Introduction to The Chemistry of Cleaning Clothes
Have you ever wondered how a cup of detergent cleans soiled jeans, towels, socks, and other articles in your wash, or how cleaning fluid at the dry cleaner whisks away that grease stain from your suit? And why are hair spray,vinegar, club soda, and baking soda surprisingly effective dirt and stain removers in an emergency? The answers lie both in the chemical properties of various types of dirt and stains and in the ways that the cleaning agents chemically interact with them.
Dirt and stains typically consist of particles, such as minerals from soil, protein and other organic matter from living things, or bits of black carbon. The particles are trapped on cloth fibers by grease and oil, which cannot be dissolved in water. Anything that can loosen the grease and oil from the fibers and disperse (scatter) these substances in the wash water or dry-cleaning solution will remove the dirt and stains.
How Soaps and Detergents Work
Soaps, detergents, cleaning fluids, and many emergency stain removers are effective cleaners because they can dissolve and emulsify (break up and suspend) the oil and grease that holds dirt in place. The grease and trapped soil particles then can be carried away in the water or dry-cleaning solution. But these agents differ widely in their dirt-fighting activities and the conditions under which they work. To understand why, we must understand the chemical nature of soaps and detergents.
The words soap and detergent are often used interchangeably, but the two cleaners differ considerably. Soaps are generally made from natural fats and oils. Soaps are excellent for cleaning our hands, face, and body, because the loosened dirt is rinsed away immediately. But soaps have definite drawbacks for cleaning laundry. For example, soaps often allow dirt lifted from clothes to redeposit on the clothes before the wash cycle is finished. And in hard water (water containing high levels of minerals), soaps react with minerals to form scum, called soap curd. Soap curd does not dissolve. It is difficult to remove from fabrics, and it makes the fabric feel stiff.
Because of these drawbacks, laundry soaps have largely been replaced by detergents. Detergents are synthetic (artificial) mixtures of ingredients that not only clean clothes but also prevent redeposition of dirt, discourage scum formation, and possess other useful properties. The most important advantage of detergents is the ability to clean effectively in hard water.
The Importance of Surfactants
Both soaps and detergents contain cleaning ingredients known as surfactants. Surfactant compounds are molecules (linked groups of atoms) attracted to the boundary between two liquids that normally do not dissolve in each other, such as oil and water. One end of the surfactant molecule is attracted to water but not oil, and the other end is attracted to oil but not to water.
This dual nature of surfactant molecules boosts the “wetting” ability of water. This means that water containing surfactants can more easily penetrate and disperse dirt and stains. One end of the surfactant molecule dissolves and emulsifies the grease that traps soil particles on fabrics. The other end dissolves in the surrounding water. As a result of this action, one portion of the molecule pulls away from the other, and this force pulls the grease from the clothes and suspends it in the form of tiny droplets. Washing machine agitation also helps loosen the greasy soil. After the soil droplets are suspended in the water, the thin layer of surfactant molecules around them keeps them separated from the fabric and prevents them from resettling on the clothes. The suspended droplets and the soil clinging to them are then easily rinsed away by the water.
The same principle enables hair spray to remove ink and certain other stains from clothes. Some hair sprays contain alcohol, which behaves chemically in a way similar to surfactants in detergents. One portion of the alcohol molecule penetrates and emulsifies the oils that hold the ink pigments in place. Another portion of the alcohol molecule dissolves in the alcohol solvents also found in hair spray. In this way, hair spray loosens the ink pigments, which can then be removed by conventional laundering with water and detergents.
How Enzymes Help In Cleaning
Compounds called enzymes enhance the cleaning action of surfactants. Enzymes are complex molecules made by living organisms. Often called “biological catalysts,” enzymes promote certain chemical reactions without themselves being changed. Enzymatic action is similar to digestive juices in the stomach, which break down food in preparation for digestion in the intestines. Detergent enzymes, made by bacteria in factory production vats, react with and break up stains that are made of proteins. Such stains include blood, meat gravy, milk, eggs, and grass. Enzymes break down these substances into simpler forms that can be removed by other components in the detergent.
If you're out of enzyme detergents and the stores are closed, try using a meat tenderizer on a protein stain. The tenderizer contains enzymes intended to partially digest proteins in meat before it is cooked. But when poured onto clothing or carpets, the enzymes can also break up protein molecules in stains. (But make sure to rinse the stain with water to wash away the salt, spices, and coloring that are included with the enzymes in the tenderizer.)
The Role of Builders
Another group of chemical compounds used in detergents are called builders. Builders typically make up more than half the weight of a box of detergent. Their principal function is to soften hard water. These chemicals react with and remove from wash water certain minerals, particularly those containing calcium and magnesium. Such minerals can react with surfactants to form scums that deposit on clothes and interfere with cleaning action. Minerals can also promote redeposition of removed soil particles.
