Detergents and soaps
by Chris Woodford. Last updated: June 22, 2016.
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?
Photo: Soap: the detergent we know best. This one
describes itself as "pure" because it contains no added chemicals or perfumes.
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originally made from purely natural products like goat's fat and wood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
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
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
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
- During the wash cycle, the surfactant (represented here by the orange blob) mixes with water.
- 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.
- During the rinse cycle, water molecules (blue blob) moving past attach themselves to the opposite, water-loving ends of the surfactant molecules.
- 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?
Photo: Soap and water can clean almost anything thanks to detergent action. Photo by Joshua Scott courtesy of US Navy.
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.
Photo: Detergent makers are doing their best to encourage low-temperature washing.
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
- 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.
- 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.
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?
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.
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.
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.
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 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."
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.)
Find out more
Find out more
On this website
On other sites
- Green Cleaning for Dummies by Elizabeth B. Goldsmith and Betsy Sheldon. New York: Wiley, 2008. Considers the environmental impact of cleaning products and how you can clean in greener ways.
- Handbook On Soaps, Detergents & Acid Slurry by Niir Board. Delhi, India: National Institute of Industrial Research, 2006. A detailed book about the manufacture of detergents and soaps, and their chemical ingredients.
- Soaps, Detergents, and Toiletries by P. K. Chattopadhyay. Delhi, India: National Institute of Industrial Research, 2003. Another detailed volume covering everything from shaving foam and after shave to tooth paste and hair shampoo. Covers manufacturing and testing processes and chemical ingredients.
- Taking the water out of detergent: BBC News, 4 February 2009. Unilever chemist Richard Craven shows how to make concentrated clothes washing detergent in a laboratory, going through all the ingredients step by step. Amazing how many different ingredients there are!
- How to Make Soap with Lye by Becky's Homestead. An interesting and practical guide to making your own soap at home using lye, distilled water, and lard. (22 minutes)
- How to make soap at home using all natural ingredients by The Do It Yourself World. A more condensed video, but essentially using the same three-ingredient process. (9.5 minutes)
If you liked this article...
You might like my new book, Atoms Under the Floorboards: The Surprising Science Hidden in Your Home, published worldwide by Bloomsbury.
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