Why does soap work so well on SARS-CoV-2?

This is a guest post from Prof Palli Thordarson of the Uni of New South Wales. It was previously posted in a Twitter thread and on Facebook and has been reprinted here with the author’s kind permission and with some light editing and formatting by VDU’s Editor in Chief.

Why does soap work so well on the SARS-CoV-2, the coronavirus and indeed most viruses? Because it is a self-assembled nanoparticle in which the weakest link is the lipid (fatty) bilayer.

Figure 28.2 from Fields Virology 6th Edition, from Edward Nirenberg’s blog, SARS-CoV-2 and the lessons we have to learn from it.

The 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. Viruses can be active outside the body for hours, even days.

Disinfectants or liquids, wipes, gels and creams containing alcohol (and soap) have similar effects but are not really quite as good as normal soap. Apart from the alcohol and soap, the “antibacterial agents” in these products don’t affect the virus structure much at all. Consequently, many antibacterial products are basically just an expensive version of soap in terms of how they act on viruses. Soap is the best but alcohol wipes are good when soap is not practical or handy (e.g. office receptions).

But why exactly is soap so good? To explain that, I will take you through a bit of a journey through supramolecular chemistry, nanoscience and virology. I try to explain this in generic terms as much as possible, which means leaving some specialist chemistry terms out. This is a rather long post, but hopefully, you enjoy it.

I point out to that while I am expert in supramolecular chemistry and the assembly of nanoparticles, I am not a virologist. The first image here is from an excellent post here which is dense with good virology info: https://t.co/73TurPhxOE?amp=1

I have always been fascinated by viruses as I see them as one of the most spectacular examples of how supramolecular chemistry and nanoscience can converge.

A bit about viruses

Most viruses consist of three key building blocks: RNA [EiC – there are also DNA viruses], proteins and lipids. The RNA is the viral genetic material – it is very similar to DNA. The proteins have several roles including breaking into the target cell, assist with virus replication and basically to be a key building block (like a brick in a house) in the whole virus structure.

The lipids then form a coat around the virus, both for protection and to assist with its spread and cellular invasion. The RNA, proteins and lipids self-assemble to form the virus. Critically, there are no strong “covalent” bonds holding these units together.

Instead, the viral self-assembly is based on weak “non-covalent” interactions between the proteins, RNA and lipids. Together these act together like a Velcro so it is very hard to break up the self-assembled viral particle. Still, we can do it (e.g. with soap!).

Most viruses, including the coronavirus, are between 50-200 nanometers – so they are truly nanoparticles. Nanoparticles have complex interactions with surfaces they are on. Same with viruses. Skin, steel, timber, fabric, paint and porcelain are very different surfaces.

When a virus invades a cell, the RNA “hijacks” the cellular machinery like a computer virus (!) and forces the cell to start to makes a lot of fresh copies of its own RNA and the various proteins that make up the virus.

These new RNA and protein molecules, self-assemble with lipids (usually readily present in the cell) to form new copies of the virus. That is, the virus does not photocopy itself, it makes copies of the building blocks which then self-assemble into new viruses!

All those new viruses eventually overwhelm the cell and it dies/explodes releasing viruses which then go on to infect more cells. In the lungs, some of these viruses end up in the airways and the mucous membranes surrounding these.

Propelled viruses and the surfaces they hit

When you cough, or especially when you sneeze, tiny droplets from the airways can fly up to 10 meters (30 ft)! The larger ones are thought to be main coronavirus carriers and they can go at least 2 m (7 ft). Thus – cover your coughs & sneezes people!

These tiny droplets end on surfaces and often dry out quickly. But the viruses are still active! What happens next is all about supramolecular chemistry and how self-assembled nanoparticles (like the viruses) interact with their environment!

Now it is time to introduce a powerful supramolecular chemistry concept that effectively says: similar molecules appear to interact more strongly with each other than dissimilar ones. Wood, fabric and not to mention skin interact fairly strongly with viruses.

Contrast this with steel, porcelain and at least some plastics, e.g. Teflon. The surface structure also matter – the flatter the surface the less the virus will “stick” to the surface. Rougher surfaces can actually pull the virus apart.

So why are surfaces different? The virus is held together by a combination of hydrogen bonds (like those in water) and what we call hydrophilic or “fat-like” interactions. The surface of fibres or wood, for instance, can form a lot of hydrogen bonds with the virus.

In contrast steel, porcelain or Teflon do not form a lot of hydrogen bond with the virus. So the virus is not strongly bound to these surfaces. The virus is quite stable on these surface whereas it doesn’t stay active for as long on say fabric or wood.

For how long does the virus stay active? It depends. The SARS-CoV-2 coronavirus is thought to stay active on favourable surfaces for hours, possibly a day. Moisture (“dissolves”), sunlight (UV light) and heat (molecular motions) all make the virus less stable.

When the surface is your skin

The 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 through both hydrogen bonds and the “fat-like” hydrophilic interactions.

So when you touch say a steel surface with a virus particle on it, it will stick to your skin and hence get transferred onto your hands. But you are not (yet) infected. If you touch your face though, the virus can get transferred from your hands and on to your face.

