So one of our readers asked us this question the other day: Why is 70-75% alcohol a better antiseptic than that of a higher concentration?
Most cells have something called “Aquaporins” which are highly specialized transmembrane proteins that act as special Toll Road lanes for water to easily get into and out of the cell. These are extremely important, because a cell regulates it’s water content by ion pumps, and let’s the water freely flow via diffusion in response.
Having ~70% allows the antiseptic alcohol to hitch a ride in the “toll lane” of water (via intermolecular hydrogen bonding) and act on the intracellular contents of the cell. Membrane structure and function is way too techy for this question. It’s about the water giving the alcohol added stability in solution, making it a more effective sterilant. Also with 70% alcohol and 30% water you get a slower evaporation rate, giving the alcohol disinfectant a longer contact time and therefore a greater period of sterilization when compared to alcohol alone.
100% alcohol will evaporate very quickly. For larger populations of bacteria the contact time of 100% alcohol wouldn’t be long enough for effective sterilization to take place. 70% alcohol is a sweet spot, and there are reasons for that.