Soap – howdya get so dope?! Answer: it has both positive and negative parts, you see, (we call this amphiphilicity) and it can break up germs before they get in you and me!
As the Covid-19 pandemic rages on, there’s been a lot of talk in recent days about “disinfectants” – many people are talking about dramatic ways to kill the SARS-Cov-2 virus that causes this novel coronavirus disease – from bleaches to alcohol-based sanitizers to UV light. I will briefly review a few of these, but the one that gets the bumbling biochemist’s highest rating for combatting this disease – good ole soap and water!
Yep, you heard me. The classic-est, “boring”-est cleaner of the bunch aces the coronavirus-killing test with some features that make it stand out among the rest! Unlike bleaches and hand sanitizers, although you still shouldn’t drink or ingest it, soap is safe to use on your skin (in fact I highly recommend you do so…), it’s cheap, you can still find it in stores, and it cleans up after itself. What more do you want? An explanation of the science of how it works? No problem!
Everything that “matters” (including soaps and their artificial versions, detergents) are made up of molecules. Molecules are made up of basic units of elements called atoms (think individual carbons, hydrogens, etc.) linked together by sharing electrons, which are negatively-charged subatomic particles that whizz around each atom’s core “atomic nucleus.” Inside the nucleus are positively-charged subatomic particles called protons that can counterbalance the negative charge of the electrons. If you have an even number of protons and neutrons, an atom or molecule will be neutral overall – but if the electrons aren’t shared fairly between the atoms of a molecule, some parts of the molecule can be partly charged while others are negatively charged in an uneven charge distribution referred to as “polarity”
This is important because water is super polar because oxygen is really electron-foggy. So water’s H’s are partly positive and its O’s are partly negative, and opposites attract, so water really likes to stick to copies of itself, forming extensive networks of water molecules. And if something else tries to get in, it needs to offer up something the water likes as much as or more than another water molecule (like polar parts or, even better, full charges). We call these water-loved molecules “hydrophilic”
If a molecule doesn’t have anything to offer the water other than a bland neutral surface, as you get with nonpolar molecules, the water molecules are like “no way dude!” and don’t let those “hydrophobic” molecules into their clique. Instead, the water molecules gang up on the hydrophobes, forcing them into as small a space as possible so that as few as possible water molecules have to interact with them. This is called the hydrophobic exclusion effect. So even though we call them hydrophobes as if these water-excluded molecules are afraid of water, it’s really just that water doesn’t like them – so stop judging!
But it’s not like all molecules are hydrophobic OR hydrophilic. Some molecules are BOTH – one part is hydrophobic and one part is hydrophilic – we call such molecules AMPHIPHILIC (aka AMPHIPATHIC) and they include soaps and detergents
They have a hydrophilic head that water’s cool with and a hydrophobic tail that water rejects. So these molecules orient themselves so that those hydrophobic tails huddle together with the heads sticking out to interact with the water. Their heads are “bulkier” so when they do this, they forms spheres called micelles with room inside for other hydrophobes to reside – if there’s greasy (hydrophobic) gunk present, micelles can form around it, coating that gunk with their hydrophobic tails facing the gunk & hydrophilic heads facing the water. So now, instead of stuck-on gunk, you have a soluble “packet” of gunk you can wash away
A lot of times we talk about this gunk in terms of things like greasy stuff stuck on a pan. But these days, we’re often talking about gunk in terms of virus particles stuck on your skin, hoping you’ll touch your nose, eyes, or mouth, so they can sneak in!
But it can’t sneak in if you wash it off first! Viruses and other microbes sitting on your skin or surfaces, when confronted with soapy water, can get caught up in those micelles too, so you can wash them away. And it’s really important that you do that scrubbing action (I strongly encourage amino-acid-song-singing to get to that 20s of hand washing….) in order to dislodge and wash away that gunky junk. https://bit.ly/aminoacidsong
Simple “dislodging” isn’t the only way soaps can combat these wannabe invaders – “viral particles” like those of SARS-Cov-2 (the virus that causes the disease Covid-19) are “just” a sack of genetic info and proteins, surrounded by a membrane made up of phospholipids. They’re similar to soaps and detergents in that they are amphiphilic, but they have multiple tails and they can form phospholipid bilayers, which are like sandwiches with the tails as the “peanut butter” and the heads as the breads. This configuration helps them cordon off the watery inside of the particle from the (often watery) outside world.
And soaps and detergents can break up this membrane, causing the virus to “fall apart” – because of their shape, soaps & detergents are more likely to form spherical micelles whereas phospholipids are more likely to form bilayers. But they’re similar enough that detergents can break up phospholipid bilayers, helping break open (lyse) viral particles. You might think that this would be dangerous because it leads to the virus releasing all its insides – but without the coat, the virus has no way to get into your cells to do any damage. And, without getting into cells, a virus can’t replicate itself – instead, its genome (genetic blueprint, which in the case of SARS-Cov-2 is a single strand of RNA) gets degraded. No harm done!
Our own cells, including those of our skin are also surrounded by phospholipid membranes – so you might wonder why soap doesn’t just dissolve us in the process – thankfully, our skin is protected by a thick layer of dead skin cells called the stratum corneum.
A quick overview of some other disinfectants and then I’ll give those who want it a closer look at soaps and detergents at the molecular level.
