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