It is widely known that alcohol solutions containing 70 percent alcohol to be antimicrobial and “effective”. But why not use 99 percent or 100 percent for maximum disinfection?
One factor contributing to the effectivity of 70 percent Isopropyl alcohol is its water concentration. The existence of water in Isopropyl alcohol is an important element in destroying or stopping the growth of microorganisms.
Is more water content better?
The water present in isopropyl alcohol functions an important role by destroying the proteins of microbial cell membranes and acting as a catalyst. The water present in Isopropyl alcohol allows the 70 percent Isopropyl alcohol to penetrate the cell wall, which eventually causes the microorganism to die. Moreover, additional water content declines evaporation rate, leading to an increase in surface contact time, therefore enhancing alcohol effectivity.
Of course, that is not to say that more water content is better, in fact, alcohol effectivity drops as alcohol concentration drops below 50 percent and water concentration rises above 50 percent. Isopropyl alcohols with alcohol concentrations lower than 50 percent are less effective because the alcohol concentration, the main factor for disinfecting, is low, therefore lower effectivity.
So why not use 99 percent Isopropyl Alcohol?
Isopropyl alcohol with alcohol concentrations greater than 90 percent coagulates the protein cell walls instantly. The coagulation of the outer layer of the microorganism will build a defensive layer for the pathogenic microorganism. This defensive layer can then act as a shield that will safeguard all the other proteins of the microorganism from further coagulation, rendering the Isopropyl alcohol solution ineffective.
However, that is not to say that Isopropyl alcohol with concentrations greater than 90 percent do not kill bacteria. Isopropyl alcohol with concentrations greater than 90 percent do kill microorganisms, yet requires longer contact times for disinfection. To add, Isopropyl alcohol solutions with concentrations greater than 90 percent permit spores to go dormant, hence difficult to kill.
In the study conducted by a group of researchers, it was found that staphylococcus aureus (a kind of bacteria) was killed in less than 10 minutes using 50 percent Isopropyl alcohol solution. On the other hand, it took. 2 hours for the 90 percent Isopropyl alcohol solution to disinfect the same bacteria, under same conditions .
To sum things up, alcohol effectivity drops sharply as alcohol concentration drops below 50 percent and water concentration rises above 50 percent. On the contrary, higher alcohol concentrations does not mean that the alcohol has more appealing anti-microbial properties.
 “Chapter 12 Alcohols.” Disinfection, Sterilization and Preservation, by Seymour Stanton. Block, 5th ed., Lea and Febiger, 1977, pp. 238–238.