When we curl our hair -- whether it's with water and styling gel or a permanent-wave kit -- we are messing with the chemical bonds that keep the protein fibers of our hair's cortex stuck together. These chemical bonds include hydrogen bonds -- a weak attachment that comes about when a hydrogen atom dangling off one protein is attracted to the oxygen atom dangling off another protein.
Water is made of hydrogen and oxygen -- it's called H2O because every molecule of water has two hydrogen atoms (H2) and an oxygen atom (O). When you wet our hair, water molecules sneak in between the proteins of the cortex and join these hydrogen bonds. Our hair swells up, absorbing up to 30 percent of its weight in water.
In wet hair, one protein molecule doesn't bond directly to another protein molecule. Instead, a protein is stuck to a water molecule, which is sticking to another water molecule, which is sticking to another protein. That's much weaker than having one protein stuck directly to the other protein -- which is why wet hair is much weaker and more likely to break than dry hair.
That's also why we can curl our hair when it's wet. If we set our wet hair in curlers or pull our curly hair straight, then let it dry in this new shape, the hydrogen bonds will reform in a new position. Of course, when our hair gets wet again, those hydrogen bonds will weaken and then reform in their original position, giving us back the hair we didn't want -- making curly hair straight, straight hair curly.
If we want a permanent change, we can perm our hair. In a perm, we don't just break hydrogen bonds, we also break the disulfide bonds that hold the proteins together. We add chemicals that break the disulfide bonds (bonds between sulfur atoms). Then we reshape our hair and add chemicals that reconstruct those disulfide bonds, holding our hair in a new shape. Since these disulfide bonds withstand water, our new hairdo will be waterproof.
Unfortunately, about 10 percent of the disulfide bonds that break when we are perming our hair don't reform again. So the proteins in permed hair are held together more weakly, and the hair is more likely to break and get split ends. That's the price we pay for messing with Mother Nature.
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