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?
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2 Answers
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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.
EDIT
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.
Dielectric Constants
- Water - 80
- Methanol - 33
- Ethanol - 24
-Organic Chemistry 11E T.W. Graham Solomons et.al.
Note
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.
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1$\begingroup$ I thought ethanol to be pretty polar? More importantly, I thought higher percentages EtOH caused microorganisms to dehydrate, killing the cells, but conserving the viable spores of spore-bearing bugs? $\endgroup$– AliceD ♦Commented Oct 26, 2015 at 1:43
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1$\begingroup$ DAMN IT @AliceD... you actually went there and you made me drag out the Organic Chemistry text book... I am hating you right now! ;-) $\endgroup$– AMRCommented Oct 26, 2015 at 2:27
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2$\begingroup$ I disagree about cost and effectiveness point. That is not correct. Main reason to add water is to denature protein faster. You can still kill bacteria with crude ethanol but with different amount of exposure time. $\endgroup$– DexterCommented Oct 26, 2015 at 3:43
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$\begingroup$ @Dexter That comment refers to the choice between using 60%, 70% all the way up to 95%, as the data you provided showed that 60% to 95% by volume had the same efficacy at killing all of the S. pyogenes. You don't go for 60% because mixing errors can make it less effective. 70% has the same bactericidal activity as 95%, but by cutting with water, you get equal efficacy at lower cost. $\endgroup$– AMRCommented Oct 26, 2015 at 3:55
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$\begingroup$ Yes, that data is for one species. But point of showing that table was not to comment on percentage dilution but to show that other dilutions can also be used with different exposure time. Cost effective point make sense if below 70% ethanol is very less effective. In that case, we could have used 60% or 50% instead. $\endgroup$– DexterCommented Oct 26, 2015 at 4:04
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General disinfectants
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
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.
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2$\begingroup$ I think that you need to explain how to interpret the figure you added. What do the pluses and minuses indicate. Does in mean that S. pyogenes survives in 50% for 20s but all are dead by 30s? Do the minuses mean that all bacterium were dead at that time point? $\endgroup$– AMRCommented Oct 25, 2015 at 19:05
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$\begingroup$ Original paper says "+" sign indicates growth of test organism, "-" sign indicates no growth of test organism while "+/-" sign indicates growth in some test and failed in another $\endgroup$– DexterCommented Oct 25, 2015 at 19:15
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1$\begingroup$ I concluded in parentheses for reason it is effective with very small exposure time. And I doubt you can comment on it's polarity. Reason you add water is to make protein denature faster. Again the concentration and exposure time you have to standardize accordingly. $\endgroup$– DexterCommented Oct 26, 2015 at 3:38
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1$\begingroup$ You are saying 50% and 100% are more efficient that 70%, but that is not what the figure shows. Plus in this instance is bad because it means that they when they plate bacteria exposed to 50% for 20 seconds, colonies formed, and at 100% they were getting bacterial colonies even when the bacteria was exposed for 1 minute. From 60 to 95% you see no growth, even when the bacteria were only exposed for 10 seconds, therefore those concentrations are the most effective. You are interpreting that chart incorrectly, assuming that what you said about + and - is correct. $\endgroup$– AMRCommented Oct 26, 2015 at 4:00
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1$\begingroup$ 1- How would adding water denature proteins faster? You would need detergents to do that. If anything water would maintain the protein conformation as the hydrophobic regions on the inside would stay on the inside. 2- Solvency is based almost entirely on polarity. That is why salts don't melt in oil, though some ionic compounds won't even dissolve in water, and you need nonpolar solvents such as hexane to dissolve nonpolar organic compounds. Lipopolysaccharides are amphipathic molecules, you need the water to solvate the polar parts and the alcohol to dissolve the less polar parts. $\endgroup$– AMRCommented Oct 27, 2015 at 11:25