Making waves: The chemistry of hair perms
I hope my previous post about cosmetic chemistry whet your appetite to know more about the mechanisms underlying the chemical processes that take place in the salon.
If it did, then I have just the thing for you.
It’s the International Year of Chemistry, and in honor of that, the chief of the CENtral Science bloggers, Rachel Pepling, has called all blogging chemists to write about their favorite chemical reaction.
If this is news to you, it’s not too late! You have until Monday, September 26th to submit your entry. Check out all the details here.
A bit of background
Before I dive into my reaction, I need to set the stage a little.
Organic chemistry was my first chemistry love. Oh, the mechanisms, the reactions, the… electron pushing!
But what really sealed the deal between me and chemistry was my first biochemistry class. I had an “Aha!” moment when my professor threw a transparency up that showed how proteins are just really stinking big molecules.
So, you mean all those colorful blobs with strange names like “Golgi apparatus” and “mitochondria” that I memorized in high school biology— those were just molecules all along?
Yup. I discovered that so many everyday occurences all boiled down to chemistry. Everything is made of chemicals. That’s right, I said it. There’s no such thing as chemical-free!
My biochem professor was great at building in examples of how science intersects with everyday life. One day we learned about the structure of hair. It’s made of keratin, a fibrous structural protein that is also found in skin and nails. Disulfide bonds between polypeptide chains in keratin molecules are what give your hair strength and rigidity.
The chemistry of perms
If you have straight hair, I know there are days when you’ve looked in the mirror and wished it was wavy. And vice-versa for the curly haired folks out there.
A century ago, you might have resorted to putting 24 pounds of heated brass rods in your hair and topped it off with a solution of cow urine and water to set the wave in place. That’s quite a price to pay for wavy hair.
The chemistry of perms. Reference: J. Soc. Cosmet. Chem. 1996, 47, 48-59.
A basic solution of ammonium thioglycolate, a.k.a. perm salt, is applied to the hair. The excess ammonia present in the solution helps the hair swell so that the reagents can work their way through each strand of hair, and also deprotonates the thioglycolate molecule, enabling it to break open disulfide bridges. At this point, the hair is wrapped around a curling rod and sprinkled with an oxidizing solution containing hydrogen peroxide. The disulfide bonds are reformed and the hair comes off the rod with a curl.
Anyone who has worked in a chemistry lab knows that yields are rarely ever one hundred percent. The same goes for the reactions on your hair. Not all the disulfide bonds that are broken by the thioglycolate will reform in the presence of hydrogen peroxide.
This decrease in the total number of disulfide bonds is why people who perm their hair often likely notice a decrease in the strength and rigidity of their hair— also known as damage. But it varies from person to person.
It’s funny because to this day, I still remember the sulfurous rotten-egg smell that would fill the bathroom whenever my mom permed her hair. I remember watching her use the solutions in sequence, wondering how it all worked.
Now I know why— it’s all just chemistry on hair.
Chemical bonds broken, chemical bonds reformed, and voila!— momma’s got a brand new ‘do.