Surfactants can be found in every cleansing product. They dissolve fat and dirt and can in many cases be drying and irritating for the skin. But there is a big difference between surfactants and surfactants. The main difference lies in the water-soluble part and its charge.
As you may remember from the last article "Understanding surfactants", surfactants are cleansing agents that can be found in all cleaning products. They consist of a water-loving (hydrophilic) head and a fat-loving (lipophilic) tail, which gives them the ability to surround the dirt and to be rinsed away with water. The water-loving head is charged in different ways, which affects the behavior of the surfactant.
Depending on the charge of the head, surfactants are divided into four different groups:
- Anionic - negatively charged
- Cationic - positively charged
- Uncharged - without charge
- Zwitterionic - both positively and negatively charged
Negatively charged surfactants having sulfate as their head group, are generally harsh on the skin. Their negative charge disrupts the structure and function of skin proteins. Depending on the size of the head group, they can to various extent penetrate the outer layer of the skin (stratum corneum) and break the oily skin barrier. This results in loss of water and water-retaining molecules (NMF) and dried out skin. Prolonged exposure breaks the cells on the basal lamina, triggering an inflammatory response, leading to red, dry and flaky skin. Despite this, this group is the most common in all cosmetic cleansing products and include sodium lauryl sulfate (SLS) and sodium laureth sulfate (SLES). How irritating a surfactant is for the skin also depends on how easy it is to wash away. Surfactants with sulfonate as their head group are more water soluble than sulfate surfactants. This is why sulfonate surfactants such as Disodium laureth sulfosuccinate and Sodium lauryl sulfoacetate, are much less irritating to the skin than Sodium lauryl sulfate and Sodium laureth sulfate, despite the names being confusingly alike.
Regular soap is also part of the group anionic surfactants since soap surfactants have a negative carboxyl as their head group. Soap surfactants such as Sodium palmitate and Sodium laurate are less irritating than sulfate surfactants but much more irritating than real traditional soap or handcrafted soap, manufactured by cold or hot process of various fats and therefore naturally containing glycerin and superfat oils that hydrates the skin. However, no matter the fabrication method, the carboxyl group in soap surfactants have the ability to bind calcium ions (Ca2+), which affects the lather of the soap in hard water. This is also the reason why soap is difficult to use as a shampoo in hard water. Hair is negatively charged and binds strongly to the soap-calcium complex, which makes it difficult to rinse out. Washing your hair with soap usually requires a mild acid rinse afterwards, with for example vinegar.
Cationic surfactants are positively charged and bind strongly, both to hair keratin and to the lipid layer of the skin, which are both negatively charged. Therefore they are not at all appropriate as cleansers and cationic surfactants are in fact used in hair conditioners. When you wash your hair with anionic surfactants you also wash away natural oils and other positively charged substances bound to hair keratin. This leaves the hair dry and static since the exposed negative charges pushes each other away. Cationic surfactants create a thin oily film around each hair, which has a softening and anti-static effect. Common cationic surfactants in hair conditioners are Cetrimonium chloride, Distearoylethyl dimonium chloride and Behentrimonium chloride.
Uncharged surfactans on the other hand, are generally mild and clean without destroying the structure of skin proteins. However, they are usually too mild cleaners to be used on their own, except in baby products and facial cleaners. Commonly, a mixture of various uncharged surfactants is used, sometimes together with zwitterionic surfactants, to obtain a better cleansing and lathering effect. Most uncharged surfactants are either sugar-based, for example Coco glycoside, Lauryl glycoside and Polysorbat 20 or are fatty alcohols such as Cetyl alcohol and Lauryl alcohol.
Zwitterionic surfactants, also called amphoteric surfactants, contain both a positive and a negative charge. They are milder than anionic surfactants but more efficient than the non-ionic ones. They are rarely used alone. Normally they are either mixed with non-ionic surfactants to boost their efficiency or together with anionic surfactants to reduce the skin irritation. Examples of common zwitterionic surfactants are Cocamidopropyl betaine, Sodium lauroamphoacetate and amino acid-based surfactants such as Sodium cocoyl glutamate.