How Does Soap Work? A Molecule With Two Opposite Ends

A soap molecule is a long chain of carbon and hydrogen atoms with a charged group at one end. The chain is a fatty acid, the same kind of molecule that fats and oils are built from, and it dissolves happily in grease and not at all in water. The charged head is the opposite: it is attracted to water molecules and repelled by grease. Chemists call such molecules amphiphilic, loving both, and the class of cleaning agents built on the principle is the surfactants.

In water, soap molecules arrange themselves to keep their tails out of the water. At the surface they line up with heads in the water and tails poking into the air, which lowers the water's surface tension and lets it spread into cracks and fabric instead of beading up. Below the surface they gather into spheres called micelles, tails pointing inward and heads facing out, a few dozen molecules each.

When soapy water meets a greasy plate, the tails of the soap molecules bury themselves in the grease while the heads stay in the water. Rubbing breaks the grease into droplets, each of which ends up wrapped in a layer of soap with its charged heads facing outward. The droplet is now, from the water's point of view, a water-friendly object, and it can be rinsed away. Because all the coated droplets carry the same charge, they repel each other and do not merge back into a film.

Hot water helps because grease softens and the soap molecules move faster; agitation helps because the droplets have to be broken up before they can be surrounded. Neither is strictly necessary, which is why cold water and soap still work, only more slowly.

Soap is made by boiling fat with an alkali, a reaction called saponification in which the fat's fatty acids are split off and given their charged heads. Soap-like material from about 2800 BCE has been found in Babylonian clay vessels, a written recipe survives on a clay tablet from a few centuries later, and Roman, Arab and medieval European makers used animal fat or olive oil with ash from wood or seaweed, which contains potassium and sodium carbonates. The city of Aleppo has made olive and laurel oil soap for over a thousand years, and Marseille and Castile gave their names to the trade.

The chemistry became an industry in the nineteenth century when soda ash could be manufactured cheaply, and a public health measure when the link between washing and disease was established. Detergents, invented in the twentieth century, do the same job with synthetic surfactants made from petroleum, and they have one advantage over true soap: they do not react with the calcium in hard water to form the grey scum that soap does.

Many viruses, including influenza and the coronaviruses, are wrapped in an envelope made of fat, taken from the membrane of the cell they came from. To soap, that envelope is just another grease droplet. The tails of the soap molecules force their way into the fatty layer and pull it apart, the virus falls to pieces, and the pieces are rinsed away. Alcohol-based sanitisers work on the same envelope by a different route, dissolving it, and are less effective on soiled hands because they cannot lift dirt.

This is why the standard advice for hand washing is twenty seconds with soap. The time is for the soap molecules to work their way into every crease of the skin and every viral envelope on it, and the rubbing is the agitation that breaks the grease up. Water alone rinses off some particles; soap dismantles them. What happens at the sink, in order:

• •Soap lowers the water's surface tension so it wets the skin thoroughly
• •Soap tails embed in grease and in the fatty envelopes of any viruses
• •Rubbing breaks the grease into droplets and tears the envelopes apart
• •Soap heads coat each fragment so water can carry it
• •Rinsing removes the whole lot

Pure water is poor at forming bubbles, because its strong surface tension pulls any thin film together until it snaps. Soap lowers that tension and does something more. A bubble's wall is a thin layer of water sandwiched between two layers of soap molecules, their heads in the water and their tails pointing out into the air on either side. When the film is stretched in one spot, the soap there thins out, the local surface tension rises, and the film pulls back, healing weak points before they break. That self-repairing quality is why soapy films last.

The swirling colours come from the film's thinness. Light reflects from both the outer and inner surfaces of the wall, and the two reflections interfere, reinforcing some colours and cancelling others depending on how thick the film is at each point. As water drains downwards and evaporates, the film thins and the colours shift, until the top of the bubble looks almost black just before it bursts. Adding glycerine or sugar slows the evaporation, which is why home bubble recipes include them and why their bubbles survive longer.

Bacteria are not built like enveloped viruses. Most have a tough cell wall around their fatty membrane, so ordinary soap does not reliably burst them, and it does not need to. Its main job on your hands is removal: the soap loosens the oils and grime that bacteria cling to, and rubbing and rinsing carry them down the drain. Late in the twentieth century manufacturers began adding germ-killing chemicals such as triclosan to many hand washes and body washes, promising to kill what plain soap merely washed away, and antibacterial versions came to fill supermarket shelves.

The promise did not survive testing. In 2016 the United States Food and Drug Administration ruled that consumer antibacterial washes containing triclosan and eighteen other ingredients could no longer be sold, because their makers had not shown that they prevented illness better than plain soap and water. Regulators were also uneasy about the chemicals building up in rivers and about the possibility of encouraging resistant bacteria. In ordinary hand washing, a few seconds of contact with an added antiseptic achieves little that thorough scrubbing does not already do. Hospitals still rely on antiseptics and alcohol rubs, but for different settings and different risks.

In areas with hard water, washing with soap leaves a grey ring on the bath and a dull film on skin and hair. Hard water carries dissolved calcium and magnesium, picked up as rain soaks through limestone and chalk, and these react with soap molecules to form a sticky, insoluble solid. The soap that forms scum is no longer available for cleaning, so more is needed to get the same lather.

This problem was one reason synthetic detergents were developed in the twentieth century. Their molecules work the same way as soap, with a water-loving head and a grease-loving tail, but their heads do not combine with calcium and magnesium to form scum. That is why washing-up liquid, shampoo and laundry detergents work well even in hard water, while a traditional bar of soap struggles.

In the 1840s the Vienna General Hospital had two maternity wards, and women begged not to be admitted to the first. It was staffed by doctors and medical students, and its death rate from childbed fever was far higher than that of the second ward, run by midwives. Ignaz Semmelweis, a young Hungarian doctor there, noticed that the students often came straight from dissecting corpses. When a colleague died after being cut by a scalpel during a post-mortem, with symptoms like those of the dying mothers, Semmelweis concluded that some invisible material from the bodies was being carried to the women on doctors' hands.

In 1847 he made everyone on the ward wash their hands in a solution of chlorinated lime before examining patients, and deaths fell sharply, to around the level of the midwives' ward. Many colleagues rejected the idea, partly because it implied that doctors themselves were killing their patients, and Semmelweis died in 1865 with his work largely dismissed. The germ theory of Pasteur and the antiseptic surgery of Joseph Lister vindicated him within a generation. Modern trials have since shown that simply promoting handwashing with ordinary soap in homes with young children markedly reduces diarrhoea and pneumonia, which still kill large numbers of children worldwide.

Soap cleans because its molecules have a water-loving head and a fat-loving tail, so they surround droplets of grease and let water carry them away. Made from fat and alkali for four thousand years, it lowers surface tension, forms micelles around dirt, and dismantles the fatty envelope of many viruses, which is why twenty seconds of hand washing works.