Ammonium thioglycolate, also known as scalding salt, is a salt of thioglycolic acid and ammonia. Its chemical formula is HSCH2CO2NH4 and it can be used to perm hair. [1]
Chemistry
As a salt of a weak acid and a weak base ammonium thioglycolate, ammonium thioglycolate exists in solution as the salt itself and as an equilibrium mixture of thioglycolic acid and ammonia:
HSCH2COO− + NH4+ ⇌ HSCH2COOH + NH3
Thioglycolates, in turn, are able to cleave disulfide bonds, capping one side with hydrogen and forming a new disulfide bond with the other side:
RSH + R'SSR' ⇌ R'SH + RSSR'
Used for perming[edit]
Solutions containing ammonium thioglycolate contain large amounts of free ammonia, which swells the hair and makes it permeable. The thioglycolic acid in perming solutions reduces the disulfide cystine bonds in the hair cortex. [2] In a sense, thioglycolate eliminates cross-linking. After washing, treat your hair with a mild hydrogen peroxide solution to oxidize cysteine back to cystine. These new chemical bonds give the structural rigidity needed for a successful perm. The hardening process is similar to the vulcanization of rubber, typically using polysulfide bonds to cross-link polymer chains. However, there are not as many reformed disulfide bonds as there were before the permanent bonds. As a result, the hair is more brittle than before the perm was applied, and repeated use in the same location may eventually lead to breakage.
Since polar molecules are less volatile than nonpolar molecules, the glycolate substituent makes the thiol less volatile and therefore less odorous. Another advantage is that glycolate has some solubility in water. It is almost certainly possible to use HSCH3 and ammonia to perm hair, but there will be serious olfactory consequences.
In popular culture[edit]
In the 2001 film Legally Blonde, the main character, law student Elle Woods, won her first legal case by proving that a witness who had a perm on the day of the murder could not have been showering at the time of the murder. Because “Isn’t the first basic rule of perm care that you should not get your hair wet for at least 24 hours after a perm, otherwise it will deactivate the ammonium thioglycolate?”
