Sebum
Beneath your scalp are tiny glands called sebacious glands. These glands produce a natural oil called sebum, which provides lubrication and protection to the hair and scalp. Although sebum is meant to protect the hair, it also attracts layers of dirt and dead skin cells from the scalp. To remove the dirt and other unwanted substances, shampoo uses surfactants to separate the sebum from the hair strand.
Surfactants contain both hydrophilic and lipophylic qualities. The hydrophilic qualities of the surfactant attract water, while its lipophylic qualities attract oil. As you lather the shampoo, the two qualities create a "push and pull" effect on the hair strands, loosening the sebum and dirt from your hair.
Strength
Soaps tend to have strong surfactants, while shampoos need gentler surfactants, called anionic surfactants. This is because the stronger a surfactant is, the more oil it removes from the surface it's used on. With hair, removing too much oil can leave the hair prone to damage. This is why using body soap on your hair can lead to dry and damaged hair strands.
Process
Shampoos most commonly use an anionic surfactant called sodium lauryl sulfate. Hair strands tend to repel water but stick to oils, such as sebum. This makes removal of sebum difficult with just a water rinse. Sodium lauryl sulfate helps reduce the surface tension of the sebum on the hair shaft, making it easier for the oil and its attached dirt and grime to be removed. The oil and grime are then solubilized (dissolved) into the shampoo and can be rinsed out of the hair with water.
Other Ingredients
Shampoos employ a host of other ingredients to deal with the many unique issues that can come with hair. For example, a shampoo may contain active ingredients such as selenium sulfide to treat hair damage. They may also contain amino acids or vitamins to strengthen and moisturize the hair and scalp. Some shampoos add scent aromatherapy and scent technology to cater to specific customer tastes, such as tea-tree oil, lavender, mint or floral tones.
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