And now the virus is dangerously close to the airways and the mucus type membranes in and around your mouth and eyes. So the virus can get in…and voila! You are infected (that is unless your immune system kills the virus).

If the virus is on your hands you can pass it on by shaking someone’s else hand. Kisses, well, that’s pretty obvious…It comes without saying that if someone sneezes right in your face you are kind of stuffed.

So how often do you touch your face? It turns out most people touch the face once every 2-5 minutes! Yeah, so you at high risk once the virus gets on your hands unless you can wash the active virus off.

Soap: destroyer of viruses

So let’s try washing it off with plain water. It might just work. But water “only” competes with the strong “glue-like” interactions between the skin and virus via hydrogen bonds. The virus is quite sticky and may not budge. Water isn’t enough.

Soapy water is totally different. Soap contains fat-like substances knowns as amphiphiles, some 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 of the skin (see picture).

The soap molecules also compete with a lot other non-covalent bonds that help the proteins, RNA and the lipids to stick together. The soap is effectively “dissolving” the glue that holds the virus together. Add to that all the water.

The soap also outcompetes the interactions between the virus and the skin surface. Soon the viruses get detached and fall apart like a house of cards due to the combined action of the soap and water. The virus is gone!

The skin is quite rough and wrinkly which is why you do need a fair amount of rubbing and soaking to ensure the soap reaches every crook and nanny on the skin surface that could be hiding active viruses.

Soap not handy? Alcohol to the rescue

Alcohol-based products, which pretty includes all “disinfectants” and “antibacterial” products contain a high-% alcohol solution, typically 60-80% ethanol, sometimes with a bit of isopropanol as well and then water + a bit of soap.

Ethanol and other alcohols do not only readily form hydrogen bonds with the virus material but as a solvent, are more lipophilic than water. Hence alcohol does also dissolve the lipid membrane and disrupt other supramolecular interactions in the virus.

However, you need a fairly high concentration (maybe +60%) of the alcohol to get a rapid dissolution of the virus. Vodka or whiskey (usually 40% ethanol – see picture), will not dissolve the virus as quickly. Overall alcohol is not quite as good as soap at this task.

Nearly all antibacterial products contain alcohol and some soap and this does help killing viruses. But some also include “active” bacterial killing agents, like triclosan. Those, however, do basically nothing to the virus!

The wash-up

To sum up, viruses are almost like little grease-nanoparticles. They can stay active for many hours on surfaces and then get picked up by touch. They then get to our face and infect us because most of us touch the face quite frequently.

Water is not very effective alone in washing the virus off our hands. Alcohol-based product works better. But nothing beats soap – the virus detaches from the skin and falls apart very readily in soapy water.

Here you have it – supramolecular chemistry, the way molecules interact with each other (see picture) and nanoscience tell us not only a lot about how the virus self-assembled into a functional active menace, but also how we can beat viruses with something as simple as soap.

Thank you for reading. This started off as a similar post in Icelandic that then took off! So I decided to put in on twitter where it has skyrocketed again. For those that don’t use twitter and can’t read Icelandic (!), I hope you find this useful. With apologies beforehand for any minor (hopefully) mistakes or over-simplifications that I have made on the way.

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47 thoughts on “Why does soap work so well on SARS-CoV-2?”

    1. Since soap causes the virus to fall apart, is there any way to deliver a soap-like material into the lungs like an aerosol? Inhaled medications for asthma and COPD are common. Might there be a way to deliver a soap-like material in small quantities to the lungs, without suffocating the patient? Taking this to an extreme, perhaps the material could be delivered and then “washed out” with another agent? Obviously, there is a need to maintain enough oxygen transfer to maintain life, so this idea may be very silly.

      1. Surfactant like that used for premature babies to prevent collapse of the alveoli in premature babies with Hyaline Membrane Disease is lipid (soapy) solution, which technicall may work in Covid-19. Problem is the amount of surfactant required to achieve the desirable effect in an adult will be presumably large, and the cost therefore massive.

  1. Great breakdown. I would have liked to hear you address the effectiveness of bleach specifically. I always heard bleach KILLS pathogens (as in totally destroys). Does it only render “inactive” as well? And does this mean that even if I treat a surface with a disinfectant and don’t also follow up with a rinse, this same virus can reassemble itself once it makes contact with something organic? I’ve been told that normal laundry washing is sufficient for clothing, but I have yet to hear if the proportion of soaps or detergents matter, likewise cold vs hot water. Normal is very subjective in the laundry world. There are a lot of variables.
    Alas I suppose I sound like I should have also studied supramolecular chemistry and nanoscience. But my mom did teach me to wash my hands.


  2. May I ask a dumb question, please? May I ask if anyone out there can point me to the, or any definitive, peer-reviewed, clinical study(ies} showing statistically valid en vivo virucidal efficacy of handwashing techniques and antiviral compounds, soaps, ethanols, etc., please?

  3. My big question is what counts as ‘soap’? I have a lot of things I think of as ‘soap’, because I wash with them, which I am now second-guessing regarding their virus-killing ability. Does shower gel/body wash work as effectively as bar soap? My liquid hand soap, upon closer inspection says it’s ‘soap-free antibacterial handwash’, which I assume is something to do with skin sensitivities to regular soap, will that work to kill viruses or do I need to get ‘real’ soap? Do things like shampoo and dishwashing detergent also do the same?

