Alcohol and water both vaporize completely, leaving no residues. 70% ethanol has been found to be most effective in killing microbes. Higher or lower concentrations are not so effective in killing microbes.
As amir from the university of Karachi states:
70% percent of alcohol is ideal to a stronger solution. Pure alcohol coagulates protein in contact. Suppose the pure alcohol is poured over a single celled organism. The alcohol will go through the cell wall of the organism in all direction, coagulating the protein just inside the cell wall. The ring of the coagulated protein would then stop the alcohol from penetrating farther from the cell, and no more coagulation would take place. At this time the cell would become inactive but not dead. Under the favorable conditions the cell would then begin to function. If 70 percent of alcohol is poured to a single celled organism, the diluted alcohol also coagulates the protein, but at a slower rate, so that it penetrates all the way through the cell before coagulation can block it. Then the entire cell is coagulated and the organism dies.
https://www.researchgate.net/post/Why_is_70_ethanol_used_for_wiping_microbiological_working_areas [accessed Jul 14, 2017].
Martin Luther from Halle Witternburg states: The effectivity of ethanol as e.g. desinfectant or antiseptic agent depends on the concentration of ethanol-water-mixture: An ethanol percentage of 50-80% destroys the cell wall/membrane of bacteria by denaturing their proteins and dissolving their lipids (effective against most bacteria, fungi and some viruses; ineffective against bacterial spores). Therefore, the ethanol has to pass the bacterial membrane/wall to get into the bacteria - if you use 100% ethanol instead, the bacteria get 'sealed' and they will survive... An other mechanism is the high osmotic pressure of ethanol/water-mixtures; and the 70% has the highest one.
Here is an interesting experiment using alcohol water ratios: