From what I understand, the water helps “penetrate” the cell walls of bacteria and deliver the lethal dose of alcohol. Take out more water and it’s unable to penetrate and murder things as efficiently.
Alcohol and water are both polar and form hydrogen bonds, but of differing strength. You want those bonds to switch, which creates conformational changes and rips the structure of proteins apart. If you just replace all the bonds with alcohol you’ll develop a new, although denatured, stable configuration which can keep the cell wall intact. Instead you want to keep developing new stresses on it until it breaks. Neither can really penetrate the phospholipid membrane of the cell because it’s nonpolar, but those conformational changes create big holes where the surface proteins are.
That seems counterintuitive, why?
From what I understand, the water helps “penetrate” the cell walls of bacteria and deliver the lethal dose of alcohol. Take out more water and it’s unable to penetrate and murder things as efficiently.
Alcohol and water are both polar and form hydrogen bonds, but of differing strength. You want those bonds to switch, which creates conformational changes and rips the structure of proteins apart. If you just replace all the bonds with alcohol you’ll develop a new, although denatured, stable configuration which can keep the cell wall intact. Instead you want to keep developing new stresses on it until it breaks. Neither can really penetrate the phospholipid membrane of the cell because it’s nonpolar, but those conformational changes create big holes where the surface proteins are.
ELI5 answer is 90% alcohol dries out the bacteria too fast and it converts into a bacterial spore with a hard shell that protects its insides.
70% alcohol gives it time to penetrate the cell wall and “dissolve” the insides of the bacteria, either killing it or rendering it non infective.
Iirc, the higher water content keeps the alcohol from evaporating off as fast, keeping it in contact with the surface for a longer period of time.