Being a teacher, I often get asked why we use alcohols as antiseptics and why it is that most of these so-called rubbing alchohols are 70% ethanol or 70% isopropanol and not 95 or 100%. Even in our labs we use 70% ethanol as disinfectants but if you go and ask your seniors, they can rarely give you a good explanation besides some variant of “it’s cheaper that way.” Well, that may be so but it is not really the right explanation for using diluted alcohol.
If you have followed me thus far, you know that we are trying to kill bacteria/germs and the simple answer is that 70% ethanol is much more efficient at killing bacteria than absolute ethanol.  I know that sounds somewhat counter-intuitive but keep up with me for a minute and you will be enlightened. I promise.
So, why is 70% ethanol/isopropanol more efficient? To comprehend that, first we have to understand why alcohols are even harmful to bacteria. That is because alcohols destroy proteins and enzymes within a bacterial cell so that bacteria can’t carry out essential functions ultimately leading to their death. In order to do that, however, alcohols have to get inside the bacterial cell. At absolute concentrations, or even 95% for that matter, they don’t have enough time to enter the cell. Instead, they just end up precipitating proteins outside the cell and evaporate before they even have time to enter the cell. At 70% ethanol, alcohol is still concentrated enough to denature proteins and because it’s diluted in water, which evaporates more slowly, it has enough time to enter the cell.
Now you know why most rubbing alcohols are diluted to 70% and if you tried using 100% thinking that you could kill even more of those pesky little germs, well, you barely killed any… and THAT is the harsh truth of life. Until later, my friends…