as an antiseptic, 70 percent of alcohol is prefered to a stonger solution. Pure alcohol coagulates protein in contact. Suppose the pure alcohol is poured ovr a single celled organism. Th alcohol will penetrate 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 prevent the alcohol from penetrating farther from the cell, and no more coagulation woul take place. At this time the cell would become dormant butnot dead. under the proper condition 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 all the cell is coagulated and the organism dies.
Although I find that hard to follow, it doesn’t seem to make the chemical more powerful when diluted, it just allows a slow process to finish, and that is the preferred action.