It’s no secret that the majority of women colour their hair. Fully embracing my lack of y-chromosome, I have been colouring my hair for years. Recently, my peers, work colleagues, and biophysics crew may have noticed that I’m sporting a slightly more tangerine mop, no thanks to a certain apprentice hairdresser who shall remain nameless. So, how does it work? I’m glad you asked.
As we all know, hair is comprised of keratin, like our fingernails. The colour is a result of the ratio we have of expressed proteins – eumelanin and phaeomelanin. Eumelanin expresses dark pigments like brown or black, while the phaeomelanin is responsible for the blonde and red colours. When we get to a ripe old age, and these melanins are no longer present at all, that’s when hair goes grey/white, unfortunately. Hair colouring has been going on for hundreds of years, with previous mechanisms consisting of simple coating the shaft of the hair in a natural pigment, for example henna. These methods are ofcourse temporary.
Hydrogen peroxide is strips the hair of its colour, in an irreversible reaction. When hydrogen peroxide is applied to the shaft in an alkaline solution (usually the weak base, ammonia), the H2O2 oxidizes the melanin molecule, liberating sulphur. The oxidised form of either melanin is colourless. The hair does not become
transparent of course, as keratin is naturally light yellow, so bleached hair is often yellow-blonde. Why the need for the basic environment? Ammonia is needed otherwise the lightening agent would be unable to react with the pigments which lie in the cortex of the shaft, under the hard cuticle of the hair. The basic environment allows for openings between the keratinocytes of which the cuticle is comprised. Once the hair is stripped of its natural pigment, new colours can be deposited into the hair shaft, and new chemical bonds can be catalysed between the colourant and the cortex of the shaft. Treatments are subsequently applied to close the cuticle, “sealing” the colour in the cortex. (Helmenstine, AM, Ph.D, 2011).
I thought it was natural.
ReplyDeleteI remember reading something about metals being important in hair colour too... do you know anything about whether that is true or not?
This may be true because many metal-based complexes can be brightly coloured. I would wager, for instance, that permanent green hair dyes contain some copper.
ReplyDeleteIt'll shock you to know I'm not a natural semi-Asian redhead.
ReplyDeleteIn doing the reading for this I tended to focus on the removal of natural pigments, but I'll be sure to follow this up with some of the chemistry of other colourants.