Re: What effect does the pH of shampoo have on hair?

Dear Emily,

You can find a good explanation of hair and shampoo here:

http://www.ex ploratorium.edu/exploring/hair/index.html

According to that article, most shampoos are slightly acidic in order to dissolve soap scum and to make your hair smooth.

Shampoo has lots of different ingredients which do many different things, so if you use it in a science experiment, it might be difficult to understand exactly what's going on. Instead, let me answer a simpler question: how does pH affect hair?

You may already know that hair is made of keratin. Each molecule of keratin is very small -- about 10 nanometers across. To give you a sense of how small that is, 10,000 keratin molecules lined end to end would be the width of your hair.

Keratin is a protein, and proteins are chains of amino acids. Different types of protein contain different amino acids, linked together in a different order. Keratin happens to contain lots of the amino acid cysteine, which is special because it contains a sulfur atom. The sulfur atoms from two cysteines can join together, forming a disulfide bond. The keratin in your hair has lots of disulfide bonds -- both within a single protein chain, and between protein chains -- and these bonds strengthen your hair.

Disulfide bonds make other materials stronger, too. For example, in the 1800's, Charles Goodyear discovered a process called vulcanization, which involves heating up rubber with sulfur. The sulfur crosslinks the rubber molecules with disulfide bonds, making the rubber stronger.

Before I tell you how pH affects keratin, let me first review what pH means. Water molecules (H₂O) occasionally break apart into H+ and OH- (and H+ and OH- will join back together to form H₂O). Things dissolved in water can release or take up either H+ or OH-, thus changing the ratio of H+ and OH- in the water. An acidic solution (pH < 7) has more H+ than OH- , and a basic solution (pH > 7) has more OH- than H+.

It turns out that OH- can split disulfide bonds, thereby removing crosslinks between keratin molecules and weakening hair. H+ does not disrupt disulfide bonds. My prediction, then, is that a strong base would weaken or perhaps dissolve hair, but a strong acid would not. But I don't really know for sure, because I haven't done the experiment! (If you do this experiment, be sure to wear safety goggles, and be careful when mixing or diluting acids or bases! Please consult your chemistry teacher for details.)

You can use these same principles to understand how a perm works. The perm solution usually includes bases and sulfur-containing chemicals. Both the base and the sulfur compounds disrupt disulfide bonds. When your hair is in curlers, the disulfide bonds have a chance to re-form, thus making the curl permanent.

Finally, let me tell you about a clever way to exploit keratin's special properties. Keratin is sometimes used to coat pills. Why would you want to do that? The keratin coating will remain intact in the stomach, which is acidic, but dissolves in the intestine, which is basic (the intestine secretes base to neutralize stomach acid, so your intestine does not get digested). This allows you to deliver the medicine directly to your intestine, where it can be absorbed into your bloodstream.

Ed