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Isopropyl alcohol (2-propanol) can be used as disinfectant.

For increasing concentration of it in water, the effect as disinfectant increases, and then decreases again.

Typical concentrations for use as disinfectant seem to be 60-75% isopropyl alcohol in water.

Wikipedia: Isopropyl alcohol - Medical mentions membrane pores of bacteria that will not open without water.

That does not feel like the full explanation to me; I expect there are bacteria with membrane pores of diverse kind. Does 100% isopropyl alcohol not disinfect at all - or just more slowly? What abour the effect on other pathogens?

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The main reason why alcohols (isopropanol and ethanol mostly) can be used as disinfectants is that they denature (bacterial) proteins. This is also the reason why they work on such a broad spectrum of microorganisms (bacteria, fungi, viruses), but not on spores, as these are better protected.

The higher the concentration of the alcohol is, the faster this denaturation is happening. And this can cause problems: When you denature the proteins of the bacterial cell wall, they will form some kind of protective layer around some of the cells which allows their survival. Diluting the alcohols makes the disinfection happening slower and thus gives the alcohols more time to get into the cell.

For more information about concentrations and also for a ton of references, have a look into the CDC's disinfection guidelines in the subchapter on alcohols. More details can be found in this review:

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So this is a good question.

70% is optimal for most cell types, especially those with a cell wall like prokaryotes.

Wikipedia is a little off here. The reason you add water is to make the solution hypotonic.

The alcohol disrupts the hydrophobic forces holding the phospholipids of the membrane while dehydrating any peptidogylcan of the wall enough to cause it to become less fluid/more brittle.

Now that you have essentially made the protection of the cell like brittle glass, the hypotonicity causes the cell to swell and now easily burst.

Bacteria aren't going to last long in 100% either. They fix instead, which although irreversible takes a little longer to occur.

It's also a little more economical and less wasteful to use 70% if it's just as, if not a little more, effective

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    $\begingroup$ Can you please add some references? $\endgroup$
    – Chris ♦
    Commented Sep 21, 2014 at 21:33
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    $\begingroup$ The sentence "They fix instead..." - it that related to nitrogen fixation? I don't understand that part. $\endgroup$
    – Volker Siegel
    Commented Sep 26, 2014 at 10:23
  • $\begingroup$ Or do they just get immobile with no way to recover? $\endgroup$
    – Volker Siegel
    Commented Feb 22, 2017 at 11:42
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    $\begingroup$ @VolkerSiegel Fixation in this context is referring to histological fixation, which can actually be one of several chemical processes. This Wikipedia page is a good starting place. $\endgroup$
    – Bryan Krause ♦
    Commented Feb 22, 2017 at 18:21

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