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Why is 70% etoh and 30% h20 more effective than 100% of either? in microbio terms

  1. Feb 16, 2009 #1
    yea so some disinfectants work better at certain concentrations such as 70% etoh as opposed to 100% etoh.

    what is the reason for this?

    this is a question pertaining to microbiology btw.

  2. jcsd
  3. Feb 16, 2009 #2
    it's assumed to penetrate better into the microbial cel. Not aware of data to this point tho have seen data showing faster kill
  4. Feb 16, 2009 #3
    It takes longer to evaporate, allowing you to cover surfaces more effectively. 70% still evaporates fast as lightning, but it's more manageable.
  5. Feb 16, 2009 #4


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    I've also heard what JorgeLobo and kingdomof mention, that it takes longer to evaporate and is thus more effective at penetrating, dehydrating, and killing microbes.
  6. Feb 17, 2009 #5


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    You want to unfold a protein and not just dehydrate it. 95% EtOH quickly dehydrates the cell, after which the microbes can be brought back to life by re-hydration. 70% EtOH contains enough water so that dehydration is not efficient, now the protein is allowed to enter the cell and unfold proteins by the action of EtOH, this is very damaging to the microbes.
    Last edited: Mar 8, 2009
  7. Feb 25, 2009 #6
    It's actually difficult to find definitive information about this question. Ultimately it's all about contact time. Pure ethanol evaporates too quickly to be effective for the reasons explained by Monique. So all above answers are correct.
  8. Mar 3, 2009 #7
    This is the answer a similar question received in the Pharmaceutical Microbiology forum:

    It is because alcohols become more efficient in two ways:

    a) By the molecular weight (hence ethanol is more effective than
    b) the way by which alcohols destroy vegetative cells is by the
    into the cell (which then causes cytoplasm leakage, denaturation of
    protein and eventually cell lysis - a combination of effects sometimes
    as membrane disruption). To be readily absorbed into the cell water is
    Thus an optimal balance between the concentration of alcohol in order to
    kill the cell and sufficient water to be absorbed is required. This
    Comes out at around 70%. With 95% alcohol there is not sufficient water for
    the bacterial cell membrane to signal that the unknown liquid can be

    For more information, the following chapter may be of interest:

    Sandle, T.: ‘Selection and use of cleaning and disinfection agents in
    pharmaceutical manufacturing’ in Hodges, N and Hanlon, G. (2003;
    updated 2008): ‘Industrial Pharmaceutical Microbiology Standards and
    Controls’, Euromed Communications, England
  9. Mar 3, 2009 #8


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    You'd really have to say 95% EtOH rather than 100%. That's the azeotrope.
  10. Mar 3, 2009 #9


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    Does the disinfectant quality extend to isopropyl and water in the same fashion? I use 50:50 water and 90% Iso as a general-purpose cleaner/disinfectant around the house.
  11. Mar 3, 2009 #10
    Most industries and hospitals use isopropanol at 60%. This is sometimes supplemented by caustic (NaOH) or peroxide.
  12. Mar 3, 2009 #11


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    I got into the habit of using that mix when I was a dispensing optician. It was wonderful for cleaning lenses before and after edging the and assembling the glasses. It is a cheap no-streak cleaner for windows and mirrors, and we use it to clean countertops, especially after handling poultry.
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