One of the most surprising things to me when I started a career as a cosmetic science was how subjective things were. There are no definitive answers about which surfactants to use, how much, or even what testing procedures make sense. Stability testing, which is an ubiquitous test that all cosmetic chemists will do, is not an exact science and decisions about whether something passes or fails are highly subjective.
This non-scientific tendency troubles my scientifically leaning mind.
However, while there is a lot of subjectivity in cosmetic science, there are some scientific principles behind the tests. Take cosmetic stability testing for example. One of the procedures for testing the stability of emulsions is to put a sample at an accelerated temperature and measure whether characteristics such as pH, viscosity or appearance significantly changes. There is a rough rule that says ….
3 months of storage at 37 – 45C equals 1 year of storage at RT
This rule is derived not only from experience but it is based on the Arrhenius equation. This equation gives a useful generalization that reaction rates of a chemical reaction double for every 10 degrees of increased temperature. So, if a formula is stable for 12 months at RT (25C), it would be stable at 35C for 6 months and 45C for 3 months.
Of course, technically speaking the Arrehenius equation is not really valid for emulsions. It predicts what happens when a specific chemical reaction happens at elevated temperatures. But when an emulsion is exposed to higher temperatures, there are reactions that occur which would not happen at room temperature. For example, at conditions above the melting point of some of the emulsified materials.
Not a waste of time
Just because it isn’t technically correct that doesn’t mean storing samples at accelerated temperatures isn’t useful or predictive. In fact, you should use elevated temperatures to predict stability. However, don’t be surprised when a formula seems perfectly stable after 3 months of high temperature storage but fails at room temperature after a year.