“Hand sanitizers” are usually alcohol-based solutions. Chemically-speaking, an alcohol is just something with a hydroxyl (-OH) group. The alcohol most people are most familiar with is the one that’s in “alcoholic beverages” – ethanol (CH₃CH₂OH). Another common one is isopropanol (aka isopropyl alcohol aka rubbing alcohol). Similarly to soaps, these alcohols can disrupt the viral membrane because they have an amphiphilic nature. And once they’ve dissolved the membrane by breaking up the lipid-lipid bonds, they go to work disrupting the bonds keeping proteins folded, denaturing the proteins so they couldn’t work even if the membrane were intact.
Then, since alcohol is volatile (evaporates easily), the liquid goes bye-bye leaving the dead virus behind – so you don’t need water to wash it off which is cool but it’s kinda like spraying ant killer on ants and then leaving the dead ants, whereas with soap and water you wash away the dead ants too.
But, when you go the hand sanitizer route, you might think the more alcohol the better – but, sin turns out if you have too much alcohol, it’ll evaporate before it can kill the virus, so you want ~60-80% alcohol to ensure adequate contact time (as well as to help the alcohol sneak up on viral molecules that are used to water, not alcohol). more here: https://bit.ly/handsanitizerss
Speaking of contact time, UV light has a contact time problem as well (in addition to having the potential to cause burns and cancer since the same high energy light waves that mess with the viral RNA can also mess with human DNA). UV light (specifically the UVC light that gets filtered out by the atmospheric ozone layer so it’s artificially made with lamps down here on earth) *can* kill SARS-Cov-2 and other germs by damaging their genetic instructions, but it doesn’t work instantly and it requires an adequate dose of radiation which depends on the strength of the lamp, how close it is to the object, and how long it’s shone. So the main uses of germicidal UV light are for disinfecting surfaces when people aren’t around and there’s plenty of time. more here: https://bit.ly/uvlightcaution
Lastly, are “bleaches” – there are 2 main types – chlorine bleaches and oxygen bleaches. Chlorine bleach is your conventional bleach. Its viral damage-doer is sodium hypochlorite (which dissolves to give you hypochloric acid), which works as a powerful oxidant. This means its an electron-stealer. It steals electrons from other molecules, even if that means breaking those other molecules. If it steals from the bonds holding together the color-absorbing parts of dyes (chromophores), it can prevent those dyes from absorbing colored light, and thus your stained clothes once again look white. And if it steals from the bonds holding together viral proteins, nucleic acids, and lipids, it can irreparably break or alter those molecules, fatally crippling the virus. more here: https://bit.ly/bleachbeware
A similar mechanism is used by “oxygen bleaches” but, instead of sodium hypochlorite, they use things like hydrogen peroxide (often “hidden” as sodium perchlorate until dissolved) as oxidants. One advantage oxygen bleaches have is that chlorine bleaches, if mixed with other chemicals, including other cleaners which contain acid and/or ammonia, can let off hazardous gases. Oxygen bleaches are therefore usually considered “safer” – but this DOES NOT MEAN SAFE TO DRINK OR INJECT OR PUT ON YOUR SKIN. Because the same oxidative damage bleaches do to viruses, they can do to you. more here: https://bit.ly/oxygenbleachteach
So, once again, we return to good ole soap, which I’ve hopefully convinced you is really pretty great!
First – the terminology – detergents are just “artificial” versions of soaps. This doesn’t mean they’re bad, it just means they’ve been synthesized, which offers opportunities for molecular “enhancement.”
Both have a hydrophobic tail made up of a long hydrocarbon chain – the linked-together hydrogens (H’s) and carbons (C’s) & C share electrons equally, giving you a long nonpolar part (how long depends on how many Cs & Hs, and the length helps determine “latherability”
Where soaps and detergents diverge is in the hydrophilic heads. SOAPS are derived from natural fatty acids, so they all have same head (carboxylate, which is ➖). But DETERGENTS can b synthesized to have different heads, which can be ➖ or➕, or even neutral, but polar (nonionic detergents)
Different detergents form micelles with different numbers of detergent molecules – the aggregation number – and in order for micelles to form you have to have enough detergent molecules for them to be able to find one another and team up – the concentration at which this becomes likely is the Critical Micelle Concentration (CMC) – it depends on the molecular makeup of the detergent (e.g. less lipophilic detergents are more less desperate to avoid water by hanging out together and minimizing their exposure so you have to get more of them together to “convince them” to -> higher CMC.
If you want to learn more about some common detergents we use in the lab: http://bit.ly/2XKuhWs
Final note: even though soaps are great, washing your hands a lot can dry out your skin because, in addition to removing viruses, you’re washing away some of the oils that help keep moisture in your skin from evaporating. Therefore, in addition to washing your hands frequently, you should be lotioning them frequently. I know, I know, *another* thing you have to do?! But, your skin will thank me! Absolutely not a paid endorsement I promise, but I have really sensitive, dry skin (under normal conditions) and one of the only lotions that works well for me is Cetaphil moisturizing cream (but please don’t go hoarding it…)
Now, more than ever, as we face an international (and biochemistry-related) crisis, I am incredibly grateful to be able to serve as Student Ambassador for the International Union of Biochemistry and Molecular Biology (IUBMB) that has helped me recruit translators and share the translated versions around the world. This post was just one in my series of weekly “Bri*fings from the Bench” which, for a while, will have to be “Bri*fings from the Bedroom…”
If you want to learn more about all sorts of things: #365DaysOfScience All (with topics listed) 👉 http://bit.ly/2OllAB0