Chemistry at the hairdresser

So, very recently I spent four (yes, really) hours sat in my hairdresser’s chair. After having read all the trash about what all those celebrities were up to and what ridonculous items of clothing I should be wearing in this so called British summer, I got to thinking about chemistry, more specifically, what transformations were actually happening to me and my hair, other than me slowly but surely looking like an alien from the planet Foil.

So what chemical processes have my ~100000 strands of hair gone through?

Hair is mainly keratin, the same protein found in skin and fingernails. The natural colour of hair depends on the ratio and quantities of two proteins: eumelanin and pheomelanin. Eumelanin gives us the brown/black hair shades (my natural colour) whereas phaeomelanin is responsible for the red based colours. Conversely, the absence of either type of melanin protein produces white/gray hair. 1

Hair dyeing by oxidation been practiced for well over 100 years and came from the observation that colourless p-phenylenediamine (shown below) produces a coloured compound when subjected to oxidation, and that this reaction could be used to colour a variety of substrates. The first patent relating to oxidation dyeing of human hair was applied for in 1883 by Monnet (F.P. 158,558).2 More scientifically put, permanent hair colouring involves the in-fibre formation of indo-dyes from colourless precursors by oxidation with hydrogen peroxide, under alkaline conditions. The primary intermediates are p-phenylenediamines (shown below) or p-aminophenols which are easily oxidised by hydrogen peroxide to form p-benzoquinone imines. 3

The use of hydrogen peroxide to develop the colour also allows for bleaching of the natural pigment by one or two shades at the same  time as the synthetic  colour is being formed.

The mechanism of oxidation dyes involves three steps:

  • The first step shows the oxidation of p-phenylenediamine (or similar stuctures, below) to the quinonediimine derivative

  • The second step involves the attack of this quinonediimine on the coupler (with chosen colour properties) by electrophilic aromatic substitution.
  • In the third and final step, the product from the quinonediimine-coupler reaction oxidises to the final hair dye.

Hair can also be dyed by bleaching of the hair’s natural pigments only. Bleaching is simply the removal of colour from hair. Bleaching can be caused by the sun’s ultra violet rays breaking bonds in the pigment molecules but hair bleaching is most commonly achieved by using hydrogen peroxide. Typically, a low volume of peroxide (5-30%) is applied to hair and left until the required amount of colour is stripped from the hair, at which point it is rinsed out. Before the bleach can change the colour of the hair, however, it must first penetrate below the surface of the hair’s cuticle. This is achieved by mixing the peroxide bleach with an alkaline solution, most commonly ammonia. The ammonia swells the hair fibres causing the cuticles to separate and open allowing the bleach to penetrate the cortex of the hair. This cuticle opening effect is also important when colour is being added to or implanted into the hair.

Hydrogen peroxide reacts with the melanin within the hair and in an irreversible reaction, the peroxide oxidises the melanin which renders it colourless. Complete bleaching tends to leave hair a pale yellow colour rather than pure white, however.

A really good review on hair dye in the modern world has been written by Christie and Morel. Well worth a read if you want to know more about dyes.

1            J. F. Corbett, Dyes Pigments, 1999, 41, 127-136.

2            J. F. Corbett, J. Soc. Dyers Colour., 1976, 285-303.

3            C. Incorporated and A. Seminar, J. Soc. Cosmet. Chem., 1984, 310, 297-310.

4            O. J. X. Morel and R. M. Christie, Chem. Rev., 2011, 111, 2537-2561.


7 thoughts on “Chemistry at the hairdresser

  1. LOVE THIS. I always wonder what is going on, on my head when I am at the hairdressers! I actually asked the hairdresser about how they mix dyes last week, if there were any restrictions (apparently not). There are so many very cool dyes and chemistry going on at the hairdressers! I’m obsessed with hair dye!

    • It is crazy that there is no real scientific way of knowing how much of a colour suits a person. People have apparently tried to make computer algorithms to come up with correct mixtures but doesn’t work. You just have to rely on the hairdresser knowing what he or she is doing. Crazy.

      Oh and I found out that your hair can go green if they use metal implements during the bleaching process.

  2. One of my senior seminar students did her presentation on hair treatments and dyes just over a month ago. Popular topic! {And ALL chemists should have a blog for their “chemistry in everyday life” musings!}

  3. Hi–great post! I’ve been following the ‘no chemicals’ conversations on Twitter about how companies have been labeling products with the term, making the ridiculous claim to attract the ‘organic’ crowd. While I see their point, I am also concerned that too often we look to a chemical solution for our bodies and environment that may be harmful, and diseases such as Multiple Chemical Sensitivity scare me.

    I’m 40…ish and about 20% of my hair is naturally grey. The last time I dyed my hair to cover it up, my scalp itched for 10 days. I keep thinking of this paper which indicated that people who dye their hair are more susceptible to epigenetic modifications, and worrying that I’m doing my body harm:

    I know this is a question that would likely require much more research, but do you know of any dyes that would be less likely to cause such damage?

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