What do surfactants do?

Oops – you spilled ketchup on your T-shirt at dinner. And your plate is also very dirty. To wash the T-shirt, you need detergent. And you can use dishwashing detergent to clean the plate. Both cleaning products contain surfactants, also known as cleansing substances. So how do these surfactants work? What do they do to the water and the dirt? These two short experiments will show you what effect surfactants have.

Before you get started

If you are experimenting at home:
1. Let your parents know and discuss the experiments with them beforehand.
2. Do not eat or drink at your workspace.
3. Wash your hands thoroughly before and after the experiment.
4. Be careful and cautious with the materials.

Graphic figure boy pouring liquid in flask

glass jars and bow with coal piece and small plastic bottle with blue liquid

You will need

Two screw-top jars ‏(empty jam jars)
Glass bowl
One teaspoon tip of soot ‏(e.g., abrasion from a piece of coal)
Dishwashing detergent
Thumbtacks
Water

Let’s get started!

three thumbtacks floating on water in a bowl

1. Surfactants lower the surface tension

Fill a glass bowl with water and very carefully place three thumbtacks flat on the surface of the water with the point facing up. Because the water has a high surface tension, the thumbtacks can float on it.
Now carefully add 1-2 drops of dishwashing detergent to the water. Observe what happens.

2. Dirty water

Fill two screw-top jars about halfway with water. Scrape some soot off a piece of coal with a teaspoon and add some to both jars.

two glass jars with soot in water and foam

3. Surfactants disperse dirt

Now drop two drops of dishwashing detergent into one of the two jars. Screw the lids onto the jars and shake both jars for about 15 seconds.
Place the glasses next to each other. What do you observe – what happens to the soot? From which glass is the soot removed better?

Did you know?
What is surface tension?

Answer:

Water consists of tiny water particles. Such particles are also called "molecules". The water molecules attract each other very strongly. As a result, they hold together tightly. So tightly, in fact, that they almost form something like a "skin" on the surface of the water. In scientific terms, they say: Water has a great surface tension.

The surface tension of water is also the reason why insects can walk across the water without sinking. Or, as in our experiment, that the thumbtacks float on the water.

When we add surfactants to water, the cohesion of the water molecules is disrupted. The surface tension drops. That is why the thumbtacks sink.