What Is a Detergent?
A detergent is a substance which has a power to cleanse. This description applies to soap, as well as to shampoos and washing powders. A detergent usually has surface activating properties, which means that it is able to reduce the surface tension of water. In the process of cleaning the detergent acts as a bridge between the solid matter and the water. Soap molecules are shaped like tadpoles. The head is soluble, but the rod-like body is composed of an insoluble fatty substance.
When mixed with water part of the soap tries to get away and the rest stays, thus breaking down the water’s surface tension. There is not sufficient room for all the soap molecules on the surface of the water. So they form bundles with the water-resisting rods on the inside. The dirt attracts the fatty part of the soap molecules which lift and surround it, while the soluble part of the molecules lifts and rinses the dirt away.
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 Soap has a long history and was originally made from purely natural products like goat’s fat and wood ash.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.
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.
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.
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:
- During the wash cycle, the surfactant 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.
- During the rinse cycle, water molecules 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.