Key concepts
Chemistry
Water
Oil
Surfactants
Foams

Introduction
We lather our scalps with shampoo all the time. But what exactly makes a good shampoo? You might be surprised to hear every new shampoo has to pass lots of scientific tests before it is considered good enough to be sold. Many different shampoo recipes are compared to decide which is best for the consumer. One of these tests assesses the foaming behavior of the shampoo. In this activity you will become a cosmetic scientist and put different shampoos to the test. Which brand do you think creates the most foam and which foam lasts the longest?

Background
Although many different shampoo products are available in stores, they all have the same purpose: to clean dirt and greasy oil from your hair and scalp. How can shampoos remove all this built-up oil? For this, shampoos contain specific ingredients called surfactants, which are the main ingredients in shampoos besides water. Surfactants have a useful chemical structure that has a hydrophobic (water-repelling) tail and a hydrophilic (water-loving) head. This property allows them to react with both water and oil, which normally doesn’t mix with water.

Although its cleaning power is probably the most important criteria for a good shampoo, there are many other aspects that decide the quality of a shampoo. Besides surfactants there are also additives in each shampoo that optimize its look, feel, scent or performance such as additional foam builders, thickeners, conditioning agents or preservatives. For example the ability to create lots of foam and control its stability is important because people associate more foam with more cleaning power. Although this is not necessarily true, a shampoo sells better if it makes more foam. Therefore additional surfactants that are able to produce extra foam are often used as foaming agents.

Scientists continue to research new shampoo recipes and have developed specific tests to assess each new product they create. This allows them to compare different shampoo recipes to find the one formula that performs best overall. In this activity you will perform such a test yourself and find out which of the shampoos in your selection creates the best and the longest-lasting foam!

Materials

  • Selection of different shampoos (at least two different kinds)
  • Disposable cups (eight ounces, one per shampoo)
  • Kitchen scale
  • Tap water
  • Spoon
  • Measuring spoon (tablespoon)
  • Tall, narrow glass, jar or vase (larger than eight ounces), preferably with lid
  • Stopwatch
  • Two permanent markers (two different colors)
  • Tape
  • Paper and pen
  • Ruler
  • A workspace that can tolerate spills


Preparation

  • Use the permanent marker to label each cup with the name of the shampoo you want to test.
  • Make a 10 percent solution of each of your shampoo samples. To do this, put one empty cup on the kitchen scale and zero it. Add 20 grams of your shampoo sample and then add tap water to the cup until the scale reads 200 grams.
  • Carefully mix the shampoo with the water by stirring with the spoon.
  • Repeat these steps for all of your shampoo samples. Clean your mixing spoon between each sample.


Procedure

  • Get your tall, narrow glass and carefully pour 50 milliliters (about six tablespoons) of the first 10 percent shampoo solution inside. When pouring the shampoo into the glass, do you already see any foam formation?
  • Have a timer ready and set it to five minutes.
  • Put the lid on the glass if you have one. (If you do not have a lid, use the palm of your hand to cover the glass and seal it off the top.) Then take the glass between your hands and shake it very hard 10 times up and down. Do your best to keep the shaking speed constant for all shampoos. What happens once you start shaking the shampoo solution?
  • Immediately after you are done shaking, start your timer. Place a piece of tape to mark on the glass the level at which the foam phase starts (at the bottom, right above the liquid) and where it ends at the top. (You can use the marker to write on the tape to note which shampoo sample it is.) How much foam did the shampoo produce?
  • To assess the foam’s stability or to determine how fast it disappears, place pieces of tape to mark the foam levels (top and bottom) on the glass after five minutes. (You can use a different color marker to distinguish this measurement.) Note: The foam levels are likely to change most at the interface between the liquid and the foam, not at the top of the foam layer. What happens to the foam over time? Do you have more or less foam after five minutes?
  • After you have marked the foam levels clean out the glass (making sure all the shampoo and foam is removed) and look at the different marks you made. Take a ruler and measure the distance between the top and bottom levels of the foam layer at the beginning and after five minutes. Write your results down on your paper. How did your foam volume change over time? What do your results tell you about how stable the shampoo is?
  • Remove the tape from your glass and repeat these steps for each of your shampoo samples. How do your results compare? Which shampoo made the most foam? How much foam disappeared for each shampoo within five minutes? Can you correlate the foaming behavior with a specific shampoo ingredient?
  • Finally, repeat the steps with pure water. Do your results differ when using just water compared with the shampoos? If yes, why do you think this is the case?
  • Extra: Find out how long your foam lasts and increase the time you monitor the amount of foam in your glass. At what time point is the foam completely gone?
  • Extra: Many natural shampoo brands reduce the amount of surfactants and additives in their shampoo recipe because these can potentially irritate the scalp. Compare a natural shampoo brand to a standard one to see if they make the same amount of foam.
  • Extra: You can also make your own shampoo from recipes you find online. How does your homemade shampoo compare with the store-bought product? Does it work well in cleaning your hair?

Observations and results
Did you create lots of foam? Most of the store-bought regular shampoo should have produced lots of foam once you started shaking it. You might have even seen a little foam formation already when you poured the shampoo solution into the glass. This is because these shampoos contain lots of surfactants and foam builders. When you shake the glass you create lots of gas bubbles that get trapped in the shampoo solution and result in the foam you see. The surfactants or foaming agents facilitate the formation of foam and stabilize the gas bubbles in the solution.

The foam, however, is not stable forever! In fact, you should have observed the amount of foam quickly decrease over time, and probably saw the bottom level of the foam quickly rise to a higher level. This is because the gas bubbles in the solution eventually collapse. The volume of foam you produce for each of your shampoos depends on the amounts of surfactants and foaming agents they contain. Solutions that do not contain any of these substances, such as water, do not create any foam, which you should have noticed in your last test.

Cleanup
Rinse out the glass, dispose of each of the cups you used and clean your work area.

More to explore
Shampoo, from How Products are Made
Make Your Own Shampoo and Test How It Performs!, from Science Buddies
Surfactant Science: Make a Milk Rainbow, from Scientific American
Science Activity for All Ages!, from Science Buddies

This activity brought to you in partnership with Science Buddies

Science Buddies