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.
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) .
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 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.
In fact dripping would work, as fat is the main component of soap.......
Also good for removing tar.
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