How do hydroxide relaxers straighten the hair?

Hydroxide Relaxers

The hydroxide ion is the active ingredient in all hydroxide relaxers, which

are very strong alkalis with a pH over 13. Sodium hydroxide, potassium

hydroxide, lithium hydroxide, and guanidine hydroxide are all types of

hydroxide relaxers, which can swell the hair up to twice its normal diameter.

Hydroxide relaxers are not compatible with thio relaxers, permanent waving,

or soft curl perms because they use a different chemistry. Thio relaxers use

thio to break the disulfide bonds. The high pH of a thio relaxer is needed

to swell the hair, but it is the thio that breaks the disulfide bonds.

Hydroxide relaxers have a pH that is so high that the alkalinity alone

breaks the disulfide bonds. The average pH of the hair is 5.0, and many

hydroxide relaxers have a pH over 13.0. Since each step in the pH scale

represents a tenfold change in concentration, a pH of 13.0 is 100 million

(100,000,000) times more alkaline than a pH of 5.0 (Figure 20-45).

Hydroxide relaxers break disulfide bonds differently than in the

reduction reaction of thio relaxers. In lanthionization (lan-thee-ohny-ZAY-shun), the process by which hydroxide relaxers permanently

straighten hair, the relaxers remove a sulfur atom from a disulfide bond

and convert it into a lanthionine bond. A disulfide bond consists of two

bonded sulfur atoms. Lanthionine bonds contain only one sulfur atom.

The disulfide bonds that are broken by hydroxide relaxers are broken

permanently and can never be re-formed. That is why hair that has been

treated with a hydroxide relaxer is unfit for permanent waving and will

not hold a curl.