70% percent of alcohol is ideal to a stronger solution. Pure alcohol coagulates protein in contact.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.

The antiseptic effect of an aqueous solution of alcohol is increasing with increasing concentration over 60%, BUT falls sharply in concentrated ethanol solution. That's why you should not treat hands with pure alcohol - this will not give the necessary disinfecting effect

100% ethanol completely coagulate the outer cells proteins and it works as defense mechanism in the cells of organism but in case of 70% ethanol, the ethanol does not completely coagulates the proteins of the cell hence penetrates in the organisms body and it dies. So the area becomes contamination free.

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.

70% isopropyl alcohol solution penetrates in the cell wall at slower rate and coagulates the all protein of the cell wall and microorganism dies.

80%, 90%, or 100% ethanol completely coagulate the outer cells proteins and it works as defense mechanism in the cells of organism but in case of 70% ethanol, the ethanol does not completely coagulates the proteins of the cell hence penetrates in the organisms body and it dies. So the area becomes contamination free.