IN FOCUS: HOW SOAP WORKS AGAINST THE CORONAVIRUS

other, eventually forming a sphere with the hydrophilic ends facing the water and the hydrophobic ends inside.

But amphiphiles are also attracted to other non-water molecules, such as dirt, grease, bacteria, dead skin cells – even viruses.

“A virus is really just a nanoscale, greasy little meat particle,” says Thordarson, “and the lipid tails on the soap molecules will try to avoid the water by sticking into the greasy virus.”

Because most bacteria and viruses have cell membranes (called “envelopes” in the case of viruses), the amphiphiles can pry open the molecules like a crowbar, spilling the contents and rendering the virus or bacteria inactive. Then the amphiphiles form little spheres around the spilled contents, ready to be washed down the drain when you rinse your hands.

Ignore all the different scents, colours and fancy packaging and you’ll find that most soaps work this same way. “The entire soap industry is 99% marketing because they’re more or less selling the same stuff,” says Thordarson.

But there’s a category of soap that’s popped up in the last few years that claims to be “antibacterial” or “antimicrobial”. While this might seem like an advertising gimmick – which it partially is – these soaps are actually a bit different in the way they work. Some soaps marketed as

antibacterial have “active ingredients” like triclosan and chloroxylenol, but these ingredients have been controversial. Back in 2016, after manufacturers were unable to demonstrate that they were safe for long-term daily use, or indeed any better than plain soap, the FDA in the US banned 17 chemicals including triclosan and another antibacterial called triclocarban – so you won’t find them in soaps any longer. However, you might still come across other ingredients such as L-lactic acid in those soaps marketing themselves as “antibacterial”.

But most antibacterial soaps now avoid using these controversial ingredients. “Some antibacterial formulations that are sold actually just have an ethanol in them,” Thordarson explains. “In a sense, they’re basically taking soap and hand sanitiser and mixing them. That’s it, really, and it works quite well.”

Ethanol works differently to soap by denaturing the spike protein on the outside of the virus. Thordarson likens it to an earthquake, where everything is shaken so much that bacterial and virus coatings just fall apart.

Having both ethanol and soap working together creates a formidable team. Although soap by itself can destroy the coronavirus no problem, other viruses and bacteria have more solid coatings, and ethanol and soap together can remove more types of bacteria and virus from your hands.

You’d think that all this cell denaturing might be an issue for us too, considering our skin is also made of cells with cell membranes, but in this case having lots and lots of cells comes in handy.

The outer layer of our skin

– the stratum corneum – is made up of around 15 layers of flattened dead cells which lack cell membranes.

Instead, they have a much hardier casing of structural proteins called a “cornified envelope” that amphiphiles don’t care for as much.

This doesn’t stop soap from doing a little damage to some of the cells, but as the stratum corneum is constantly shed, we can afford to lose a few cells here and there.

On the other hand, bacteria and viruses – with a cell count of one – don’t fare so well.

As the research into COVID-19 has progressed, we’ve learnt that washing our hands with soap or sanitiser – although good – isn’t the only way to protect ourselves. But are we likely to see something better than soap in the future? Thordarson doesn’t think so.

“It’s dirt cheap, it’s safe, it doesn’t do any damage to us, it doesn’t do any damage to the environment that we can see, and it breaks down quickly,” he explains. “I’m hard pressed to see how to make it better.”

JACINTA BOWLER writes about far-flung exoplanets, terrifying superbugs and everything in between. This is her first story for Cosmos.