In our cosmetic surfactant series we’ve talked about what surfactants are and what they do in your formulations. In this entry we’ll give a brief overview of the different types of surfactants found in cosmetic products.
Classified by Charge
If you recall in the previous article, surfactants are molecules that have both a polar portion and a nonpolar portion. Since polar molecules have an imbalance of charge, one side of the molecule has a more negative charge while the other side has a more positive charge. In the case of some surfactants this charge can be so strong that it actually forms a salt with a counterion. The nature of the counterion is the basis for one classification of surfactants. Basically, surfactants can be put into four different categories.
- Anionic surfactants - These are surfactants that have a negative charge.
- Cationic surfactants - These are ones with a positive charge
- Zitterionic surfactants - Also called Amphoteric surfactants these can have a positive or negative charge depending on the environment in which they exist.
- Nonionic surfactants - These are surfactants that have no strong charge.
A classic example of an anionic surfactant used in the cosmetic industry is Sodium Lauryl Sulfate. The surfactant portion of the molecule is Lauryl Sulfate which has a negative charge. In this surfactant, the Lauryl part is the nonpolar end while the Sulfate is the polar end. Sodium (which has a positive charge) is the counterion. These types of surfactants are great for cleansing systems like shampoos, body washes and facial washes which is typically where you find them used.
Sometimes the surfactant has a positive charge when put in an aqueous solution. A common type of cationic surfactant is Cetrimonium Chloride. In this molecule, the surfactant portion, Cetrimonium, is positively charged while the counterion, Chloride, is negatively charged. These surfactants have an ability to form electrostatic bonds with damaged protein (negatively charged) which makes the surfactant “stick” to a surface. This makes them great for products that are meant to condition a surface of skin or hair.
Some surfactant molecules have the ability to form both positive and negative charges when put in a solution. This is due to having both a positive ion such as an amide group on the molecule and a negative ion like a carboxylic acid. Depending on the pH of the overall solution, the surfactant may have an overall positive charge or a negative charge. A classic example of an amphoteric surfactant is Cocamidopropyl Betaine. These surfactants are typically used as secondary surfactants because they can boost foam and reduce irritation. They may also be used for gentle cleansing but other surfactants work better.
Finally, there are some surfactants that do not form ions when placed in an aqueous solution. These are nonionic surfactants. Depending on the molecular structure these surfactants are more or less polar without charge. Typically, they contain a number of -OH groups. These groups will strongly associate with water molecules while the rest of the molecule contains long chain hydrocarbons which associate with the nonpolar materials in the solution. Nonionic surfactants are used for solubilizing oils and fragrances but they can also be the basis for low foaming, gentle cleansing systems like baby shampoos.
The different classes of surfactants all have the ability to form structures in solutions so on some level they can all work for the uses of surfactants mentioned in the previous article. But some of these surfactants certainly work better for some applications than others. In a future post we’ll go into more detail about each of the different types of surfactants and when they are used in formulating cosmetics.