The antimicrobial properties of ethanol have been known for over a century. It was soon realised that it worked more effectively as a disinfectant at less than 100 per cent concentration, that is, when there was some water present. This is because denaturation of proteins proceeds much more effectively in the presence of water. (Recall that moist heat is more effective than dry heat for the same reason.) It is important, however, not to overdo the dilution, as at low percentages some organisms can actually utilise ethanol as a nutrient! Ethanol and isopropanol are most commonly used at a concentration of 70 per cent. As well as denaturing proteins, alcohols may act by dissolving lipids, and thus have a disruptive effect on membranes, and on the envelope of certain viruses. Both bacteria and fungi are killed by alcohol treatment, but spores are often resistant because of problems in rehydrating them; there are records of anthrax spores surviving in ethanol for 20 years! The use of alcohols is further limited to those materials that can withstand their solvent action.
Alcohols may also serve as solvents for certain other chemical disinfectants. The effectiveness of iodine for example, can be enhanced by being dissolved in ethanol.
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