23 Mar 2020

Covid-19: the science of soap

From Our Changing World, 7:00 am on 23 March 2020

One of the best protections we have against SARS-CoV-2, the coronavirus that causes the severe respiratory disease Covid-19, is to wash our hands, well and often.

That’s because when it comes to virus-busting, soap is an oldie but a goodie. And it turns out that soap is particularly effective against coronaviruses.

Bar of wet white soap

Photo: Picture Partners

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“We’ve got to find relatively benign molecules that are deadly to viruses - and of everything that is out there soap is a damn good one,” says chemist Professor Allan Blackman, from the Auckland University of Technology.

Know your enemy

“A virus is a very intriguing package of nucleic acid that is wrapped up in a protein exterior and then, in some cases, surrounded by a lipid – or a fat - envelope.”

That’s according to University of Otago virus expert Professor Kurt Krause, who says the lipid envelope is important for maintaining the integrity of viruses such as those in the coronavirus group.

In the case of a coronavirus, however, that fatty envelope is both a strength – and a weakness.

The reason why comes down to chemistry.

Transmission electron microscope image shows SARS-CoV-2, the virus that causes COVID-19—isolated from a patient in the U.S. Virus particles are shown emerging from the surface of cells cultured in the lab. The spikes on the outer edge of the virus particles give coronaviruses their name, crown-like.

This transmission electron microscope image shows SARS-CoV-2 -also known as 2019-nCoV, the virus that causes COVID-19—isolated from a patient in the U.S. Virus particles are shown emerging from the surface of cells cultured in the lab. The spikes on the outer edge of the virus particles give coronaviruses their name, crown-like. Photo: CC-BY-2.0 NIAID-RML

The chemistry of fat and water

Grease and water, as we all know, don’t mix. That’s because water is a polar molecule, while grease or oil is a nonpolar molecule.

“What we mean by polar is that there is some separation of charge,” says Blackman. “One end of the molecule is slightly positively charged and the other is negatively charged.”

So a polar molecule such as water has both a positive and a negative end, just as a battery does, while a nonpolar molecule has its charge evenly spread.

“And there’s a rule in chemistry,” says Blackman, “that ‘like dissolves like’. Polar molecules like other polar molecules … so polar molecules will dissolve quite happily in water.”

You can happily stir sugar into your tea, for example, because both water and sugar are polar molecules.

Try and wash anything oily or greasy off your hands with just water, however, and it won’t work. The nonpolar oil is insoluble in water and much prefers sticking to your slightly greasy skin.

Add soap, however, and the oil washes away.

That’s down to the unique molecular structure of soap, and it works just as well against a virus protected by a fatty envelope as it does against any greasy object.

Novel Coronavirus SARS-CoV-2 ; This scanning electron microscope image shows SARS-CoV-2 (round gold objects) emerging from the surface of cells cultured in the lab. SARS-CoV-2, also known as 2019-nCoV, is the virus that causes COVID-19. The virus shown was isolated from a patient in the U.S.

Novel Coronavirus SARS-CoV-2 ; This scanning electron microscope image shows SARS-CoV-2 (round gold objects) emerging from the surface of cells cultured in the lab. SARS-CoV-2, also known as 2019-nCoV, is the virus that causes COVID-19. The virus shown was isolated from a patient in the U.S. Photo: CC-BY-2.0 NIAID-RML

The secret of soap’s success

The basic recipe for soap hasn't changed for nearly 5,000 years. Mix fat or oil with something alkaline, usually sodium hydroxide or lye, and the process of saponification combines the two to make soap.

Blackman says you can describe soap as a surfactant or amphiphilic compound. What this means is that soap is both water-loving and fat-loving, and this is the secret of its success.

You can picture a soap molecule as having a head – the water-loving bit – and a long tail – the fat loving bit – made up of a chain of hydrocarbons.

“If you make a molecule that’s got a polar end and a nonpolar end,” says Blackman, “the polar end really wants to be in the water and the nonpolar end really wants to be in the grease.”

“What really is going on there is [the soap molecule] drags the grease into the water,” says Blackman, “it kind of makes the grease soluble in water.”

That is why a good soapy lather will wash oils and fats off your hands.

Coronavirus busting

But when it comes to coronaviruses, soap actually goes one better.

Professor Krause says that the lipid envelope of a coronavirus is its Achilles heel, because it is readily broken down or busted apart by soap.

“It’s one of those viruses that a good scrubbing with soap and water does quite a good job in breaking down its composition,” says Krause.

That’s right. A 20-second hand wash with soap and water is not just washing any viruses off your hands; it’s actually destroying the viruses in the process.

Turns out that viruses are tough, yet fragile, and that old-fashioned soap is very effective at removing viruses from your hands before they have a chance to infect anyone.

Soap Q&A

Is it important to wash your hands for 20-seconds? Yes, you need to wash your hands thoroughly to be sure you have reached every virus, as the viruses like sticking to human skin. You also need to dry your hands thoroughly afterwards.

Is antimicrobial soap better? No.

Does detergent work the same way? Basically, yes.

What about hand sanitizers? If you don’t have access to soap and water, then a vigorous hand-clean with a good amount of an alcohol-based hand sanitiser with an alcohol concentration of at least 60% is a good alternative.

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