Shampoo chemistry

"Why do normal shampoos sting my eyes, but baby shampoos don't?" This is what listener Donald wanted to know, so we asked Cambridge University chemist Ljiljana Fruk...

Ljiljana - Before I answer this question, let’s look at the eyes first (ha, pun intended). To make sure we can focus well on the images around us, the surface of the eye needs to be smooth. Any imperfections or surface debris would interfere with this, possibly blurring vision. So to maintain a healthy eye surface, our tear glands produce a complex mix of water, salts, enzymes and other components to create a tear film which is constantly maintained by us blinking to protect the eye and provide soluble nutrients for the cells to thrive.

Embedded within the cornea are sensitive nerves, that can detect slight changes in the environment and make the eye blink or tear up to remove dust, or bacteria and viruses. These nerves are particularly sensitive to changes in pH: noticing if the liquid became more acidic or basic. The optimal pH of the eye is around 7, we say it is neutral.

Now….an ordinary shampoo is usually slightly acidic to be able to remove the dirt without damaging the proteins within the hair, which are sensitive to more alkaline conditions. And…( I am looking at my shampoo at the moment) they contain surfactants: these are molecules composed of two distinct chemical parts; one is attracted to water and the other one to oil. Basically, a surfactant acts as bridges between two liquids that are not mixing, and reduces surface tension, which allows a shampoo to spread nicely over a larger area and also produce a good foam. A commonly used surfactant is sodium lauryl sulfate, obtained from palm oil, which can cause an eye irritation, because surfactants can disrupt our cell membranes a little bit, and can cause a slight unfolding of our proteins, which we can experience as stinging or pain.

‘No tear’® and other baby shampoos contain different surfactants so you might notice that they do not foam that much, and are formulated to have a pH closer to 7. That also means they do not clean the hair as well as an ordinary shampoo. Funnily, urban legends claimed that baby shampoos contain desensitising medication, which numbs the eyes… which is absolutely not true. Ideally any surfactants should be kept away from the eyes. They should all be labelled as "keep away from eyes" and to rinse eyes thoroughly with clean water if this occurs.