  4. Good article! Wonder if you can add something comparing dilute chlorine (hypochlorite) bleach to soap and alcohol. Seems to be similarly effective and not particularly toxic.


  5. I’m just a little confused by “fat-like” interactions as being “hydrophilic”. Fat-like interaction is hydrophobic or lipophilic. Is this is an error in the article or an error in my understanding?

    1. Does it become hydrophilic because it’s allowing the fat to dissolve in water by adding a detergent or soap?

    2. In my humble opinion, “fat like” interactios are “hidrophobic” not “hidrophilic” (they repel water).
      Dear author: please clarify this point.

  6. Thank you so much for your detailed discussion of surfaces! I have been wondering about this very issue because I interact so much with books, paper, and cards. I’ve been wondering if it is okay to loan books to a neighbor–I was thinking of setting up a tiny lending library on our apartment building stair landing, and have been thinking about the surfaces of books and the pages within: the pages are often ordinary paper, while book covers often are covered with some sort of laminate. Your discussion above makes me think it’s not likely for a virus to remain on the page for very long, but perhaps the covers should be wiped off with soapy water and then dried with a towel. (I’m sure someone will suggest wiping them with wipes, but no one in my neighborhood has or can obtain wipes right now, and it sounds to me like a fairly rough washcloth dipped in soapy water would be more effective.)

    The same issue also comes up if one is thinking of sharing things like cards or board games. And I know a lot of people have been wondering about paper with regard to having groceries delivered in paper bags, and with regard to sending cards and letters in the mail.

    But, it also occurs to me that what happens to a virus on a surface is also at the heart of the debate about masks. We have been frequently told that there is no evidence to suggest that wearing a cloth mask is beneficial to the public, that they don’t fit properly and that they don’t contain copper. And at the same time I’m also hearing that many health care workers are having to reuse masks–so, are they washing the masks with soap and water before reusing them? Or creating a sort of mask quarantine where they go back to a mask that was worn several days earlier, not to the one that was worn yesterday?

    Moreover, why wouldn’t it be helpful for members of the public to wear a mask while grocery shopping, for example, if they made the mask themselves from materials at home? For that matter, why wouldn’t it be helpful, for the public generally, to simply wear a scarf across their nose and mouth? And–sorry to keep throwing questions at you, but–we are also often told, if exiting a burning building, to dampen a scarf, put it over our nose and mouth, and then crawl out of the building, doing all of this to avoid smoke inhalation. I realize that smoke is different from a virus. But how and why is it different, and would a cloth mask be more or less effective when damp? I assume it would be less effective, because I seem to be hearing that health care workers should change out masks (if supplies are adequate to allow them to) when a mask becomes damp from one’s breath. If you have time to write a post discussing this, I would love to know more about it. I know two women who are sewing masks at home that they hope to donate to hospitals or clinics, and I don’t know if that’s at all a helpful thing to do or not. Thank you again for your fascinating post and blog!

  7. Question: Can you say something about direct sun light and UV-bulbs? Does it work, how long do you recommend? —- AND: THANK you so much for competent scientific advice. This really does make a difference.

    1. UV does degrade virus but it takes some time to be sufficiently exposed; I’d expect a bit longer in the sun but you’d have the added drying and heat effects.

  8. Would exogenous pulmonary surfactant work on SARS CoV2 inside the lungs as a hydrophobic or lipophylic agent, and/or is the virus “out of reach” in the cells? What if surfactant were administered early in the course of the disease before ARDS became acute?
    Thank you for this interesting discussion.

  9. I use ammonia household cleaner on countertops. … it leave a streak free (grease free) surface behind. Would a blast of ammonia cleaner work on our hands too?

    1. You’d be better off sticking to recommend disinfectants for both surfaces and hands.

      1. What are the recommended cleaners? If one uses a product that removes grease would this work on the lipids in the cells structure?
        You say alcohol works, but isn’t as effective as soap and water, is it not effective enough, did we not use alcohol then if it wouldn’t destroy all the virus on our hands?

  10. Echoing the comment above:

    – which soaps are [most] effective?
    a) traditional fat/alkali soap?
    b) carbolic soap?
    c) teepol detergent?
    d) washing-up liquid?

    For the detergents to be effective, what is the optimal dilution ratio in water? I am assuming you also need polar water for the hydrophilic ends of the soap molecules to be attracted to, and break the virus apart.

    – Adam

  11. In cases that we have access to liquid soap but not water nor alcohol will it be effective to use liquid soap or maybe some soap in a water spray to kill or inactivate the virus on our hands even if we don’t have enough water to rinse off the liquid soap?

  12. I need to clean wooden pews between services, what is my best bet for effective cleaning without ruining the wood?

  13. Loved that, I personally belive this virus should be the challange to chemical engineers as well as virologist. If this virus dissolves in soap that means it’s reactive to strong alkali solutions. We must search better solvents actually since cleaning food vegitables is a challange.

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