Sam Lemonick has a great piece in Chemical & Engineering News on hair removal, with a focus on depilatory creams:
Creams are a popular option for at-home removal. To understand how these work, you first have to understand hair. Hair is made of fibrous proteins called keratin, twisted like yarn or rope into long bundles. Keratin strands are cross-linked by covalent disulfide bonds and weaker hydrogen bonds. These are depilatory creams’ targets.
The active ingredients in brands Veet and Nair are salts of thioglycolic acid like potassium thioglycolate or calcium thioglycolate in combination with bases such as calcium, sodium, or potassium hydroxide. The bases serve two purposes. They cause the hair to swell, opening its keratin fibers to allow thioglycolate to penetrate. The bases also remove the proton on thioglycolate’s thiol group. Once thioglycolate’s proton leaves, its sulfur atom is free to attack the hair protein’s disulfide bonds. Break enough of those, and the hair degrades completely and can simply be wiped away.
Because of this mechanism of action, chemical hair removers are remarkably selective, studies have shown. Researchers tested Nair on thin, thick, and medium hair, and on cotton, rayon, and polyester fibers. All three strands of hair broke within 10 minutes, but the remover had no effect on the other fibers, none of which contain disulfide bonds.
Other experiments have shown that cream hair removers should have a pH between about 12.0 and 12.5 to make sure the products work quickly but aren’t so caustic that they burn the skin, which has a pH of 4.5–5.5. Dermatologist Meghan Feely says cream hair removers can cause chemical burns for some people. They should be used according to their directions to minimize risk.
Because these chemicals are so effective, the book is basically closed on finding new depilatory agents, says Heike Hanau, a marketing manager for Merck & Co., which used to supply calcium thioglycolate for hair removers. But she says chemists are still working to improve depilatories’ smell. One by-product of thioglycolate’s reaction with disulfide bonds is hydrogen sulfide, which smells like rotten eggs.
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