THE CHEMISTRY OF SOAP
A few minor cosmetic tweaks aside, the basic recipe for soap has endured for millennia. JACINTA BOWLER looks at its literal love-hate relationship with water, its potency against a planet-busting virus, and why any attempts to improve upon it just won’t w
JACINTA BOWLER breaks down the humble household bar that is rinsing the COVID virus.
Show of hands: who has become infinitely, intimately more familiar with soap recently? If 2020 was the year we learnt to wash meticulously in between our fingers and all the way up our arms, it was also the year we discovered just how long 20 seconds in front of a bathroom mirror can be.
But have you ever stopped to wonder how the humble bar of hand soap actually works? How does that slippery ‘cake’ invented thousands of years ago remain on par with hand sanitiser or any of the other modern-day cleaning products we wield against a virus that’s become a pandemic? At its most basic, hand
soap is just a combination of fat or oil with an alkaline substance. The first people we know of to lather up like this were the ancient Mesopotamians, who mixed animal fat with water and wood ash to produce a substance that, while no doubt greasy, smelly and hugely unpleasant, could also spirit away dirt and grime in a manner that must have appeared borderline miraculous.
At its most basic, the wood ash splits the animal fat or oil (called triglycerides) into molecules called amphiphiles. These amphiphiles have one end that loves water, and another that hates it.
“One end is usually bulky and shorter; we say it’s hydrophilic – it interacts strongly with water,” explains University of NSW chemistry professor Pall Thordarson. “Then there’s a longer hydrophobic end; we call it
a ‘greasy tail’.”
A very similar type of molecule – called a phospholipid – is what forms our cell membranes.
Those Mesopotamians invented a product we’re still using 5000 years later. Today’s soaps, admittedly augmented with exotic conditioners, oils, colours and scents, still utilise the same basic mechanism – amphiphiles.
You may not know Thordarson from a bar of you-know-what, but he actually went viral in early 2020 thanks to a Twitter thread on how soap works against virus particles, so he knows a thing or two about the topic.
“It was quite an experience,” he explains. “I’ve been interested in lipids ever since my pre-phd days.”
In his viral thread, which was originally a Facebook post in Icelandic, the scientist broke down the supramolecular chemistry behind soap and water’s effectiveness.
Imagine you could see down to a molecular level while washing your hands.
Once you rinse your hands with water and lather them up with soap, the amphiphiles get to work. The hydrophobic ends want to avoid the water at all costs, so they start to bunch up
next to each