Stains on clothes can be a pain to shift – luckily chemistry is on hand to help out! A range of different molecules are present in stain removers and detergents to help shift grease and dirt, and they can work in different ways. This graphic takes a look at how we can categorise different types of stains, and how the molecules that help remove them work.
Stains themselves can be roughly grouped into several classes: enzymatic, oxidisable, greasy, and particulate. This is really something of a simplification – in reality, a particular stain will have a number of components, which may fall into more than one of these categories. For example, a stain from something like a tomato pasta sauce would have a coloured, oxidisable component, but would probably also be a little greasy. As such, stain removers tend to be a mixture of all of the agents we’ll discuss, to tackle these multi-component stains.
Enzymatic stains include blood stains and grass stains, which are both largely the result of proteins. Enzymes in stain remover formulations can be used to break these down. Specifically, proteases will break down proteins by breaking the larger molecules into smaller, soluble chunks. Human sweat stains can also be removed by proteases. Other molecules that can be broken down by enzymes include fats, broken down by lipases, and starch, broken down by amylases.
Brightly coloured stains often fall into the oxidisable stain category. These include stains like tea and coffee, as well as red wine. Throwing white wine on a red wine stain won’t help at all – but throwing a bleach-based stain remover on it might. These stain removers contain bleaching agents, commonly hydrogen peroxide, which breaks down colour-causing sections of chemical structures, removing the appearance of the stain. The hydrogen peroxide is usually present in the form of sodium percarbonate, which releases hydrogen peroxide when combined with water.
One issue with hydrogen peroxide is that it doesn’t function quite as well at removing stains below 40˚C. Not a problem if you’re washing at or above that temperature, but if you’re washing lower, or just wanting to use your stain remover on carpet or furnishings, the hydrogen peroxide is going to need some help. It gets this in the form of the addition of tetraacetylethylenediamine, or TAED for short. TAED reacts with hydrogen peroxide to produce peracetic acid, an even stronger bleaching agent than hydrogen peroxide.
Whilst oils and grease can be broken down by lipase enzymes, they are primarily removed by the use of surfactants. These are commonly long carbon chain compounds with a charged water-soluble ‘head’ and an oil-soluble ‘tail’. Generally, they’ll appear listed as either ‘cationic surfactants’, ‘anionic surfactants’, and ‘nonionic surfactants’ on the stain remover bottle. These simply refers to the charge (or lack of) on the molecule’s ‘head’. A cationic surfactant has a positive charge, an anionic surfactant a negative charge, and a nonionic surfactant has no charge.
These surfactants remove oil and grease by forming structures called ‘micelles’ around them. The oil-soluble parts of the molecule dissolve in the oil or grease, forming a spherical structure around the oil droplet. The water-soluble parts of the surfactant molecule are then sticking outwards, meaning that the micelles are able to dissolve in water, allowing the greasy stain to be washed away.
Finally, for particulate stains, compounds referred to as ‘builders’ are used. These compounds primarily help to soften the water during washes by removing positive metal ions (mainly calcium and magnesium ions) from the water. They are also very helpful in removing soil stains from clothes, as these stains are often bound to fabrics by calcium ion bridging. Removing the calcium ions therefore helps remove the dirt.
Washing detergents used to commonly use sodium triphosphate as a builder, but due to concerns about its excessive release into the environment (it can cause eutrophication) many companies have now replaced it with other agents. Some of these can include sodium carbonate, polycarboxylates, and also zeolites. Zeolites are crystalline aluminium silicates, inorganic structures with pores that can incorporate calcium and magnesium ions. They also possess a number of other advantages over other builders, such as inhibiting dye transfer during washes. Generally, builders also increase the cleaning action of other chemicals in the detergent, by preventing cations interfering with other charged molecules, such as surfactants. They can additionally help prevent the redeposition of dirt onto fabrics once it has been removed.
References & Further Reading