|Photo by: Bill Selak|
Alcohol disinfects by denaturing proteins. In concentrations over 90%, alcohol instantly coagulates the protein on the just the outside of the cell. This forms a protective shell (think Magic Shell on ice cream) that prohibits the alcohol from further penetration, so it can not effectively kill the cell.
70% alcohol still coagulates the proteins in the cell wall, but it does so slower. Because it takes its time, 70% alcohol is able to penetrate the bacterial cell wall and do it's business inside of the cell.
Of course, to properly disinfect, the alcohol must come into contact long enough; which means a quick spritz won't cut it. A minimum of 30 seconds of wet contact is necessary. Since alcohol can degrade the glue holding bristles in place, as well as ruin some rubbers and plastics, be careful to submerge only the bristles and lay the brush flat (never upright) to dry.