Are other concentrations (say 80%) less effective,or is this just for convenient manufacturing? Is the concentration chosen only because it is less volatile than 100 percent ethanol and hence safer?
70% ethanol is the minimum concentration when used in a lab setting for aseptic technique. You don't want to go lower, because it will not kill the microorganisms you are trying to kill.
However the solution requires water to be effective at doing its job. The 30% water is necessary to provide sufficient polarity to the solution.
A rough indication of a solvent's polarity is a quantity called the dielectric constant. The dielectric constant is a measure of the solvent's ability to insulate opposite charges (or separate ions) from each other...
Water is the most effective solvent for promoting ionization, but most organic compounds do not dissolve appreciably in water. They usually dissolve, however, in alcohols, and quite often mixed solvents are used. Methanol - water and ethanol - water are common mixed solvents for nucleophilic substitution reactions.
- Water - 80
- Methanol - 33
- Ethanol - 24
-Organic Chemistry 11E T.W. Graham Solomons et.al.
Dexter's answer provides experimental evidence for why 70% ethanol is used. My answer is based on my experience and understanding and the concentration that I have used in the lab. I would add that 70% was likely chosen as a balance between cost and effectiveness. If you are a little off on mixing the solution from 95% Ethanol stock, then you are still at a bactericidal level, where as if you err at 60% concentration, you could start to let a few bacteria survive.
70% ethanol is still flammable and should be handled with care. The issue with ethanol greater than 98% pure is that the only way to distill the last 2% of water out is to add benzene, and benzene is a known carcinogen. There will always be trace amounts of benzene in 100% reagent grade ethanol.
You need two things for good aseptic agent or disinfectant,
- It should be able to kill most of microbes (Spectrum)
- It should do it in short time (Speed)
Many chemical disinfectants including ethanol are non-specific antimicrobial. Their predominant mode of action is by protein coagulation or by denaturation. This results in disruption of cytoplasmic integrity, cell lysis etc. Protein coagulation occures at optimum alcohol concentration. In presence of water proteins are denatured fast compare to without water. Alcohol induced coagulation occurs at cell wall involving various plasma proteins.
Ethanol is highly efficient in killing non-sporulating bacteria and mycobacteria but inefficient against bacterial spores. Ethanol is effective bacteriostatic at 10% (Vol/Vol). It is bacteriocidal at concentration above 30% depending on species, exposure time and water content. Following images taken from (Seymour 2001) shows how it is effective within given parameters for one specific species. ("+" sign indicates growth of test organism, "-" sign indicates no growth of test organism )
In 1950, people have standardized set of parameters which are most efficient for killing wide spectrum of microbes with very short duration. Hence ethanol can be used in different concentration but you have to fine tune exposure time accordingly (You can see in above figure, that 50% and 100% can still act as a bactericide, they are less efficient than 60%-95% and require longer to kill all of the S. pyogenes.). 70% ethanol helps in killing wide spectrum of microbes with very less time.