Getting Clean: The Science of Soap
Credit: Hand washing via Shutterstock

Public health officials encourage handwashing with soap and clean, running water to prevent the spread of germs and infections. If clean water is not available, using any water with soap is essential, according to the Centers for Disease Control and Prevention. Soap, then, is the key element in hand hygiene.

The ancient Babylonians invented soap around 2800 B.C. Their recipe of animal fats, wood ash and water has been found carved onto clay containers. The rough concoction was mainly used for washing wool and cotton in order to prepare the materials to be woven into cloth. Ancient Romans, Greeks and Egyptians also independently developed recipes for soap. Arabic chemists made the first soap from vegetable products in the seventh century. 

The basic recipe for soap has not changed in thousands of years. Soap is made by combining fats and oils with an alkali, according to Soap History. Oils are plant-based, such as olive oil or coconut oil, and the fats come from animal sources (beef tallow, for example). Alkalis traditionally were obtained from wood ash; other sources exist today, such as sodium hydroxide (lye or caustic soda) or potassium hydroxide (caustic potash). Chemically, an alkali is a base — the opposite of an acid — that reacts with and neutralizes acid. 

Soap making entails a chemical process called saponification, according to the American Cleaning Institute. During the process, the alkali causes the fats or oils to split into fatty acids and glycerin. The sodium or potassium part of the alkali joins with the fatty acid part of the fat or oils. This mixture is called soap or the potassium or sodium salt of the fatty acid, according to Soap History. 

There are two ways that soap cleans:

  1. Soap lessens the surface tension of the water so that it more readily wets what needs to be cleaned rather than simply balling up on the surface. 
  2. Soap molecules are like a bar magnet — but instead of north on one side and south on the other, the molecules are hydrophilic (water-attracting) on one side and hydrophobic (water-repelling) on the other.

Surface tension is what causes water to form droplets or beads on surfaces. This phenomenon slows the process of water wetting many surfaces and can also slow the cleaning process, according to the American Cleaning Institute. The bond that each water molecule makes with other water molecules creates the surface tension, in which all molecules are pulled into the water droplet. Soap is a surface active agent, or surfactant, according to the Essential Chemical Industry, help to reduce the surface tension of the water so it can spread and wet the surface and speed up the cleaning process. Some people describe this as soap making "water wetter." 

Soap molecules are long and thin with one end being hydrophilic and the other hydrophobic. According to the Royal Society of Chemistry, the hydrophobic ends of the molecules are attracted to dirt and oil. The dirt and oil particles are surrounded by many soap molecules and being held onto by the hydrophobic ends while the hydrophilic ends stick outward waiting to be rinsed away by water.

These two processes work together to clean surfaces. Soap may also contain other ingredients, according to the Cleaning Institute, which are added to increase the effectiveness of the soap or to adjust the pH (to either control or modify the pH to make it safer and gentler). Some soap contains abrasives to help scour surfaces to better remove stubborn dirt and grime; water softeners to help additionally with making the water easier to clean with; and enzymes to help with biological stains such as grass or blood by digesting proteins, fats, and carbohydrates.

Some soap is labeled antibacterial or antimicrobial. These soaps go beyond washing away dirt, oil and anything else on your hands and kill any bacteria or other microbes (such as yeast, fungi, and possibly viruses) on your skin, in your clothes, or on your dishes, according to Clean Link

According to University of California, Santa Barbara’s Science Line, antibacterial soaps are made just like regular soap with the addition of triclosan or triclocarban. These are hydrophobic molecules that can penetrate through fatty substances such as bacteria cell membranes, where the molecules then poison the bacteria. The residues are held in suspension and rinsed away with water. Triclosan and triclocarbon have antibacterial and antifungal properties that are not only commonly used in soaps, but in toothpaste, cosmetics, and some plastics used for toys and kitchen utensils. 

However, the Food and Drug Administration has banned companies from marketing hand soaps containing triclosan or triclocarban. In announcing the ban, the FDA cited questions about the antibacterials' safety for long-term use. In addition, there is no evidence these chemicals add any benefit to people's heath beyond those of regular soap, the agency said.

"Consumers may think antibacterial washes are more effective at preventing the spread of germs, but we have no scientific evidence that they are any better than plain soap and water," Dr. Janet Woodcock, the director of the FDA's Center for Drug Evaluation and Research (CDER), said in a statement. "In fact, some data suggests that antibacterial ingredients may do more harm than good over the long term."

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