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?
Answer
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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