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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 they 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!

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 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.