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Biochemistry Cosmetic Chemistry

The chemistry of henna

The Muslim festival of Eid al-Adha started on Sunday evening and runs through until Thursday. As part of the celebrations, some create impressively intricate art on their hands using henna. Here we take a look at the molecules in henna that allow it to be used as a dye, and how this process works.

Henna dye comes, unsurprisingly, from the henna plant. The particular molecule in the plant’s leaves that allows it to be used as a dye is lawsone, also known as hennotannic acid. Lawsone can be extracted by mixing powdered leaves with a slightly acidic solution, such as lemon juice.

This preparation is necessary because lawsone isn’t present as a free molecule in henna leaves, but is derived from compounds called hennosides. The mild acidic conditions and subsequent oxidation used in preparing the dye bring about this transformation.

Once a paste is obtained, it is applied to the skin and left there to dry for up to 6 hours. The longer it is left on the skin, the darker the colour left behind will be. The lawsone in the dye preparation migrates into the outer layer of the skin and binds to the skin’s keratin protein. This produces a red-brown stain which can last for up to six weeks until the outer layer of skin is naturally shed.

The above-described preparation is known as ‘red henna’. In recent years, ‘black henna’ has become increasingly popular. This also contains lawsone, but in addition contains the dye paraphenylenediamine, or PPD for short. PPD is also found in permanent hair dyes.

There’s concern over the use of PPD in black henna because it can be present in high levels to give a black colour more quickly. Some people can become sensitised to PPD, which can lead to severe allergic reactions. Consequently, the use of black henna is discouraged.

On a lighter note, there are some incredibly creative henna designs out there – including the fantastic chemistry henna in the tweet below!

https://twitter.com/Finetraces786/status/1160654457656500229?s=20

Got your own chemistry-themed henna to share? Tweet it at @compoundchem!

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