Simply put, water is required for the denaturization of proteins, which is how isopropyl alcohol works as a disinfectant.
Back in microbiology we used alcohol in various concentrations to disinfect surfaces, and then cultured
Them in petrie dishes. The 70% had the least growth.
Perhaps someone with a chemistry background could explain how this works.
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.
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.
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.
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