Why is 70% alcohol solution a much better disinfectant than a 90% alcohol concentration? Wouldn’t a higher concentration mean better?

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Why is 70% alcohol solution a much better disinfectant than a 90% alcohol concentration? Wouldn’t a higher concentration mean better?

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Evaporates slower so more time in contact with surface and the water helps to permeate the cells of the bacteria.

90 and above % alcohol causes the outer membranes of the bacteria to shut down and coagulate forming a protective barrier. The outer proteins in viruses are not soluble in pure alcohol so they remain protective, in 70% alcohol they are disrupted and more likely to open up and let the virus break apart.

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As I once posted:

70% alcohol has 30% water, and that water is necessary for the alcohol to interact at all with the cells it’s killing.

It’s like cooking pancakes. You know how when your pan is really hot and you put in pancake batter, it cooks the outside really fast? And then you can flip it, but it does the same thing to the other side and the middle doesn’t cook very well? 90% alcohol is like that. It doesn’t penetrate well into cells or clumps of microbes because it just fries everything it touches on the outside. The 70% alcohol is like cooking on medium heat with a moderately hot pan. It contacts the outside, too, but the water helps it penetrate to cook the inside (denature proteins deeper) as well.

More info and sources here:

https://www.reddit.com/r/explainlikeimfive/comments/erfec8/comment/ff3dlhk/

Everyone has given correct answers but I will reiterate simply.

70% alcohol has more water content than 90, and as such it’s better able to penetrate cell walls and kill the bacteria/pathogen, while 90% can’t. This is a case where higher isn’t always better.

The water allows the alcohol to access the cells more easily to kill them, less water means it’s less able to do that

90% is better for cleaning electronics because it contains less water.

*edited for grammar

Because too much alcohol dries the cells, the same way salt does.
This make some bacteria sporulate before killing them.

Also, isopropanol is very hygroscopic (it readily absorbs atmospheric moisture) and alcohol solutions (including anhydrous isopropanol) absorb water until they reach equilibrium at about 65% alcohol / 35% water. So even if higher solutions were more effective, the average person wouldn’t be capable of maintaining the higher percentage once their container is opened.

There’s probably some awesome biological reason that the most effective concentration of isopropanol is found at it’s hygroscopic equilibrium, but I don’t know what that reason is!

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Other than what the top comment explained, 70% is the magic number if you don’t want your disinfectant to evaporate before it actually disinfects. Those who have problems with dry skin also use 60% if needed.

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90% evaporate too fast, germ(infacting agent) can still survive after all the solution evaporated. 70% evaporate at slower rate so germ will die before the solution completed its evaporation.

All disinfectants (bleach, alcohol, ect.) have a “soak time” to be effective at killing viruses and bacteria. This is the amount of time the surface being disinfected has to remain moist and undisturbed to penetrate the cell walls and actually break down the virus or bacteria.

90% alcohol evaporates quickly and does not have the time to fully “soak” into the cells it’s trying to destroy. That’s also why using a single wipe of a bleach wipe doesn’t actually do anything. (Most I’ve seen require enough wipes to keep the surface wet for at least 30 seconds.)
If you’re just removing grime you can see, the concentration doesn’t matter. It’s the microscopic grime that takes time to kill.

The pancake example currently at the top of this thread is perfect!

Cells are hydrophilic. This means the molecules they are made of attract water. So when the cells come into contact with alcohol mixed with water, the cells will act like a sponge and absorb the water, and also brings with it the alcohol.

If it’s just alcohol, the sponge effect won’t work so well on the cells.

90% evaporates too quickly to be as effective – it will still disinfect, just not as completely. The 70% stuff takes a bit longer to evaporate, so longer exposure time means more time to do its job and therefore more effective.

90% alcohol evaporates quicker than 70% alcohol, so the 70% alcohol is able to stay on the bacteria longer and attack it more

I use 70% alcohol for my mcology hobby. I’ve also used 90% alcohol. The rule of thumb is that things need to soak for three minutes and if you spray it on, 90% evaporates much too quickly. Other than that some are commenting on the mechanism of action where 90% causes cells to coagulate and protect themselves but I don’t know about that. IMHO 90% would work just as well if you reapplied it and waited the three minutes.

Alcohol is like Hulk who beats up bad guys. And water is the vehicle Hulk uses to reach thebad guys.

With 90% alcohol and 10%water, alcohol can’t reach the bacteria to beat them up. But 70%alcohol and 30% water can reach all the bacteria

Absolute alcohol has a very low flash point and it’s easy to accidentally ignite it from a naked flame used to sterilize needles and scalpels, so 70% was considered to be the best compromise. Also it doesn’t evaporate too quickly so when it’s sprayed onto a surface it has sufficient contact time to kill pathogens.

90 & 99% evaporates far too fast. It’s very well suited for use on cleaning electronics, not disinfecting.