Why does soap work so well on the Sars-CoV-2, the coronavirus and
indeed most viruses? The short story: because the virus is a
self-assembled nanoparticle in which the weakest link is the lipid
(fatty) bilayer. Soap dissolves the fat membrane and the virus falls
apart like a house of cards and dies – or rather, we should say it
becomes inactive as viruses aren’t really alive.
The slightly longer story is that most viruses consist of three key
building blocks: ribonucleic acid (RNA), proteins and lipids. A
virus-infected cell makes lots of these building blocks, which then
spontaneously self-assemble to form the virus. Critically, there are
no strong covalent bonds holding these units together, which means you
do not necessarily need harsh chemicals to split those units apart.
When an infected cell dies, all these new viruses escape and go on to
infect other cells. Some end up also in the airways of lungs.
When you cough, or especially when you sneeze, tiny droplets from the
airways can fly up to 10 metres. The larger ones are thought to be the
main coronavirus carriers and they can go at least two metres.
These tiny droplets end on surfaces and often dry out quickly. But the
viruses remain active. Human skin is an ideal surface for a virus. It
is “organic” and the proteins and fatty acids in the dead cells on the
surface interact with the virus.
When you touch, say, a steel surface with a virus particle on it, it
will stick to your skin and hence get transferred on to your hands. If
you then touch your face, especially your eyes, nostrils or mouth, you
can get infected. And it turns out that most people touch their face
once every two to five minutes.
Washing the virus off with water alone might work. But water is not
good at competing with the strong, glue-like interactions between the
skin and the virus. Water isn’t enough.
Soapy water is totally different. Soap contains fat-like substances
known as amphiphiles, some of which are structurally very similar to
the lipids in the virus membrane. The soap molecules “compete” with
the lipids in the virus membrane. This is more or less how soap also
removes normal dirt from the skin.
The soap not only loosens the “glue” between the virus and the skin
but also the Velcro-like interactions that hold the proteins, lipids
and RNA in the virus together.
Alcohol-based products, which pretty much includes all “disinfectant”
products, contain a high-percentage alcohol solution (typically 60-80%
ethanol) and kill viruses in a similar fashion. But soap is better
because you only need a fairly small amount of soapy water, which,
with rubbing, covers your entire hand easily. Whereas you need to
literally soak the virus in ethanol for a brief moment, and wipes or
rubbing a gel on the hands does not guarantee that you soak every
corner of the skin on your hands effectively enough.
So, soap is the best, but do please use alcohol-based sanitiser when
soap is not handy or practical.