Final answer:
The longevity of bonded hair revolves around the stability of disulfide bonds, which are covalent and give hair its structure. Heat can temporarily break these bonds, but they eventually reform, which is why heat styling is not permanent. Chemical treatments can cause longer-lasting changes by breaking and reforming these strong bonds.
Step-by-step explanation:
The longevity of bonded hair is influenced by the stability of various types of bonds that its proteins contain. The most significant of these are the disulfide bonds, which are covalent bonds and are much stronger than hydrogen bonds. Disulfide bonds give hair its shape and texture and are responsible for the permanence of hairstyles like curls from permanent waves.
Using heat styling tools, like hair straighteners, can break these disulfide bonds temporarily, resulting in a change in hair shape, such as the loss of curls. However, because these changes are not permanent, the disulfide bonds eventually reform, returning hair to its original state. This is why hair straightening with heat is not a permanent method.
On a molecular level, the bond strength, bond energy, and the environmental conditions such as pH and temperature play a role in how long a hair bond will last. Substances such as chemicals used in permanent dyeing or perming can produce changes in the hair by affecting these bonds, typically by breaking and reforming disulfide bonds to alter the hair's structure.