Final answer:
The second common ingredient in most shampoos is the primary surfactant or detergent, typically sodium lauryl sulfate or sodium laureth sulfate. These substances serve as detergents by reducing the surface tension of water, allowing the shampoo to remove grease and dirt from the hair.
Step-by-step explanation:
The second ingredient most shampoos have in common is the primary surfactant, also known as a detergent. Shampoos typically use surfactants like sodium lauryl sulfate or sodium laureth sulfate, combined with a co-surfactant such as cocamidopropyl betaine. These surfactants act as detergents, which are substances that contain molecules with a hydrophilic (water-attracting) 'head' and a hydrophobic (water-repelling) 'tail'. Because of this duality, surfactants are able to reduce the surface tension of water, helping to remove grease and dirt from the hair by emulsifying and suspending oil in the water.
While other ingredients like preservatives and fragrances are included in shampoo formulations for various functions, it is the sodium lauryl sulfate or sodium laureth sulfate that serves as the main cleaning component. These ingredients function similarly to soap but are more effective in hard water conditions. The presence of a surfactant as the second ingredient in shampoos helps to effectively cleanse hair by loosening sebum and other debris, making it easier to rinse away.