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Selection of household detergents

Detergents and soaps

by Chris Woodford. Last updated: July 22, 2014.

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 Soap has a long history and was originally made from purely natural products like goat's fat and wood ash.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

What effect do detergents have on the environment?

We all love clean clothes, but most of us also love a clean planet. Do the two things go together? Look at the ingredients label on a typical bottle of detergent and you'll see a chemical cocktail. What are all these things and what do they do? More to the point, do they have any harmful effect on our health or the planet on which we all depend? There's very good reason to think so. That's why some detergent brands deliberately position themselves as eco-friendly, not by comparing themselves to soap and water (the basic dynamic-duo of the detergent world) but by drawing attention to the potentially harmful chemicals used by their rivals.

What harm do detergent chemicals do?

An informative label about insect pollination on a bottle of Ecover clothes detergent.

You might think this is a matter of opinion; mostly it's a matter of science: the effects of detergent chemicals are well documented. What's less well understood is that all chemicals are added to detergents for a specific purpose (watch the BBC video in the links below to learn more), and some of the additives actually reduce the harmful impacts that detergents would otherwise have.

Surfactants

As we've already seen, these play a crucial part in helping water to attack and remove dirt. But once they flush away down the drain, surfactants don't stop working: they start to play similar tricks on aquatic life, for example, attacking the natural oils in the mucus membranes of fish, stopping their gills from working properly, and increasing their risk of attack from other chemicals in the water. Some surfactant ingredients (including one called nonylphenol ethoxylate or NPE) produce what are called endocrine-disruptors, which can affect the hormonal balance of animals (including humans), causing a variety of health problems and sometimes changing their sex characteristics. Although surfactants can be toxic to fish and other aquatic life (some are even listed as persistent organic pollutants (POPs)—ones that remain in the environment for many years without breaking down), most surfactants biodegrade relatively quickly in sewage treatment plants before they can do much harm to the natural world.

Phosphates

When the phosphates in detergents enter freshwater, they can act like fertilizers, promoting the growth of tiny plants and animals. The biggest problem they can cause is a huge growth of algae, known as an algal bloom, which kills fish life by reducing oxygen. Although phosphates enter water in many different ways, detergents contribute significantly to the problem.

Enzymes

Enzymes are catalysts, which means they're chemicals that help to make chemical reactions happen more quickly or easily. Generally, they're added to detergents to make them more effective at tackling tricky forms of dirt that ordinary detergents struggle with. They also help to lower the environmental impact of detergents by reducing the need for surfactants. Although it's widely believed that enzymes can cause skin problems, a recent scientific review by David Basketter et al in the British Journal of Dermatology suggested that's a myth: "the irritating and allergenic hazards of enzyme raw materials do not translate into a risk of skin reactions."

Perfumes

Fragrances in detergent serve no purpose other than to make your clothes smell nice. But the oils from which they're made can cause rashes and skin allergies. (The New Zealand Dermatological Society has a good page about fragrance allergies.)

Photo: "The last thing a loved up butterfly needs...." The label on this Ecover washing detergent bottle tells us one of the potential hazards of using chemicals in detergents—and what Ecover does instead to reduce the problem.

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Text copyright © Chris Woodford 2008, 2012. All rights reserved. Full copyright notice and terms of use.

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Woodford, Chris. (2008) Detergents and soaps. Retrieved from http://www.explainthatstuff.com/detergents.html. [Accessed (Insert date here)]

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