hair trimming is usually a purely physical process, but hair removal cream is an exception. Applying hair removal cream to the skin for a few minutes can suddenly make tough hair vulnerable to a single blow. As long as it is gently scraped with a plastic sheet, it will fall off one after another, and chemical reactions actually take place in the process.

hair removal cream is aimed at keratin in hair, specifically, a disulfide bond in protein molecules, which is very important for maintaining the structure of proteins.

(a simple schematic diagram of protein disulfide bonds, which are formed between cysteine residues)

and a substance called mercaptoacetic acid (or its salt) is usually added to hair removal creams. This substance is specifically used to break the disulfide bond. It reacts with disulfide bonds in hair keratin to "break" these connections (disulfide bonds are represented by R-S-S-R). As a result, the structure of the hair is destroyed and can be easily removed.

this is a reversible reaction in which mercaptoacetic acid is involved in the form of anions. In order to ensure that enough disulfide bonds are broken, a sufficient concentration of anionic reactants is needed. In order to achieve this condition, depilation cream will also add substances such as calcium hydroxide and potassium hydroxide to keep the local alkalinity high (look at the data pH is about a little more than 12). The surface of the skin also contains keratin, but the hair removal cream still destroys the hair much faster than on the skin, and will not have much effect on the skin in a few minutes.

(composition table of depilation cream, in which the most critical reactant is mercaptoacetic acid)

for a hand cripple like me, the force of chemistry can prevent the blade from scratching the skin. But hair removal cream destroys keratin and requires strong alkaline properties also determine that this thing is certainly not very mild to the skin. If you have paid attention to the packaging of this kind of products, you will also find that merchants always use a lot of space to write instructions and warnings. The most important principle is to do small area tests first, and don't let them stay on the skin for too long at any time, especially for people with sensitive skin.

another disadvantage of hair removal cream is that it always smells a strange smell, which comes from mercaptoacetic acid (the producer has actually tried very hard to cover it, but it can't). The smell is reminiscent of a barbershop, and in fact the same ingredients are used in perms (perms can be seen as a process of disulfide bond destruction and reconstruction). Of course, different uses of products have different formulations, not to say that hair removal cream can be used to perm hair (and no one will do so _ (: hair "∠) _).

looking at the composition table of depilation cream two days ago, I suddenly thought of this, which is also an example of the application of chemistry in daily life.

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reference: http://eng.mst.dk/media/mst/69114/31.pdf