How Shampoo Works

Shampoo cleans by stripping sebum from the hair. Sebum is an oil secreted by hair follicles that is readily absorbed by the strands of hair, and forms a protective layer. Sebum protects the protein structure of hair from damage, but this protection comes at a cost. It tends to collect dirt, styling products and scalp flakes. Surfactants strip the sebum from the hair shafts and thereby remove the dirt attached to it.

While both soaps and shampoos contain surfactants, soap bonds to oils with such affinity that it removes too much if used on hair. Shampoo uses a different class of surfactants balanced to avoid removing too much oil from the hair.

The chemical mechanisms that underlie hair cleansing are similar to those of traditional soap. Undamaged hair has a hydrophobic surface to which skin lipids, such as sebum, stick, but water is initially repelled. The lipids do not come off easily when the hair is rinsed with plain water. The anionic surfactants substantially reduce the interfacial surface tension and allow for the removal of the sebum from the hair shaft. The non-polar oily materials on the hair shaft are solubilised into the surfactant micelle structures of the shampoo and are removed during rinsing. There is also considerable removal through a surfactant and oil “roll up” effect.

How to use: Lather, Rinse, Repeat!

Lather, rinse, repeat is a phrase that is a common part of the instructions on shampoo bottles. It is sometimes also used as a witty way of saying that a certain set of instructions should be repeated until an explicit or implicit goal is reached, or as sarcastic commentary on some people’s practice of taking descriptions, instructions or expressions literally and without common sense. Often the phrase is shortened to simply “Rinse and repeat”.

The found humor inherent in the phrase is that the instructions, if taken literally, tell the user to lather their hair, rinse the shampoo off, then repeat the process, but they do not say when to stop this cycle. In theory, a person could possibly keep washing and rinsing their hair endlessly (or until they ran out of shampoo) as they repeated the instructions continuously. In contrast, a person using common sense would consider that the word repeat does not refer to itself, but only to the two previous steps, so the instructions actually mean to repeat the process just once, for a total of two washings.