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.4
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.