How Does Soap Work? A Molecule With Two Opposite Ends
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Water on its own does a poor job of cleaning, because most dirt is stuck on with grease and grease does not mix with water. Soap solves the problem with a molecule that has a foot in both camps: one end that clings to water and one that clings to fat. Given both, it surrounds each droplet of grease with a coat that water can carry away. The same trick is why soap and water are the most effective defence against many viruses, including the one that causes COVID-19.
A head that loves water and a tail that hates it
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.
Lifting the grease
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.
Four thousand years of fat and ash
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. The earliest recipe survives on a Babylonian clay tablet from about 2800 BCE, 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.
Why soap kills viruses
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
The takeaway
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.