Appliance Science: The clean chemistry of laundry detergents

There's one thing that most people don't realize about the past: it was a filthy, filthy place. Even ignoring the people themselves, the clothes that we have worn for most of human history have been stained, dusty and crusted with all sorts of unpleasantness. We've worn them for a long time, getting them sweaty, dirty and generally covered in all sorts of icky stuff. Fortunately, much of humanity doesn't have that issue anymore, thanks to the discovery of laundry detergents. Today, we can throw some socks in a washing machine and have them clean in a few minutes, thanks to the remarkable efficiency of laundry detergents at getting the dirt out of your clothes.

Modern laundry detergents contain a huge array of chemicals that help the cleaning process and make your clothes look nice, including chemicals that digest stains, clean the water and perform many other tasks. We'll discuss more of these in future columns, but for right now, let's focus on the two main ingredients of all laundry detergents: the water conditioners and the detergent that does the dirty work.

You might think that detergents are simple: add them and the dirt magically disappears. But, as anyone who has tried to scrub a small child clean can tell you, you need more than just soap or detergent to get things clean. To work, laundry detergents (and small child cleaners) need three things: water, warmth and agitation. All of these are essential parts of the process, and modern laundry detergents are created to make the most of all of them with a huge range of fabrics and materials.

In most detergents, the bulk of the weight is made up by chemicals called water conditioners. Tap water is full of things like calcium and other metals that interfere with the washing process, so these water conditioners attract these and bind them, keeping them out of the way. This is why in areas with hard water (which contains lots of these metals) you often have to use more laundry detergent, because the more metals there are in the water, the more water conditioners are needed to grab them and stop them messing with the washing process. The most commonly used water conditioner is a chemical called sodium carbonate, which loves to grab metals and hold them tight. Older laundry detergents used sodium triphosphate, which was more effective, but which caused problems by promoting the growth of algae and similar organisms in rivers, choking other wildlife and killing native plants and fish (a process called eutrophication). So, sodium triphosphate was replaced by the less effective (but less ecologically damaging) sodium carbonate. These water conditioners form more than half of the content of the detergent by weight.

The second biggest ingredient of detergents is a chemical called a surfactant, which is one of the main active ingredients that lifts and removes stains. These surfactants have a conflicted nature. Most modern detergents are a class of chemicals called linear alkylbenzonesulfates, which are made of long chains of a chemical called a dodecane, composed of long chains of Carbon and Hydrogen. This chemical readily forms long chain molecules, quite similar to petrochemicals like oil. Attached to this is a benzene ring, with a sulfate molecule attached. These two parts fundamentally disagree about something: how they feel about water. The dodecyl chain hates it, doing all it can to get away from it. The benzosulfate bit, however, loves water and wants to get close to it. Chemists call these properties hydrophobic (water-hating) and hydrophilic (water-loving), and this conflicting nature is what makes detergents so powerful. Dodecyl chains hate water, but like each other, and also like other chemicals like fats, sugars, proteins and others. In other words: all of the things on your clothes that you want to get off. Dodecyl chains also like each other: give them a chance, and they will gather together and complain about how much they hate water.

When you put a small amount of detergent in water, these two parts of the detergent molecules argue. The benzosulfate bit is happy to swim about loosely bonding with water, while the dodecane chain hates water. If these molecules bump into something they feel better about (such as, say a bit of fat from milk on your shirt), they reach a compromise, with the dodecyl chain attaching to the fat, and the benzosulfate chain happily hanging out with the water. As things slosh about, the bit of fat will attract the dodecyl chain of more detergent molecules, covering it. The benzosulfate bit is still trying to hang onto the water, though. Eventually, the two reach a compromise by creating a small globe around the bit of fat or oil, which then floats off the shirt into the water. When it is like this, the dodecyl chains are happily hanging onto the fat and chilling with their neighbor dodecyl chains, and the benzosulfate is hanging with the surrounding water. And that's how the stains are lifted from your shirt: they are surrounded and freed by these enclosing globes of detergent.

These enclosed globes (usually called micelles) are very small: usually composed of just a few hundred molecules in each globe, floating in the water. Chemists call this type of solution a colloid, because the oils aren't really dissolved in the water: they are instead held in thee microscopic micelles .To you, it looks like the stains are dissolved in the water, and when the water is drained, it takes the stains with it down the drain.

That, of course, is only part of the story. These surfactants form about 30 to 40 percent of the weight of the detergent, and they are accompanied by chemicals such as enzymes and others that break stains down and help the process run quickly. We'll look at these in a future column, and look at how temperature and agitation affects the chemistry of cleaning your socks.

(One interesting bit of historical grossness: the Romans didn't use detergents, although soaps were known to them. Instead, they used urine. For the aristocracy of Rome, the best way of getting your wollen toga washed after it got grubby with a few weeks wear was to give it to the Fullo, the washer and scourer of cloth and linen. This dedicated class of laundry workers used urine and other chemicals to remove stains. To obtain the urine, they placed large pots on street corners and encouraged passers by to relive themselves. The clothes were then washed in this mixture, which was agitated by young boys treading on it for hours. The fabric was then scoured in fuller's earth and dried. So, the next time you are watching Spartacus or a particularly gripping performance of Julius Caesar, consider that the hero's outfit would have been washed clean of the blood stains afterwards in urine, and that the streets these heroes trod stank of urine. Believe me, it gives you a whole new perspective on history, and how utterly icky most if it was.)