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How soap kills the COVID-19 virus

Dr Dan Corbett from Queen’s School of Chemistry and Chemical Engineering explains why old-fashioned soap is an unlikely weapon in the war against COVID-19.

As scientists around the world screen libraries of drugs searching for a combination with the potential to stop the COVID-19 pandemic and supermarket shelves are stripped of anti-bacterial hand gel and disinfectant, one highly effective way to kill the SARS-CoV2 virus on your skin has emerged: old-fashioned soap and water.

Dr Dan Corbett from Queen’s School of Chemistry and Chemical Engineering says, “As the global coronavirus pandemic continues, we’re receiving rapidly developing advice, however, one piece of advice has been clear from the start: we should be frequently and correctly washing our hands, using soap and water.

“Simple, good old-fashioned soap and water is exceptionally effective at removing the coronavirus from your hands - more so than harsh chemicals or disinfectants that can also damage the skin,” adds Dr Corbett. 

How does a simple bar of soap have the power to kill a virus powerful enough to spread round the world and send billions of people into lockdown?

The science of soap

“Soap is a surface-active agent (or surfactant for short),” explains Dan. “These substances are special in that they have a hydrophobic (water-hating) part, as well as a hydrophilic (water-loving) part. We regard these structures has having a ‘head’ (normally the hydrophilic bit), and a ‘tail’ (normally the hydrophobic bit).

“When we mix these molecules with water, they exist in two main forms: either on their own, or in groups, which we call micelles. Micelles are spherical structures within which all of the tails face inwards, away from the water, and all of the heads face outwards, coming into contact with water. If there is enough of the soap, this is the form that it likes the most, as it is the most energetically favourable, and all the parts of the molecule are exactly where they want to be.”

So what happens when these micelles interact with the coronavirus?

“When we look at the structure of the coronavirus in more detail, we see that it is surrounded in a phospholipid layer – a structure which consists of lots of molecules which are not unlike those within soap,” says Dr Corbett.

“In the case of the virus, this coat is actually a bilayer – consisting of two layers of these phospholipids with tails facing in, and heads facing out. When we wash our hands with soap and water, the tails of the soap molecules start to look for an area where there isn’t any water, and as they do so, they start to surround the virus particles. As they continue to move around, the tails can wedge themselves into the virus’ outer layer, trying to get to the bit in the middle, where there is no water. This effect is similar to popping a balloon with a pin. As the soap molecule penetrates into the virus coat, it splits it apart, breaking the virus open, releasing its contents into the surrounding soapy water, which eventually carries them away,” says Dr Corbett.

Soap versus hand gel

The reason that soap is such a powerful agent against viruses like COVID-19 is because of its multiple mechanisms of action.  “Soap molecules can pierce the virus, but can also surround it, trapping it in a micelle and defeating those strong chemical bonds between the virus and our skin,” says Dr Corbett.

So how does the power of soap compare against antibacterial hand gel?

“The truth is that soapy water is much more reliable than liquids and gels containing high quantities of alcohol,” says Dr Corbett. “The reason for that is pretty simple: despite the fact that alcohol also works to break viruses apart, they miss out on the important aspects that help to carry things away from the skin. This means that whilst hand sanitisers do work well in a pinch, you might still be leaving some of the active virus on the skin. With all of this being said, alcohol-based hand sanitising products are a good substitute for soap and water when it’s not available!”

WHO handwashing guidelines

School of Chemistry and Chemical Engineering