Final answer:
The primary surfactant is the second ingredient most shampoos have in common. Surfactants act as heavy-duty soap that helps trap oil and grease in the hair without stripping it too much.
Step-by-step explanation:
The second ingredient most shampoos have in common is the primary surfactant. Surfactants are substances that disrupt intermolecular interactions between molecules of a polar liquid, reducing the surface tension of the liquid.
In shampoos, surfactants like sodium lauryl sulfate or sodium laureth sulfate act as heavy-duty soap that helps trap oil and grease, making it easier to remove unwanted build-up of sebum in the hair without stripping it too much.