Surfactants in Hair Care Applications
What are Surfactants?
Surfactants are an essential part of most hair care products. They are widely used in washing, conditioning and styling formulations to perform the following functions:
- In shampoos as cleansing and foaming agents
- In hair-care products as conditioning agents
- In hair styling products as wetting agents
As the role of surfactant varies from one hair-care product to another, the type of surfactant suitable for the application also varies. A surfactant molecule consists of a hydrophobic part and a hydrophilic part. The hydrophobic part usually consists of a hydrocarbon chain of variable length between 8 to 18 carbon units long. The hydrocarbon chain length strikes the balance between mildness and detergency of the surfactant. Surfactants with shorter chain lengths have stronger grease removing properties; a longer chain length leads to increased mildness and lower lathering properties. The hydrophilic part of the molecule consists of various functional groups which determine the nature of the surfactant and its key properties.
Surfactants suitable for major hair-care applications
Shampoos
Shampoo is the basic hair care product with the largest market share of the hair care cosmetics market. Most hair contaminants, such as excess sebum (natural skin oil), dandruff, environmental contaminants, and residues of other hair care products are insoluble in water. These contaminants cannot be effectively removed by water alone. A good shampoo cleanses the scalp and hair by lifting these contaminants so that they may be washed off. Shampoo surfactants cause the formation of micelles which wash away oil and dirt from hair with water. The hydrophobic part of shampoo surfactants is typically short in length so that it does not cling to the hair and is easily washed off. Shampoo surfactants are generally anionic or amphoteric in nature. Anionic surfactants provide most of the detergency and lather in shampoos. Amphoteric surfactants are used to decrease the irritancy of shampoo formulations. They are also useful for increasing the active content level of the product and for improving the quality of lather. Mild, tearless shampoos may be based entirely on amphoterics. Non-ionic surfactants are generally not used for cleansing in shampoos due to their harsh detergency which may lead to excessive degreasing and cause scalp irritation. Some mild non-ionics are however suitable for use as foam-stabilizers and thickeners.
Conditioners
Conditioner surfactants are meant to deposit onto hair rather than being washed off like shampoo surfactants. A longer carbon chain length gives the surfactant molecules greater ability to cling to the hair surface. Consequently, they are not easily washed off during the rinsing process and form the basis of conditioning. Cationic surfactants (positively charged) are generally used in hair conditioning formulations. Shampoo surfactants leave a small negative charge on hair which lifts the hair cuticles. The positive charge on cationic surfactants helps remove the negative charge smoothing down the cuticle. The table below summarizes the benefits of cationic surfactants used in hair-conditioner formulations.