Being the salt of a
weak acid and
weak base, ammonium thioglycolate exists in solution as an
equilibrium mixture of the salt itself as well as thioglycolic acid and ammonia: :HSCH2COO− +
HSCH2COOH +
NH3 Thioglycolate, in turn, is able to cleave
disulfide bonds, capping one side with a hydrogen and forming a new disulfide with the other side: :RSH + R′SSR′ ⇌ R′SH + RSSR′
Use in perms A solution containing ammonium thioglycolate contains a lot of free ammonia, which swells
hair, rendering it permeable. The
thioglycolic acid in the perm solution
reduces the disulfide
cystine bonds in the
cortex of the hair. In a sense, the thioglycolate removes crosslinks. After washing, the hair is treated with a mild solution of
hydrogen peroxide, which oxidizes the
cysteines back to
cystine. These new chemical bonds impart the structural rigidity necessary for a successful perm. The rigidification process is akin to the
vulcanization of rubber, where commonly polysulfide linkages are used to crosslink the polymer chains. However, not as many disulfide bonds are reformed as there were before the permanent. As a result, the hair is weaker than before the permanent was applied and repeated applications over the same spot may eventually cause strand breakage. Since polar molecules are less volatile than nonpolar ones, the glycolate substituent makes the thiol non-volatile and hence less odorous. An added advantage is that the glycolate confers some solubility in water. One could almost certainly use
HSCH3 and ammonia to give a perm, but there would be serious olfactory consequences. ==In popular culture==