethanolamine, the liberated free air bubbles may be trapped in the newly formed gel because of the high viscosity and yield value of the Carbopol gel. Thus, no matter how much care is taken to avoid ex- ternally entrained air, it is still possible to form air bubb!es by this mech- anism. To obtain a bubble-free gel, one can disperse Carbopol in a preblend of alcohol and water and neutralize the dispersion only after all the excess bubbles are freed. Since unneutralized Carbopol dispersions have very low viscosities, air liberated before neutralization can readi!y escape from the system. To hasten the bubble liberating process, the un- neutralized dispersion should be agitated with a mixer. A more rapid way to remove the excess bubbles is to pass the unneutra!ized dispersion through an ultrasonic machine. By a cavitation process, the excess air can be removed quickly and the gel obtained by neutralizing such a dispersion can be completely bubble-free, provided that no external air is entrained during the neutralization process. Finally, another possible mechanism of bubble formation as a result of a physical change is due to a density change. As some melted waxes are solidified, a density increase may occur and result in a product shrink- age. Sometimes, the wax mixture may harden before the full shrinkage takes place and result in a gradual formation of bubbles. An example would be the solidification of paraffin-based hair pomades. In some formulations, the full shrinkage may take several days resulting in the pulling away of the product from the sides of the jar or formation of many bubbles. Sometimes a gradual cooling rather than a rapid cooling * Carboxy vinyl resins, B. F. Goodrich Chemical Co., Cleveland, Ohio.

GAS BUBBLE FORMATION 337 will minimize the problem but frequently a reformulation will be re- quired to solve the proble•n completely. CONCLUSIONS Control of gas bubbles in cosmetic preparations is a difficult but ex- tremely important problem in making stable and aesthetically pleasing products. Although there is deaeration equipment * designed to remove gas bubbles from the products, it is generally best to prevent the gas from getting into the products in the first place. As has been shown, once the mechanisms and sources of bubble formation are correctly identified, it is generally possible to prescribe solutions which will prevent or minimize the problem. (Received October 21, 1970) REFERENCES (l) Martin, A. N., and Banker, G. S., Rheology, in Bean, H. S., Beckett, A. H., and Carless, j.E., A•'dvanc•s in Pharmd•euticM Sciences, Vol. 1, Academic Prfiss, New York, 1964, p. 58. (2) SterbJcek, Z., and Tausk, P., Mixing in the Chemical Industry, Pergamon Press, Oxford, 1965, pp. 278-82. (3) Lin, T. J., Rheology Fundamentals and Applications in Cosmetic Industry, in deNavarre, M. G., Chemistry and Manufacture of Cosmetics, Vol. 1, Van Nostrand, Princeton, N.J., 1962, pp. 332-6. (4) Lin, T. J., and Donnelly, It. G., Gas bubble entrainment by plunging laminar liquid jets, AIChE ]., 12, 563-71 (1966). (5) Lin, T. J., Ph.D. dissertation, Wayne State University, Detroit (1963) Univ. Microfilms, N. 64-9539, Ann Arbor, Mich. (6) Lin, T. J., Bubble formation in hydroalcoholic gels, ]. Soc. Cosmet. Chem., 20, 795-805 (1969). • Examples, Versatot© from Cornell Machine Co., Springfield, N.J., or Sontrifuge© from Teknika Inc., Dayton, Ohio.