PRODUCT STABiLITY--PART II 317 homo•enization pressure. The presence of internal wax did not help, althouõh the density adjustment did in some cases. The emulsions were stored at room temperature and 45øC to hasten deterioration, after which a microscope was used to measure the diameters of 900 to 1000 õ1obules to obtain the size frequency classification. The õ1obules were divided into size õroups, and the midpoints of these õroups were consid- ered to be the average diameters. Considering the õ1obules to be spheres, the volume and area were calculated, and these were then used to obtain the area per unit volume or the specific interfacial area. When plotted aõainst time this parameter, of course, showed a decrease. Sherman (33) pointed out the influence of the kinetics of globular coaõulation or aggregation (precursors to coalescence) on the rheoloõical properties of aging emulsions. His goal was to predict viscosity changes over Ionõ aging periods. Since the chanões depend on õ1obule size, he made emulsions of various sizes to õeta picture of what would happen in the future. He noted that this technique may be better than centrifuõa- tion or hiõh temperature storaõe, even thouõh such stresses may produce an increase in averaõe õ1obule size. He interrelated many measure- ments and developed a parameter, called the inhomoõeneity factor, which increases with time. The factor essentially converted data ob- tained by measurinõ 2000 õ1obules to a distribution parameter which accounts for globule number, size, and area. Also as a further and more detailed comparison, Sherman calculated the averaõe distance between õ1obules which is also related to the averaõe diameter. Finally, this averaõe distance of separation was related to viscosity data, which means that õ1obule diameter is related to viscosity. Thus, if õ1obule diameter increases and has an effect on viscosity, both old and fresh emulsions of the same õ1obule size should have the same viscosity. Sherman found that when relative viscosity was plotted aõainst the distance separatinõ the õ1obules, or aõainst time, the data from both new and old emulsions could be superimposed thus viscosity chanões over long aging periods could be predicted. This discussion of finished emulsions and coalescence is somewhat reminiscent of the coalescence time test of Cockbain and McRoberts (34). In essence, their test requires that the emulsion be put toõether in two parts: the oil phase is layered over the water phase after which an oil drop is introduced from the bottom underneath the water. After the drop rises to the interface, one checks to see how Ionõ it takes for it to coalesce. Of course, the lonõer this time, the more stable the completed emulsion is considered to be.

318 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Lloyd (35) related the per cent reflectance of color and the surface average diameter obtained microscopically in a study of the change in distribution pattern of the globule sizes of Of W emulsions. A straight line plot resulted when the log per cent reflectance was plotted against the log surface average diameter. The latter quantity, which of course is inversely related to interfacial area, is then also capable of relating stability to reflectance. We note also that interfacial area simulates a concentration factor as it can be expressed as interfacial area per unit volume of oil. Lloyd found that a plot of log diameter rs. time gave a curve which rose steeply in first order fashion and then reached a plateau which indicated limiting of the coalescence. Menczel, Rabinovitz, and Madjor (36) used a selectively soluble dye (soluble in only one of the phases as used for the differentiation of emul- sion types) to color their test emulsions with an exact amount of such a dye. They then developed a colorimetric method to determine volumes of separated internal phase at various times. The rate of this separation is then equated to the rate of de-emulsification. They employed a dye soluble in the internal phase and used a separatory funnel to collect the separated portion from the emulsion bulk. The internal phase was thus collected at various times from a series of preparations its volume was determined colorimetrically by measuring the dye concentration with a photoelectric colorimeter. The volumes were converted to per cent separation. A straight line log-log relationship was found by plotting the log of the per cent separation vs. log time. A family of lines re- suited the position of any particular line of the family then is an indica- tion of the stability rank of the particular emulsion. Void and Groot (37) developed an ultracentrifugal method to de- termine emulsion stability. They investigated the utility of an ultra- centrifuge which permitted observation of the emulsion while it was being rotated. When the per cent of oil separated was plotted vs. the time of centrifugation, a plateau curve was obtained. It was noted that the separation rate was very rapid at first, then slower and more-or-less constant while 20 to 60% of the oil separated, and finally, the rate was slow. Naturally, increased speeds caused higher rates of oil separation. Centrifugation speeds up to 56,100 r.p.m. were employed. Garrett (38) also studied the use of the ultracentrifuge in predicting pharmaceu- tical emulsion stability, especially as it pertains to oil flotation or cream- ing. A linear graph resulted when flotation rate was plotted rs. the square of the centrifuge revolutions per minute. The flotation rate is represented by the slope of the line which results when the log distance of