Detergents and soaps
Last updated: May 2, 2008.

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.
With the possible exception of water, 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. The two Ecover products on the left
are
made with plant-based ingredients to reduce their environmental impact.
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originally made from purely natural products like goat's fat and wood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.
How detergents 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
The cleverest part of a washing
machine is the detergent (soap
powder or liquid) you put into it.
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 mud on your favorite jeans. No problem! Throw
them into the washing machine with some detergent and this is what
happens:
- During the wash cycle, the surfactant (represented here by the
green
square and hand) mixes with water.
- The grease-loving ends of the surfactant molecules start to
attach
themselves to the dirt on your jeans.
The tumbling motion beats your jeans about and breaks the dirt and
grease
into smaller, easier-to-remove pieces.
- Water molecules (blue drop with hand) 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 rinse and spin, the dirty water flushes away, leaving your
jeans clean again!
Additives
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.
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 was an expensive luxury until French chemist Nicolas
Leblanc (1742-1806) found a cheaper way of making it using salt.
- 1800s: Soap-making became popular in the United States and North
America, where people mixed the ingredients in large "soap kettles".
Soap kettles were 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 were introduced in the
United States.
- 1950s: Synthetic detergents are developed to counte rsoap
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 led to the development of the first biodegradable
surfactants.
Text copyright © Chris Woodford 2008. All rights reserved.

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