Some of us have naturally curly hair but want it straightened; others have it naturally straight
but want it curly. But whatever the
style you like to wear, there's chemistry involved in it!
The structure of hair
Hair is made
mostly of a protein called keratin, which is also present in nails. In hair,
keratin molecules are arranged in straight bundles. These bundles are held
together by disulphide bonds (-S-S-), which give strength to the hair.
Disulphide bonds are made by the amino acid called cysteine. The cysteine of one keratin molecule forms a disulphide bond with the cysteine of the neighbouring keratin molecule. The more disulphide bonds there are in a strand of hair, the straighter it is.
Disulphide bonds are made by the amino acid called cysteine. The cysteine of one keratin molecule forms a disulphide bond with the cysteine of the neighbouring keratin molecule. The more disulphide bonds there are in a strand of hair, the straighter it is.
Ammonium thioglycolate:
the perm salt
Ammonium thioglycolate (HSCH2CO2NH4)
is a compound that can break disulphide bonds. This is because it contains a
thiol group (-SH). The thiol group replaces one of the sulphur atoms in the
disulphide bond, like this:
Keratin-S-S-keratin + 2HS-CH2CO2NH4 --> -HO2CH2CS-SCH2CO2H + 2NH3 + 2HS-keratin
When the disulphide bond is broken, the keratin bundles come apart, and hair is weakened. Ammonium thioglycolate is therefore used widely in beauty parlours when customers want their hair re-styled. However, if you use too much of it, or if the reaction is left for too long, you could end up going bald. So do not, ever, try it at home!
Keratin-S-S-keratin + 2HS-CH2CO2NH4 --> -HO2CH2CS-SCH2CO2H + 2NH3 + 2HS-keratin
When the disulphide bond is broken, the keratin bundles come apart, and hair is weakened. Ammonium thioglycolate is therefore used widely in beauty parlours when customers want their hair re-styled. However, if you use too much of it, or if the reaction is left for too long, you could end up going bald. So do not, ever, try it at home!
If you have watched the film Legally Blonde, you'll have seen
Reese Witherspoon (acting as a law student) win her first case by her knowledge
of the chemistry of perms!
How to get your hair re-styled
If you'd like your
hair curled, it's called a perm. If you like it straightened, that's rebonding.
In both cases, the steps are very similar.
First, the beautician will wash your hair thoroughly to clean it. Then she applies perm salt (ammonium thioglycolate solution) to it for a short while. It releases ammonia, which loosens the hair and allows the glycolate to seep through. If you want a perm, your hair will be tied around curlers to get the curls you want. If you want it rebounded, it is pressed firmly among flat irons till it becomes straight.
When the hair is shaped to your satisfaction, it needs to be strengthened again so that the style becomes permanent. For this, an oxidation lotion is applied. This contains hydrogen peroxide (H2O2), which reconstitutes the disulphide bonds.
2Keratin-SH + H2O2 --> Keratin-S-S-keratin + 2H2O
Another way to get your hair rebounded is to use lye soap or lye cream. This contains 5-10% of sodium hydroxide, which breaks the disulphide bonds. The hair loosens up, causing the curls to disappear. After treatment with lye soap, the hair is washed with water and conditioner to remove the sodium hydroxide, which can otherwise corrode the scalp.
First, the beautician will wash your hair thoroughly to clean it. Then she applies perm salt (ammonium thioglycolate solution) to it for a short while. It releases ammonia, which loosens the hair and allows the glycolate to seep through. If you want a perm, your hair will be tied around curlers to get the curls you want. If you want it rebounded, it is pressed firmly among flat irons till it becomes straight.
When the hair is shaped to your satisfaction, it needs to be strengthened again so that the style becomes permanent. For this, an oxidation lotion is applied. This contains hydrogen peroxide (H2O2), which reconstitutes the disulphide bonds.
2Keratin-SH + H2O2 --> Keratin-S-S-keratin + 2H2O
Another way to get your hair rebounded is to use lye soap or lye cream. This contains 5-10% of sodium hydroxide, which breaks the disulphide bonds. The hair loosens up, causing the curls to disappear. After treatment with lye soap, the hair is washed with water and conditioner to remove the sodium hydroxide, which can otherwise corrode the scalp.
http://humantouchofchemistry.com/the-chemistry-of-perming-rebonding.htm
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