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

Question

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

In: 5202

Answer 1

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!