I have gotten asked from some curious students why we use alcohols as antiseptics and why it is that most of these so-called rubbing alcohols 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 better explanation than some variant of “it’s cheaper that way.” While that is also true, it is not really the primary reason for using diluted alcohol.
You would think that more concentrated the alcohol, the more lethal it will be, but the truth is that 70% ethanol is just much more efficient at killing bacteria than absolute (100%) ethanol. I know that sounds counter-intuitive but keep up with me for a minute and hopefully, by the end of it you will understand why.
Before we go any further, we must understand why alcohols like ethanol are even harmful to bacteria. Alcohols, to put it simply, ‘destroy’ proteins and enzymes within bacterial cells so that bacteria can’t carry out their essential survival functions like break down food, etc, which ultimately leads to their death. Notice that for alcohols to be effective they have to get inside the cells. And this is where our answer lies.
Alcohols are highly volatile substances – they evaporate easily at room temperature – compared to water. At absolute concentrations, or even 95% for that matter, alcohols evaporate before they even have time to enter the cell. They just end up precipitating proteins outside the cell, which is obviously not ideal if you are trying to kill some bacteria.
What you have to try to do is reach a happy mix in which the alcohol is concentrated enough to have an effect and, at the same time, stable enough so it has time to enter the cell. 70% seems to be that magical number. It is still able to denature proteins and because it’s diluted in water, which evaporates more slowly, it has enough time to enter the cell. That is basically why all rubbing alcohols are 70%.
Now you know why most rubbing alcohols are diluted to 70% and if you tried using 95% thinking that you could kill even more of those pesky little germs, well, you barely killed any.
Questions, thoughts, disagreements, complaints? Please leave a comment.