While it doesn’t come up too often when creating cosmetic formulations, it is useful for cosmetic chemists to know the concept of Traube’s Rule. At the very least it will help you understand why some of your raw materials contain one surfactant over another and could give you a clue on how to fix potential stability problems.
Traube’s Rule is a relationship between hydrocarbon chain length and surfactant activity. It states that for every extra CH2 group in a surfactant molecule, the surface activity approximately triples.
So, shorter chain length molecules have less surface activity than higher chain length molecules. In terms of cleansing or emulsification that means you can get away with using a much lower concentration of a longer chain length surfactant.
Let’s look at the example of Sodium Lauryl Sulfate (C12) and Sodium Cetyl Sulfate (C16).
Using Traube’s Rule we know that the Sodium Cetyl Sulfate will have 81 times the surface activity of Sodium Lauryl Sulfate at the same concentration. Theoretically, that would be a big cost savings if you switched over your formulas!
But alas, it doesn’t quite work that way. You see Traube’s Rule only applies to dilute aqueous solutions of surfactants. Solutions that are so dilute that the surfactants don’t form micelles. That wouldn’t apply to most cosmetics.
However, this represents the edges of scientific research and it might be interesting to do an experiment to see what the lowest level of surfactant you could use to have a functional, consumer acceptable cleansing product. Maybe Traube’s Rule could help.
Decreasing surface activity
Incidentally, there is a similar but opposite effect if you increase the length of the hydrocarbon chain using ethylene oxide which is hydrophilic. That means molecules with higher numbers of PEG will be more water soluble than ones with lower numbers.
Interested in more detail about this and other surfactant subjects? Check out this link.