350 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS important to be sure that the mechanisms involved in causing coalescence under ultracentrifugal forces are the same as those operating under normal conditions. In my opinion it is really doubtful that this is ever the case since ultracentrifugal forces can distort droplets and place stresses on the emulsion barrier in a way not normally encountered. It may be argued that survival under such high stress assures survival under other less strenuous conditions, and thus assures an acceptable shelf-life. There is no doubt that the barrier at the droplet interface must be very good if it prevents coalescence under ultracentrifugal forces. However, it tells us nothing about the time period over which we can be assured of stability and it actually may eliminate normally acceptable emulsions. This is an example of a situation which exists in any accelerated test, i.e., a tendency to "overkill" the emulsions because the test used introduces a new mechanism of instability or causes an unreasonably high stress. By far the most widely used stress in emulsion testing is temperature. The rationale for increasing temperatures to predict instability comes from the well known relationship between the rate constant, k, for a chemical reaction and temperature, T, expressed as the Arrhenius equation. Simply plotting k vs. 1IT allows one to determine k at any temperature, as long as the mechanism of the reaction has not changed. Herein lies the critical question concerning the use of high temperatures to accelerate instability in emulsions. Is it proper to assume that high temperatures don't change some of the basic mechanisms involved in the instability process? Consider first some of the ways in which temperature may affect emulsions. These include changes in the viscosity of liquid phases solubility partitioning of molecules between both phases the melting and freezing of various materials, particularly waxes and the hydration of polymers and colloidal solids. Given that one or more of these processes is the primary factor in stabilizing the emulsion, it would not be surprising if a higher temperature abruptly eliminated this as the important stabilizing factor normally observed at lower temperatures. An example of this is seen in Figure 3, where an o 40,000 •'0,000 -- I0,000 - tu 6,000 - o• 4.,0oo - • 2,000 '"' 1,000 - 60O 4OO 2OO I00 • 0.04 tO øG • 25 øG 5oC oC 0.1 0.4 I 4 I0 40 I00 ELAPSED TIME (DAYS) Figure 3. Logarithm of Apparent Viscosity vs. Logarithm of Time for an o/w lotion at various temperatures (22).

STABILITY ASSESSMENT OF EMULSIONS 351 emulsion subjected to higher temperatures undergoes a dramatic decrease in apparent viscosity, presumably when the melting point of some of the waxes present is reached. The partitioning of surfactants between phases, likewise, can change with changing temperature in such a way as to create a situation unrelated to that at room temperature. This is not to say that stability testing should not be carried out at higher temperatures. Indeed, if these temperatures are or could be actually encountered such tests must be carried out. What I am saying, however, is that prediction of long-term stability from such high temperature studies is highly unlikely, particularly for emulsions containing waxes, polymers, and colloidal solids. Again, as in the case of centrifugation, survivors may be taken to be stable systems, but we still may reject good systems in the process and not really have a good handle on actual expected shelf-life. Freezing of emulsions is also a widely used form of stress, particularly in the form of a freeze-thaw cycling test. In addition to the effects due to temperature change discussed above, in this test we also have the freezing of water to form ice crystals. In general there are 3 factors which are important to recognize in interpreting the meaning of such tests (8,9). These are concentration and solidification of free liquid water concentration and precipitation of dissolved substances and thinning and disruption of the emulsifier film by ice crystals. The critical part of the test comes as one thaws the sample since thawing permits the release of water and its rapid movement throughout the emulsion. If the emulsifier film can "heal" itself upon release of the stresses induced by ice crystals before coalescence occurs, the system will survive the test. However, if the rate of redissolution of ingredients is too slow, for example, instability may occur in a manner not related to normal usage at higher temperatures, so use of this technique for predictive purposes still suffers from the uncertainties already discussed. In summary, with regard to predictive testing of emulsions, I would conclude that there really is no clear cut basis for expecting that accelerated studies allow extrapolation to normal storage conditions and quantitative expiration dating. Careful analysis of how increasing or decreasing temperatures might mechanistically effect various processes of importance in stabilizing emulsions could help one to choose procedures which best reflect anticipated effects. If, for example, we know waxes are present, and that their degree of crystallinity is important, we should build that into the type of stress test we develop. This obviously calls for much more research on the thermal properties of emulsions and their isolated parts. RHEOLOGICAL EVALUATION OF EMULSIONS Since the properties of cosmetic emulsions, be they pourable liquids or semi-solid creams, depend primarily on the extent of structure developed through particle-particle interactions, any change in this structure can be considered a form of instability or an indication of some type of instability. Changes in this structure may be brought about, for example, by such processes as creaming sedimentation, coalescence fiocculation crystallization or melting of waxes hydradon changes in colloidal solids like clays or changes in the solution state (aggregation) and concentration of surfactants and polymers in the two bulk phases. In view of the importance of structure in emulsion system, it follows that any technique used to monitor instability should be carried out on the finished product rather than on isolated parts of the system or after the system has been diluted so as to change the real