Soaps & detergents

Greasy fingers after the fries barely come clean with plain water. One drop of soap, and suddenly it works. How does the soap manage that? The trick lies in its particles, and once you understand it, you also understand how every detergent works.

Fat and water don't mix

Pour oil into water and it floats on top as its own layer. Stir hard and the oil breaks up into many small droplets for a moment, but as soon as you stop they merge back into one layer. Fat and water simply don't go together. That is why plain water hardly helps against greasy dirt. It beads off the fat instead of taking it up. This calls for a go-between.

Soap particles with two ends

A soap particle is built like a pin: the head is water-friendly, the long tail is fat-friendly. That makes the soap the perfect go-between. Its fat tail sits in the fat droplet, its water head points into the water. Many particles wrap around a fat droplet like this and carry it into the water. This forms an emulsion: tiny fat droplets that stay finely spread through the water.

Cleaning power and the environment

This is exactly why washing-up liquid dissolves fat from a plate: it surrounds the greasy dirt and flushes it away with the water. Laundry detergents work on the same principle. All of it ends up in the waste water and runs through the treatment plant. Whatever is not broken down there travels on into rivers and lakes. That is why care is taken today that the substances break down well and burden waters as little as possible. Dosing sparingly helps the environment and still cleans well.