Why is 70% ethanol the recommended concentration for disinfection?

I thought the higher the concentration the better? But why do researchers recommend 70% ethanol instead of a higher concentration, say… 100%?

If you will think about it, by logic, you can reason out that “oh, because 100% ethanol will evaporate quickly”. Hmm.. It makes sense. But let’s understand it in a more scientific way.

We use ethanol (a simple alcohol commonly used — there are different types of alcohol) to disinfect surfaces, including our hands. These days, you will usually see people with those small bottles (of cute designs) filled with 70% alcohol (plus moisturiser and fragrance) hanging on their bags. Tadaan!! A portable alcohol spray. One of the clever inventions of humans. We do this because we believe that this solvent will kill germs. Going back to the question, why not use 100% ethanol instead?

A germ cell that looks like a watermelon πŸ™‚

Okay, let’s talk about germs. Again, we use ethanol because we want to get rid of these germs. The image above is an example of a germ cell. It has a lot of important parts, but let’s focus first on the outer covering. I kinda zoomed out the image so you can imagine what’s in these tiny creatures. The germ has a cell wall, which serves as its protection (much like our skin). Right after the cell wall, you will find a quite thicker layer which is called protein layer or protein coat. This layer is the secondary protection of the germ. Now this protein coat is made up of many strong bonds which ensure the integrity and stability of the cell wall. So basically, all these parts protect the inner/inside of the germ cell.

Again, we will apply alcohol because we want to kill this germ. In this case, we will use 100% (or absolute) alcohol. What will happen then?

100% ethanol applied to kill the germ cell.

Ethanol is represented as CH3CH2OH in chemistry (the numbers are subscripts, ok? :D). Ok, no worries. I won’t discuss too much chemistry here. So what does ethanol do? Basically, ethanol’s job is to tear down all the defenses of the cell so it can enter the inside of the germ cell leading to cell death. How will it perform the job? By cutting down the bonds that stabilize the cell wall and protein coat of the germ. Just like the image below:

Degraded/cut bonds.

There is this term called “denaturation“, which means “destroying the structure of the protein”. Once the structure of the protein is destroyed, it’s initial function will never gonna happen again. That’s why they say “protein denaturation is irreversible“. You can never put it back again to its original form. Still sound confusing? Just a quick explanation for this. Imagine an egg (example of protein). A raw egg looks slimy, it is transparent and it has a yellow thingy (yolk) inside. But once you cook it, it becomes a different form: opaque white with yellow-orangish center (if you are cooking a sunny-side up egg). Can you still transform it to its original slimy and transparent form once it has been cooked? No right? That’s irreversible. πŸ™‚

So, going back to 100% ethanol (so sorry. sometimes I get carried away explaining things), all the bonds will be degraded. If you think about it, it’s good right? I mean, just keep destroying every single bond. Yeah! Just cut it down! But, but, but… in doing this, you are actually keeping the germ cell alive. Huh?! Yes yes yes! Look at the image below:

Pairing of the bond, and another bond. Ugh.

Huh? What happened all of a sudden? Okay. Here’s what happened. When the ethanol cut down the bonds, the “cut bonds” started pairing with other bonds (which they usually call as side chains“). This is called as coagulation of proteins, or simply say “clotting“. Why? Because that’s their nature. Now, once they all started pairing at the same time, what do you thing will happen? Look at this:

Blockage!!!!

All these bonds will clot (connect to each other). And guess what? These coagulated proteins will serve as a BARRIER/WALL, making it really impossible for ethanol to enter the inner part of the germ. What will happen next? Since it is 100%, the germ might probably just be inactive for a moment, but not dead. The destruction of the protein layer is way too fast, making the coagulation also fast. The goal is to coagulate ALL PROTEINS (meaning including those inside the cell) to kill the germ. But because it was too fast, only those proteins within the protein layer (under the cell wall) were coagulated.

Bottomline is… 100% ethanol will not kill the germ. 😦

So, back to the question: Why is 70% ethanol the recommended concentration?

To prepare 70% ethanol, you will need to add 70 milliliters (mL) of 100% ethanol and 30 mL of water. Unlike the effects of 100% ethanol, the denaturation and coagulation of proteins in the protein layer will be slower. Yup, slower because this time, we have added water in it. This will give time for the ethanol to penetrate the cell. So what will be the outcome? The ethanol will successfully enter the germ cell. And taddaaaan!! GERM CELL will REST IN PEACE. πŸ™‚

Is it clear now? Hmm.. you may ask, but what about 75% or 80%? Okay. Good question. Well, this concentration will still kill the germs. But the 70% is the proven and tested concentration, as this is what scientists use in the lab. And besides, we do not want our hands to become sooooo dry, right? So yeah, 70% ethanol is the recommended concentration. πŸ™‚

Hope you find this article useful. So the next time you buy your alcohol from the supermarket, you will not anymore wonder why they do not sell 100% ethanol. πŸ™‚ Take care everyone! Adios!

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