Another function of builders is to make the wash water alkaline. Alkali builders are chemicals that neutralize acids in the water and aid the breakup of oil and fat molecules by rupturing their chemical bonds. Some builders act as buffering agents to maintain the proper alkaline level in the wash water.
Because ordinary baking soda contains an alkali—sodium bicarbonate—baking soda is handy for neutralizing and removing acid stains such as those made by toilet-bowl cleaners. Because vinegar contains acetic acid, which is mildly acidic, it is useful for breaking up and dissolving such alkaline stains as hard-water residues. You also could use club soda on these residues, because it contains weakly acidic carbonic acid, a compound not found in plain water.
Some builders also boost the action of surfactants. For example, certain builders help surfactants suspend loosened dirt and keep it from settling back on clothes. Other builders help surfactants emulsify greasy soil by breaking the oily particles into tiny globules.
The Declining Use of Phosphates
In the 1960's, chemicals called phosphates were the most common builders in detergents. Phosphates remove minerals from hard water by combining with them. The compound thus formed is then rinsed away with the water after the clothes are washed.
But phosphates in waste water were found to harm the environment. Detergent phosphates ultimately ended up in streams and lakes, and because phosphates are nutrients for algae, the chemicals overfertilized the streams and lakes. The result was excessive growth of algae. Eventually, the abundance of algae clogged streams and lakes, setting in motion a process that could kill most of the life in the water. Because of this, detergent manufacturers drastically reduced the phosphate content of their products and began using builders that were less harmful to the environment.
Suds and Bleaching
Interestingly, there is no relationship between a detergent's sudsing action and its cleaning ability. Nevertheless, manufacturers may recommend the use of low sudsing detergents for front-loading tumbler-type washing machines because high levels of suds would cushion clothes as they drop back into the water after being lifted out in the tumbling action. Such a cushioning effect would interfere with the machine's washing action. To appeal to consumers who prefer various amounts of suds, detergent manufacturers include in their formulas special sudsing modifiers. These compounds are long-chain molecules, made from natural fats, that can either boost or depress levels of suds made by dissolved detergents.
Bleaches do not remove dirt particles but make them colorless or nearly colorless. Liquid chlorine bleach is the most powerful of the chemical bleaches used as laundry aids. Chlorine bleach not only whitens clothes, but also disinfects and deodorizes them. It can, however, remove color from clothes.
A less powerful chemical bleach is oxygen bleach. Because it is safe to use on most fabrics, oxygen bleach is the one most frequently added to detergents. It is also used in presoak products to aid in cleaning heavily soiled clothes or in helping to remove stubborn stains before clothes are put through a normal washing machine cycle.
Some presoak products use enzymes, but these require more time to work than do products using only oxygen bleach. Also, enzyme presoak products should not be used at the same time as chlorine bleach, because chlorine bleach destroys enzymes. By using the products separately, you will get the maximum benefit of each.
Other Additives
Other laundry aids also do not remove dirt or stains, yet they can make clothes appear cleaner. Whiteners, also known as optical bleaches, consist of organic (carbon-containing) molecules that can absorb invisible forms of light and, through a complex process at the atomic level, reemit it as visible blue light. Clothes treated with these compounds come out of a wash looking both brighter and whiter than they did before being washed.
The hydrogen peroxide found in many medicine cabinets for the treatment of wounds behaves comparably to the bleach we add to washes. When applied to blood stains, for example, peroxide liberates oxygen atoms, which turns red blood pigments into less brightly colored stains. Denture cleaning tablets, which contain oxygen in the same form found in oxygen bleaches, can similarly decolorize stains made by tea and coffee.
Unfortunately, laundry cleaning agents function only in water, a medium that can damage some natural fabrics, such as silk and wool, which are water-sensitive. When they are wet, water-sensitive fibers swell in diameter and shorten, causing the garment to shrink.
Dry Cleaning
The most effective way of removing dirt and stains from water-sensitive articles is dry cleaning. Dry cleaning is a process in which a liquid other than water is used to dissolve and flush away oil and grease along with underlying soils. The most useful solvents in dry cleaning are water-insoluble liquids derived from petroleum, particularly a carbon- and chlorine-containing compound called perchloroethylene. Dry-cleaning solvents, unlike water-based detergents, do not repel oil and grease molecules. Instead, the solvents surround and dissolve these molecules. Many commercial dry cleaners also add special detergents to their solvents to further loosen soil particles.
A primitive form of dry cleaning is possible using vegetable shortening as a “solvent.” The shortening dissolves oils—for example, those in deep-fried snacks or those that hold ink pigments in place. Then a follow-up treatment with hair spray will remove the dissolved oils and pigments.
So, the next time you're studying late on a Sunday night, and your pen slips from the paper onto your white pants, where a greasy snack fell without your knowing it, remember your chemistry. A little shortening and hair spray could prevent an ugly stain.