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Selection of household detergents

Detergents and soaps

by Chris Woodford. Last updated: January 24, 2012.

When you're young, "bathtime" is another word for "torture" and a harmless block of soap can seem like an offensive weapon. Fortunately, most of us soon grow out of that little problem and learn to recognize soap and water for what they are: a perfect way to shift the daily grime.

Soap seems like the simplest thing in the world. Just splash it on your face and it gets rid of the dirt, right? In fact, it's quite a cunning chemical and it works in a really interesting way. Let's take a closer look!

Photo: Some typical household detergents. All of them, except for the soap, are liquids. Environmentally friendly detergents, such as those produced by Ecover, are made with plant-based ingredients to reduce their environmental impact.

What are detergents?

A bar of pure soap

Often we use the words "soap" and "detergent" interchangeably, but really they're quite different things. A detergent is a chemical substance you use to break up and remove grease and grime, while soap is simply one kind of detergent. Soap has a long history and was originally made from purely natural products like goat's fat and wood ash. Today, detergents are more likely to be a mixture of synthetic chemicals and additives cooked up in a huge chemical plant and, unlike traditional soap, they're generally liquids rather than solids. Detergents are used in everything from hair shampoo and clothes washing powder to shaving foam and stain removers. The most important ingredients in detergents are chemicals called surfactants—a word made from bits of the words surface active agents.

Photo: Soap: the detergent we know best. This one describes itself as "pure" because it contains no added chemicals or perfumes.

How surfactants work

You might think water gets you wet—and it does. But it doesn't get you nearly as wet as it might. That's because it has something called surface tension. Water molecules prefer their own company so they tend to stick together in drops. When rain falls on a window, it doesn't wet the glass uniformly: instead, it sticks to the surface in distinct droplets that gravity pulls down in streaks. To make water wash better, we have to reduce its surface tension so it wets things more uniformly. And that's precisely what a surfactant does. The surfactants in detergents improve water's ability to wet things, spread over surfaces, and seep into dirty clothes fibers.

Surfactants do another important job too. One end of their molecule is attracted to water, while the other end is attracted to dirt and grease. So the surfactant molecules help water to get a hold of grease, break it up, and wash it away.

How detergents work

Artwork explaining how detergents work

The cleverest part of a washing machine isn't the drum or the drive belt, the electric motor that spins it around or the electronic circuit that controls the program: it's the detergent (soap powder or liquid) you put in right at the start. Water alone can't clean clothes because it won't attach to molecules of grease and dirt. Detergent is different. The surfactants it contains are made of molecules that have two different ends. One end is strongly attracted to water; the other is attracted to oily substances like grease.

Suppose you got some grease on your favorite jeans. No problem! Throw them into the washing machine with some detergent and this is what happens:

  1. During the wash cycle, the surfactant (represented here by the orange blob) mixes with water.
  2. The grease-loving ends of the surfactant molecules start to attach themselves to the dirt on your jeans (shown by the brown blob on the leg). The tumbling motion beats your jeans about and breaks the dirt and grease into smaller, easier-to-remove pieces.
  3. During the rinse cycle, water molecules (blue blob) moving past attach themselves to the opposite, water-loving ends of the surfactant molecules.
  4. The water molecules pull the surfactant and dirt away from the jeans. During the final spin, the dirty water flushes away, leaving your jeans clean again!

This is why soap and water clean better than either one of these things alone.

What other chemicals are in detergents?

Surfactants aren't the only thing in detergents; look at the ingredients on a typical detergent bottle and you'll see lots of other chemicals too. In washing detergents, you'll find optical brighteners (which make your clothes gleam in sunlight). Biological detergents contain active chemicals called enzymes, which help to break up and remove food and other deposits. The main enzymes are proteases (which break up proteins), lipases (which break up fats), and amylases (which attack starch). Other ingredients include perfumes with names like "limone", while household cleaning detergents contain abrasive substances such as chalk to help scour away things like burned-on cooker grease and bath-tub grime.

Sailors scrubbing the deck of a boat with brushes, soap, and water

Photo: Soap and water can clean almost anything thanks to detergent action. Photo by Joshua Scott courtesy of US Navy.

A brief history of soaps and detergents

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Text copyright © Chris Woodford 2008. All rights reserved. Full copyright and legal notice.

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