Cosmetic chemists have at their disposal a seemingly endless supply of emulsifiers from which to choose. By far, the most popular category of emulsifiers is “soap” – the reaction product between a fatty acid typically stearic acid) and an alkali earth metal (typically sodium hydroxide, occasionally potassium hydroxide).
Sometimes either an amine (such as triethanolamine) or ammonium hydroxide is used, though ammonium hydroxide is used less often due to its odor and volatility. This volatility was actually used to an advantage years ago by the Noxell Company when the ammonium hydroxide was used to neutralize stearic acid to form the O/W emulsifier.
Over time, the ammonia volatilized off to leave an excess of free stearic acid. This stearic acid crystallized to solidify the emulsion. The result was the “crunch” when you stuck your finger into the Noxzema cream. Additionally, the excess stearic acid gave a pearlescent appearance, creating great marketing potential. The product as sold was quite stable, however when the cream was stirred, an unstable lotion was formed.
The lotion was unstable because there was no significant concentration of emulsifier (ammonium stearate) remaining. Emulsions that are formed by using sodium or potassium stearate tend to have a high pH (7.5-8.5) and have a tendency to build in viscosity over time. This is probably due to the formation of a gel network of liquid crystalline nature in the external phase. The reader should keep in mind that monovalent soaps form O/W emulsions while polyvalent soaps form W/O emulsions. Remember Bancroft’s Rule: the phase where the emulsifier is most soluble will become the external phase.
Soap emulsifiers have many positive attributes:
• They are quite powerful (efficient).
• They are cost-effective (inexpensive).
• They are reliable. (They will emulsify almost any oil.)
• They are temperature insensitive.
They also have a few signifi cant negatives:
• They must be used at a high pH (or else you don’t have much soap).
• They form emulsions that gel over time.
• They are incompatible with many (cationic) conditioning agents.
• They can dramatically increase the TEWL (Trans Epidermal Water Loss) by mobilizing the interstitial skin lipids.
• They can cause eye sting.
• They can cause skin irritation.
Nonionic emulsifiers (particularly the ethoxylates) address many of these negatives but have a few negatives of their own:
• They are very temperature sensitive. (Remember the PIT or Phase Inversion Temperature issue.)
• They associate with paraben preservatives (through hydrogen bonding between the para-hydroxyl group of the paraben molecule and the pendant oxygen from the ethylene oxide), increasing the likelihood for microbial attack.
• They work better for lotions than creams.
• They tend to be a bit inefficient.
• They can be a bit expensive.
Excerpt Only This is a shortened version or summary of the article that appeared in the Dec. 1, 2002 issue of Cosmetics & Toiletries magazine. The full content is not currently available online.