Good morning, science fans! Today sees the advent of a new series of posts, The Friday Question. Every fortnight, I’ll be answering an everyday question about science: from the ridiculous to the fascinating, from wonderful to bizarre. So, if there’s a question that’s always bugged you, or something you just don’t understand, drop me an email at firstname.lastname@example.org (I am always mega excited to receive mail). For now, onto a question that has regularly made me wonder…
I was in the shower the other day, and I got shampoo in my eye. It. Was. Agonising. Painful to the point where, once I had finished stumbling blindly about my shower room, splashing water in the general direction of my face and yelping, I wondered why it hurts QUITE so much. What is it about shampoo and your eyes, that means they can’t get along and play nicely, like shampoo does with my skin and hair and all the rest of me?
Well, it turns out it’s all about pH; pH being a measure of how acidic or alkaline something is. The liquid that lubricates your eye is approximately neutral in pH, and because eyeballs are such delicate and sensitive little flowers, even tiny changes in the environment upset them. Your eyes are the ultimate divas, and they don’t like it when things don’t go their way.
If something that is not neutral comes into contact with the surface of your eye, the diva tantrum kicks off. The change in pH is detected by the sensory nerve endings of the ciliary nerve. The ciliary nerve is a branch of the much larger trigeminal nerve, which is responsible for transmitting sensations of touch, temperature, and pain from your face to your brain. Once a change in pH has been detected by the ciliary nerve, your diva starts to shriek. A message sprints at full speed to the central nervous system, which interprets the stimulation as a sharp, painful burning sensation. This triggers a reflex pathway, sending another message shooting back to the eye, specifically to the lachrymal glands, telling them to produce water fast, to wash away the irritant in your eye, and calm the diva down. This is why you can help stop the pain by rinsing your eye with clean water.
So why not just have neutral shampoo, then? Would that not save us all some agony? Well, actually, you can get neutral shampoos, which are less likely to sting your eyes, but are also slightly less effective at the whole cleaning element.
Shampoos work by using surfactant molecules to clean your hair. Surfactants are made from two parts; they’re a cleaning duo, the Kim and Aggie of molecules. Except in this case they’re not ladies in rubber gloves, they’re a fat soluble region and a water soluble region. The fat soluble half of the team gets down and dirty and attaches itself firmly to oil and other bits and bobs wherever it finds them. This is stage one of the cleaning. When you rinse your hair, the water-soluble part allows the whole team, including all the dirt it’s clinging to, to be swept away, out of your hair. Surfactants that are slightly acidic or slightly alkaline are better at their job, perhaps neutral surfactants just don’t care about the dirt as much.
Shampoos tend to be acidic rather than alkaline though; this is because an acidic environment encourages the cuticle layer of your hair to lie flat, giving you smoother and shinier hair. And there’s no real problem with having a slightly acidic shampoo, as long as you keep it away from your sensitive little eyes!