40 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS preciably influence the stability of the completed emulsion. With this in mind, it was decided to study the small and often overlooked variations in techniques used in lotion manufacture and to try and establish with a degree of certainty the best way of preparing cosmetic emulsions. It is no secret that the best way to establish uniformity of products is to eliminate all variables in their manufacture, and considering the many possible variable factors existing in emulsion production at least ten chief factors which may affect the final product with respect to its physical prop- erties are found: the purity of ingredients, the method of mixing (includ- ing the speed of stirring, duration of stirring, type mixer used, and order of mixing), the temperature of the two phases of the time of emulsi- fication, the types of emulsifiers used, the type of emulsion formed, the ratio of the internal to the external phase, the amount of entrapped air, and the size of the batch made. Obviously, to study all of these variables on any one formula would en- tail years of experimentation, and even then any results would have to be confined to the particular type of emulsion studied. Our investigations were conducted with a simple formula containing mineral oil as the dis- persed phase and using a non-ionic emulsifier. The formula was made as simple as possible and was developed for this particular experiment. It was not intended to produce an ideal emulsion because we wanted to be able to detect and measure changes in the product, either as improvements or as degradations as they were evidenced by altering some of the previously mentioned factors. The formula does produce an average o/w emulsion which has cosmetic possibilities. A stearic acid emulsion was first consid- ered, but was rejected as a test emulsion because the chief objective was to study physical factors only without the introduction of variables due to chemical reaction in forming a soap in the emulsion. The formula for the emulsion used in all tests is as follows: Heavy Mineral Oil ..... Lanolin Anhydrous .... Cetyl Alcohol ......... 35.0% 1.0% 1.0% Span 80 ............. 2.1% Tween 80 ........... 4.9% Distilled Water ...... 56.0% The specific factors studied were: the order of mixing the two phases, the temperature at the time of emulsification, and variations in the speed of stirring. These objectives were sought: (1) to determine whether the oily phase should be added to the aqueous phase or vice versa, or whether all ingredients could be put together at once and emulsified (2) to determine whether continuous stirring or intermittent stirring produces the more stable emulsion and (3) to determine the effect on stability of an emulsion by emulsifying it hot against that of emulsifying it cold. A total of seven emulsions was prepared from the above formula, and each emulsion was made three times. In each instance, the finished products were allowed

FACTORS ON THE FORMATION OF COSMETIC EMULSIONS 41 to stand for a twenty-four hour period before any readings were taken. The following properties were then determined: viscosity, surface tension, pH, specific gravity, per cent creaming, time of creaming, particle size dis- tribut'ion, and over-all appearance and stability. Measurements were taken for each of the three samples made by each method and the figures reported are an average. In no case, however, did the readings vary more than 5 per cent. The seven different methods of making this emulsion may be briefly outlined as follows: 1. I/F to 0 Slow. The oil and water phases, each containing a preferential emulsifier, were heated separately to 75øC. and the aqueous phase was poured slowly into the oily phase with continuous stirring with a power mixer at a speed of 600-700 r.p.m. The pouring was done over a 1S-second period while the total mixing time was one minute. 2. I/F to 0 Fast. All procedures were the same as those used in the first method except that the speed of stirring was increased to 1300-1400 r.p.m. 3. I/F to 0 Intermittent. The oil and water phases were heated to 75øC. as in the previous methods, and the aqueous phase was poured into the oily phase with simultaneous mixing at 600-700 r.p.m. for 15 seconds and then discontinued for an equal period of time. This was followed by another IS-second period of mixing and another period of rest for a total mixing time of one minute. 4. 0 to IF Slow. The exact procedure used in method (1) was followed except that the order of mixing was reversed so that the oily phase was poured into the aqueous phase. 5. 0 and IF Combined. All of the materials were put together cold in one container, heated to 75øC., and then mixed with the power mixer at a speed of 600-700 r.p.m. for one minute. 6. IF to 0 Cold. The oil and water phases were heated separately to 75øC. and allowed to cool to room temperature. The water was then added to the oil phase over a 1S-second period with steady stirring at 600-700 r.p.m. for a total mixing time of one minute. 7. The same procedure used in (6) was followed except that the reverse order of mixing was employed, pouring the oily phase into the aqueous phase. The speed of stirring was measured with a stroboscope. A label was affixed to the shaft of the three-bladed propellot and when the printing on the label appeared to stand "still" the shaft speed could be determined from the stroboscope scale. A varaic was used to adjust the speed of the propellot. However, the shaft speed could not be closely controlled due to apparent fluctuations in the circuit. The propellot speed could be controlled within limits and the speeds were chosen as shown above. After the emulsions had stood for twenty-four hours, those physical prop- erties which might conceivably be affected by variations in techniques were