Chemistry of Cosmetics- Part II

If you didn’t see my last post on the chemistry of nail varnish, you can see it here.

I decided to do a post on hair dye for the second part of this topic as it might be a little more relatable to everyone rather than exclusively to those who use nail polish or make-up.


What is hair?

Hair is made up mostly of keratin, a fibrous structural protein. α-Keratin is the form that is found in mammals (hair, skin, wool, nails, claws, hooves), whilst β-keratin is found in birds, amphibians and reptiles (nails, scales, shells, feathers beaks).




Curly Hair vs. Straight Hair

The basic difference between curly and straight hair is simply that curly has more disulfide bonds in the filaments so they coil more.

What are disulfide bonds, you ask? Well, I’m glad you did ask! Disulfide bonds are bonds between two sulfur atoms. These disulfide bonds are present between two α-keratin strands which form protofilament and the filament.


Hair Colour

So what makes up our hair colour?

Your hair colour is determined by two types of melanin (a type of natural pigment): eumalanin and pheomelanin. This in turn is determined by genetics. Certain genes suppress and activate eumelanin.

More eumelanin = darker hair colour

Less eumelanin = lighter hair colour


*Fun fact:

Shades of hair colour is categorised by the Fischer-Saller scale (named after Eugen Fischer, the father of eugenics, and Karl Saller).


Eumelanin has two further sub-types, brown and black.

Orange / yellow hair – pheomelanin

Blonde hair – low concentration of brown eumelanin

Brown hair – high concentration of brown eumelanin

Gray hair – low concentration of black eumelanin

Black hair – high concentration of black eumelanin


Hair Dye and Bleach

Bleach, usually sodium hypochlorite, is a strong oxidant. It is also commonly used as a disinfectant.

An oxidant is a substance that removes electrons from another substance, called an oxidation reaction.

So, to bleach your hair, bleach undergoes an irreversible oxidation reaction with melanin, thereby breaking the bonds in the melanin molecule, preventing it from absorbing visible light. This is why the sun and chlorine in pool water can act as a bleach.



Bleached hair, before & after


Now, you might have noticed that some people who have bleached their hair to blonde will find that their hair will go a bit orange after a while. This is because bleach reacts much more readily with eumelanin (the dark pigment) rather than pheomelanin.

Permanent hair dye works be physically opening the outer layer of the hair strand (the cuticle). The dye then reacts and bonds with the cortex (the inner part) of the hair. Most permanent hair dyes will contain a small percentage of bleach in order to remove any existing colour and to open up the cuticle in order to insert the dye into the cortex.

Semi-permanent dye will temporarily bond with the hair cortex and can be dislodged with shampoo (which is just a detergent). Semi-permanent and temporary dye do not contain any bleach.



Hair cross section


So now you know how bleach and hair dye works, you can determine if you really want your hair to undergo these harsh reactions in order to attain that desired hair colour. I think it is worth it, as long as you let your hair rest, take good care of it and let it grow back to its natural state once in a while.




Image References (in order of appearance):

*All images licensed under Creative Commons


4 Responses to “Chemistry of Cosmetics- Part II”

  1. Eileen says:

    @Kathryn Well, the orange colour does come from certain concentrations of pheomelanin 🙂 it’s why if I ever bleached my hair (my hair is quite dark) it would go orange because of the excess pheomelanin ^^

  2. Kathryn says:

    Interesting! You forgot about us redheads, though 😉

  3. Eileen says:

    @bspicer I don’t think there are any scientific sources on hair care, but you can presume that (and I know it works from practice) that hair masks (usually oil-based) are great for your hair. Shampoos are literally detergents, so they strip the natural oils from your hair. Conditioner just pastes the split/broken strands of hair back together. Better quality shampoos will be less harsh on your hair and will have oils in them to replace the ones that have been washed out, whilst good quality conditioners do a better job of smoothing and fixing the cuticle of the hair strands.

    All I know for sure is that, advertisement for cosmetics is not regulated very well, so they can pretty much say whatever they want.

  4. bspicer says:

    I didn’t realise the difference between hair curliness is so straightforward! (pun unintended) That’s neat.

    I’ve always felt a little lost when it comes to hair care. I don’t really trust the companies who are selling something to give me the right information, but (as far as I’m aware?) there aren’t rigorous scientific sources on hair care.

    Or maybe there are?