Cleansing Power: How Does Dishwashing Liquid Work?
Have you ever wondered why it takes more than just water to get your clothes and dishes clean? Detergent dissolves in water and attaches to things like dirt and whatever is staining your clothes to help dissolve them into the water to be washed away. But how does dishwashing liquid work, and where does its cleansing power come from?
Most detergents are amphiphilic, which means they are partly hydrophilic and partly hydrophobic. Hydrophilic means a molecule is polar, and thus is attracted to water. Hydrophobic describes to non-polar molecules that don’t like to mix with water.
Objective: Observe how detergent interacts with oil in water.
What will happen when you add detergent to an oil-water mixture?
- Laundry detergent
- Olive oil
- 4 clear glasses or jars
- Spoon (for stirring)
- Labeling tape
- Fill the 4 clear glasses until they are mostly full with water. Make sure each cup has the same amount of water.
- Label the first glass as your control. This glass will contain only clean water so that you can compare it with the other cups.
- In glass 2, add 1 tablespoon of detergent.
- In glass 3, add 1 tablespoon of olive oil.
- In glass 4, add 1 tablespoon of detergent and 1 tablespoon of olive oil.
- Stir each cup vigorously and record your observations.
- Repeat the experiment using different detergents, like shampoo and soap. Record your observations and compare your results.
Cup 2 will be mostly clear with some bubbles produced by the detergent. Cup 3 will have obvious phases, as oil and water do not mix. Cup 4 will be cloudy, as the oil will have bound to the detergent.
Because detergents are amphiphilic, they help oils become more soluble in water. The hydrophobic (non-polar) parts of the detergent molecules bind to the non-polar oil molecules. Simultaneously, the hydrophilic (polar) parts of the detergent molecules bind with the water so that the entire molecule can dissolve in water. This results in a cloudy solution of water and the molecules formed by detergent and oil.
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