To get things clean, water must penetrate deeply into the fabric. A glance at the water beads on your umbrella shows that water isn’t particularly good at wetting things. The problem is it has a high surface tension, which makes water behave as if it had a thin skin on it. That’s why insects can walk on water.
Washing powders and detergents work to make water wetter by lowering its surface tension. This enables the water to carry detergent molecules deep into the fibres, where they can come into close contact with the dirt.
Detergent molecules are like little tadpoles. The heads are attracted to the water molecules by a small electric charge, while the tails, which are chemically very similar to grease, are repelled by water molecules. So the tail of the detergent molecule attaches itself to the grease. With their heads attracted to water, and their tails embedded in the grease, the detergent molecules hold the water and dirt together, until they float away as the clothes are agitated.
Washing powders may involve other things besides detergent. These include bleach, enzymes to break down protein stains like blood and perspiration, and bluing agents, which make the laundry appear whiter. Bluish dyes absorb some of the reflected yellow light. This brings it more into balance with the blue light, so that the material appears whiter to the eye.
So next time you wear a white top, spare a thought for the hard working molecules that drag out the dirt.