Detergents: How do soaps and detergents bust the dirt from your clothes?

Detergents and soaps

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 waterfor 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!

What are detergents?

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 Soap has a long history and was originally made from purely natural products like goat's fat and wood ash.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

What are surfactants?

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.

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.

What effect does temperature have?

From the explanation above, you can see two different effects that are helping to get your clothes clean: there's the chemical action of the detergent and the mechanical action of the washing machine. While the water and detergent work together to remove dirt, all that tumbling and bashing also plays an important role. And there's a third factor too: thermal action, from hotter water, speeds up the chemical reactions.

Thinking about laundry as a scientific problem, we're most likely to see it as a matter of chemistry. But we can also understand how the three cleaning actions in a clothes washing machine are a problem of physics—connected with a basic law called the conservation of energy. Consider this: if there's a certain amount of dirt in your clothes, you need to use a certain, minimum amount of energy to remove it, which will be part chemical, part mechanical, and part thermal. In theory, you can reduce any one of these, but only by increasing one or both of the others. So, for example, if you use less detergent (or none at all), you'll need to use hotter water (more thermal energy) or wash for longer (more mechanical energy). Similarly, if you reduce the temperature of your wash (less thermal energy), you'll generally need to use more detergent. The laundry detergents that are specifically designed for low temperatures are either more concentrated or have a completely different "recipe" of ingredients that clean using less thermal energy and coat fibers to stop them getting so dirty in future.

If you're environmentally minded, you might have given some thought to how much energy is needed to make your clothes. In fact, during the typical lifetime of a piece of clothing, you'll use three to four times more energy for washing and drying it than was used making it in the first place; cooler washing can help to reduce that impact. The benefits of using a lower-temperature wash (at 30–40°C or about 90–100°F) are that you save energy (and therefore money) and help your clothes last longer. Typically, 75–90 percent of the energy you use washing a load of clothes comes from getting the water hot; only a quarter is used to tumble and spin the clothes and operate the machine. Cooler washing stops clothes shrinking or stretching out of shape and helps colors last longer.

A brief history of soaps and detergents

• 600BC: Historians think people have been making soap for around 2000 years, even since the time of the Phoenicians (an early Mediterranean civilization).
• 1790: Soap remains an expensive luxury until French chemist Nicolas Leblanc (1742–1806) finds a cheaper way of making it using salt.
• 1800s: Soap-making becomes popular in the United States and North America, where people mix the ingredients in large "soap kettles". Soap kettles are used for most soap-making until World War II.
• 1878: Modern branded soaps, distinguished as much by how they're advertised as what they contain, are born with thelaunch of Ivory Soap, Procter & Gamble's first national brand. It's developed accidentally when a factory worker lets his machine run too long, admitting too much air into the mixture and producing a unique soap that floats on water. People love the novelty—and Ivory Soap is soon being sold with the famous promise that it's "99 and 44/100% pure."
• 1916: German chemist Franz Gunther develops the first surfactant for detergents from coal tar.
• 1930s: Detergents based on surfactants are introduced in the United States.
• 1950s: Synthetic detergents are developed to counter soap shortages caused by World War II and soon overtake traditional soap to become our favorite chemical cleaners.
• 1960s/1970s: Concerns about water pollution from detergents building up in rivers and seas lead to the development of the first biodegradable surfactants.
• 1979: Starting with a simple lab in a rented barn, Belgian chemist Frans Bogaerts founds the Ecover company to promote eco-friendly cleaning without using potentially harmful chemicals such as phosphates.
• 2014: Procter and Gamble, one of the world's biggest detergent makers, announces plans to eliminate phosphates from its laundry products.

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?

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.)

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