Monday, March 15, 2004

How Does Shampoo Work?

Have you ever wondered while rubbing shampoo into your hair how this colorful, sometimes clear soapy substance can clean hair?

Shampoo is made up of molecules such as ammonium lauryl sulfate that bond with the dirt and sweat on your hair. This bonding action helps shampoo clean your hair of dirt. Water helps by adding pressure to the shampoo-dirt components and rinsing these components out of your hair and down the drain.

Okay, so these shampoo molecules bond with dirt. How? Well, in the case of ammonium lauryl sulfate, the chemical detergent is similar to the dishwashing or laundry detergent used to wash dishes or clothes. Ammonium lauryl sulfate is a harsh chemical as it needs to bond aggressively with the dirt on your hair to clean it. Sodium laureth sulfate is also a detergent found in shampoos, but it is a little gentler to hair. Guar hydroxypropyltrimonium chloride is another type of chemical found in shampoos that adds volume and smoothes hair. This chemical helps make your hair easy to comb. Diethicone helps soften hair by coating the outer hair surface.

Shampoo is not just chemicals. It is actually 80 to 90% water. But, just using water won't really leave your hair clean, soft or comb-able. Rather, it is the 2 to 8% of detergents such as the chemicals listed above that really do the critical work of shampoo. The remaining 1% of shampoo is added fragrances or scents. The type of fragrance or scent your shampoo has really doesn't affect how clean your hair is, but it does affect the scent you smell when you are washing and brushing your hair.

So, shampoo helps clean hair; does it matter how much is used? The amount of shampoo should be about the size of a quarter. Anything less will not be enough to bond to all the dirt and sweat in your hair. Anything more is just wasting shampoo, water and your time. Too much shampoo can also leave your hair feeling dull as you wash away vital nutrients from your hair if you overwash it. Using a little more than a quarter may be necessary, though, if you were outside playing in a muddy creek.

One good way to tell if you are using too much shampoo is the amount of lather produced. Lather forms when the shampoo gathers around air instead of the oil from your hair. Dirt and oil actually destroy lather. If you have too much lather, you used too much shampoo. Remember, shampoo is to clean your hair, not the air.

Little Lion Experiment:


  • A piece of polystyrene clear plastic
  • A soda straw to use as a dropper
  • A little shampoo
  • A centimeter ruler
  • A toothpick, wire or some other small diameter "stick-like tool" that you can coat with shampoo


  1. Place your polystyrene sheet on a flat level surface where you can observe easily from the side and from the top.
  2. Place some drops of water on the sheet using your straw. You can do this by sticking your straw into a glass of water, placing your index finger over the hole and pulling out the straw. When you loosen your index finger slightly, you can control the amount of water that you drop out. Make some big drops and some small ones.
  3. Measure the diameters of the drops and looking from the side, sighting with your ruler, estimate their height. Finally, and again looking from the side, estimate the angle at which the water contacts the polystyrene. Why are these drops all circular? Make a plot of the height versus the diameter? What do you conclude? Make a plot of the contact angle versus the diameter. Again, what do you conclude?
  4. Take your "tool" (no shampoo yet), and push it across and through a water drop (i.e. move it parallel to the polystyrene). Describe what happens. You may want to try it several times to check what things happen every time.
  5. Dip your "tool" in your shampoo and shake off the excess (we do not want any big drops). Now push your "tool" into the edge of one of the water drops. What happens? Now push it across and through a drop. What happens? How is this different from what happened before you dipped it into the shampoo?

This experiment was described by the Science and Technology Center, University of Texas at Austin.