Washing your hands thoroughly requires soap and water. But how does soap keep your skin clean and free of germs? There is a bit of chemistry underlying how soaps work.

Soaps and detergents

By itself, water is not good at washing away dirt or grime. Most of those molecules have oils in them, and as you may have observed, oil and water don’t mix. Oils are nonpolar (uncharged) while water is polar (charged), which is why they don’t go together well. 

Soaps, on the other hand, have both polar and nonpolar chains. Its polar ends are said to be hydrophilic, or water-loving, because they will bond with water. Nonpolar chains make up the hydrophobic part of soap. 

Soaps are therefore able to emulsify water and oil. Emulsify means to combine two immiscible (unmixable) things. When making mayonnaise or hollandaise sauce, eggs are used as an emulsifier to mix oily components with water-based ingredients. 

In oil and water, soap molecules take the form of a micelle, where its hydrophobic ends are all at the center surrounding oil, while the hydrophilic ends are on the edges. Micelles can then be dissolved in water since they are polar, but they can grab oil molecules because they are also nonpolar.

Now we know how soaps work, but are the detergents the same thing? 

While similar, soaps and detergents are actually two different types of cleaners. Both contain surfactants, which reduce surface tension, in turn making it easier for micelles molecules to bind to water.

But soaps are made from natural ingredients, like plant oils or animal fats. Detergents, on the other hand, have synthetic components.

A majority of foaming cleaning products are detergents, not soaps.

This is because when soap reacts with hard water, the two will precipitate and leave a residue. This residue must be rinsed away or it will build up. Soap scum is not just annoying but also gradually ruins fabrics. 

Another difference between soaps and detergents is that soaps work best in warm water.

Detergents especially can vary in their chemical makeup. This makes detergents suitable for cleaning a number of surfaces, including laundry, dishes, and carpets.

A brief history of surfactants

Soaps have been around at least since the time of the Babylonians, dating back to 2800 BCE or more. The ancient Babylonians boiled animal fats, wood ash, and water to make soap.

This first soap was used to wash fibers prior to making cloth. Since the Babylonians, most civilizations have been known to use soap in some form. Soap was initially utilized primarily as a medicine.

Around 1550 BCE, ancient Egyptians began creating soap with oils and alkaline salts, using it to treat skin diseases and sores. The Phoenicians made soap from tallow and wood ash and sometimes traded the product with the Gauls.

However, soaps were not used to clean the body until around 100 CE, when the Greek physician Galen made note of its ability to cleanse skin.

The recipe for soap that is largely still in use was first cooked by Arabic chemists in about 600 CE. They mixed vegetable oils, aromatic oils, and sodium lye. 

Beginning around the 11th century, scientists gradually discovered that germs are responsible for some diseases. This is known as the germ theory of disease and was not widely accepted until the late 1800s. As the years went on, soap gained popularity as a bathing agent as the practice of modern hygiene became more widespread.

Since the 20th century, commercial production of soap has been prevalent. Making soaps on a large scale was impossible prior to relevant breakthroughs in chemical engineering. Among these developments was the discovery of the chemistry behind soap formation.

Also known as saponification, soap forms due to the chemical reaction between fats, which split to make an alkali salt of glycerin and fatty acids. Saponification was demonstrated by the work of French chemist Michel Eugène Chevreul in 1923.

During World War I, the resources needed to make soap had to be diverted to other manufacturing processes. In place of soap, Germans developed the first synthetic detergent. As the century progressed, detergent quality improved rapidly.

Modern soaps and detergents usually have a few ingredients in addition to alkali and fats. 

Optical brighteners are found in virtually all laundry detergents. They are a type of dye absorbed during washing that aren’t washed out of the fabric when rinsed. Optical brighteners reflect light, so they make cloth appear to be brighter.

Some soaps have sequestering agents like ethylenediaminetetraacetic acid (EDTA) which chelate, or bind to, metals in water. This essentially softens the water, meaning no soap residue is formed.

And yet other soaps and detergents contain abrasive substances. These physically agitate a surface to clean it. Among the abrasives that might be seen in soaps and detergents are talc, chalk, and diatomaceous earth.

Antibacterial soaps: the good and the bad

In the 1940s, Dial introduced antibacterial soap, which originally contained hexachloropene. As the years went on, though, it was discovered that hexachloropene had numerous harmful side effects, such as brain damage in infants. Hexachloropene is therefore no longer a component of antibacterial surfactants.

Triclosan was the next antibacterial molecule used in soaps. However, triclosan has also been found to have detrimental effects.

Triclosan is an endocrine disruptor, a molecule that mimics a hormone and interferes with important endocrine and hormonal functions in the body. The chemical is also found in household sewage at high levels, where it can combine with chlorine to create chloroform. Triclosan exposure heightens the risk for some cancers as well.

Furthermore, triclosan has actually been shown to increase antibiotic resistance. Antibiotic resistance is a growing issue in the current day and contributes to the evolution of superbugs, which are bacteria that are resistant to available antibiotics. Antibacterial soaps also decrease the strength of the human immune system.

As a result of these findings, The Food and Drug Administration forbid the use of triclosan in antibacterial soaps in 2016. But other common consumer products such as toothpaste still contain triclosan. More than 75% of Americans continue to have some triclosan in their system. This is despite the fact that antibacterial soaps are no more effective than non-antibacterial soaps at preventing illness.

Soaps are a valuable item that we use constantly but probably take for granted. They help us, our clothes, and our dishes squeaky clean! The chemistry behind them is actually quite simple, so hopefully you can now teach someone something new about soaps.

Also, be sure to always wash your hands for at least twenty seconds, with water (cold or hot) and soap!

See soap in action

There is an easy way to demonstrate how soap works using materials you already have at home.

All you need are two bowls, water, a soap or detergent, and pepper.

Put water in both bowls. In one bowl, sprinkle some pepper. In the other bowl, squeeze a few drops of soap or detergent.

When you dip your finger in the pepper bowl, you might notice a few pieces of pepper sticking to your finger.

But when you put your finger in the soapy water before touching the pepper water, the soap on your finger will repel the pepper grains.

This means pepper is nonpolar – it cannot be dissolved in water, and can be washed away by a soap.

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