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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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!
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.
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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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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!
Because too much alcohol dries the cells, the same way salt does.
This make some bacteria sporulate before killing them.
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
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:
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byu/pixie_laluna from discussion
inexplainlikeimfive
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