Shampoos cleanse by utilizing synthetic detergents, also known as surfactants, which are amphiphilic. This means that the detergent molecule possesses both lipophilic, or oil-attracting, and hydrophilic, or water-attracting, sites. The lipophilic site binds to sebum and oil-soluble dirt while the hydrophilic site binds to water allowing removal of the sebum with water rinsing. There are four basic categories of shampoo detergents: anionics, cationics, amphoterics, and nonionics. Usually, a shampoo is a combination of two to four detergents with various abilities to remove sebum, produce foam, and condition the hair. Creating the perfect balance between hygiene and beautification is the goal of a successful shampoo.
Anionic detergents are the most popular cleanser in general purpose shampoos and are named for their negatively charged hydrophilic polar group. Anionic detergents are adept at removing sebum from the scalp, but leave the hair harsh, rough, subject to static electricity, dull, and difficult to detangle. Common anionic detergents include the lauryl sulfates, laureth sulfates, sarcosines, and sulfosuccinates. The second most popular detergents are the amphoterics, which contain both an anionic and a cationic group. This allows them to behave as cationic detergents at low pH and as anionic detergents at high pH. Within the amphoteric detergent category, there are several subgroups, which include the betaines, sultaines, and imidazolinium derivatives. Amphoteric detergents such as cocamidopropyl betaine and sodium lauraminopropionate are found in baby shampoos. These detergents actually numb the tissues of the eyes, which accounts for the non-stinging characteristics of baby shampoo. Amphoteric detergents are also used in shampoos for fine and chemically treated hair because they foam moderately well while leaving the hair manageable.
The main distinguishing characteristic between a bar cleanser and a shampoo is the addition of a sequestering agent. Sequestering agents function to chelate magnesium and calcium ions thereby preventing the formation of insoluble soaps, known as scum. Without sequestering agents, shampoos would leave a film on the hair, making it appear dull.
