Since a 70% ethanol solution has more water, it takes longer to dissolve and thus can actually penetrate the bacterial cell wall and kill it.
TIL 70% Ethanol (alcohol) is a more effective disinfectant because pure alcohol on a single celled organism triggers coagulation (clot) preventing it from penetrating deeper into the cell wall rendering it dormant but still alive. The water in 70% ethanol allows it to penetrate before it can clot.
Water acts as a catalyst and plays a key role in denaturing the proteins of vegetative cell membranes. 70% denatured alcohol penetrate the cell wall more completely which permeates the entire cell, coagulates all proteins, and therefore the microorganism dies.
70-75% ethanol is used for sterilization because it can dissolve microbial membranes. Ethanol has both a hydrophobic and hydrophilic portion which allows it to dissolve just about anything and this ability to dissolve organic molecules means it can strip away bacterial membranes which causes the cells to die.
Absolute ethanol, on the other hand, has a purity level of between 99% and 100%. It's also highly hygroscopic or, in other words, readily absorbs water. As a result, it makes an effective antiseptic and disinfectant because it can inactivate many species of microbes through dehydration.
100 % ethanol would evaporate very rapidly and therefore would not be able to kill the bacteria. However, if it is 70% or 80% ethanol and 20% or 30% water, the ethanol would not evaporate very rapidly. Since the ethanol is not evaporating rapidly, it will enter the cell and denature proteins inside the cell.
A solution of 70% ethanol is more effective than pure ethanol because ethanol relies on water molecules for optimal antimicrobial activity. Absolute ethanol may inactivate microbes without destroying them because the alcohol is unable to fully permeate the microbe's membrane.
Because higher concentrations tend to evaporate too quickly, and lower concentrations are not strong enough, 70 alcohol is most common. For antiseptics, ethyl is commonly used as it is less harsh on living matter and less likely to cause burns or injuries to the skin.
Ethanol is bad for cold-starting, because it doesn't burn as quickly as gasoline. (It has a higher octane, if you're interested.) Pure ethanol would be useless as fuel in the winter months. There are no passenger cars designed to take E100 (but some racing cars are) so it could damage your car engine.
Mechanisms of Action
The primary mode of action is related to coagulation/denaturation of proteins and solubility of the alcohols in lipids. Without water, coagulation cannot occur, therefore a 70% solution of isopropyl alcohol or ethanol will be more effective than higher concentrations of alcohol.
At a 70% concentration, however, the ethanol takes longer to evaporate and this longer contact time means that it is able to penetrate cells more effectively.
DNA is washed with 70% ethanol to remove some (or ideally all) of the salt from the pellet. If water was used as the wash then DNA would dissolve again and if 100% ethanol was used the salt would not wash off because sodium salts are poorly soluble in ethanol.
Hydrogen Peroxide & Peracetic Acid
Several combination hydrogen peroxide/peracetic acid products are cleared by the FDA as disinfectants in health care settings. Combining peracetic acid with hydrogen peroxide increases its effectiveness, especially against glutaraldehyde-resistant mycobacteria.
Basically, a 90 or 91 percent alcohol solution is too powerful in some cases: It fries the outside of the cell before it can get into the inside and kill the actual germ. 70 percent alcohol is just the right proportion of water and alcohol to zap the entire cell.
Water acts as a catalyst and plays a key role in denaturing the proteins of the cell membrane. At a concentration of 70%, the alcohol-based disinfectant solution penetrates the cell wall more thoroughly, soaking through the entire cell, coagulating all proteins, thereby killing the microorganisms.
In conclusion, our study results suggest that a single application of 70% IPA is the optimal antiseptic method to be used for the collection of blood samples for culture. A very low contamination rate was also noted when well-educated resident physicians performed phlebotomy, regardless of the antiseptic chosen.
You can buy rubbing alcohol with a concentration of 70% or 99% isopropyl alcohol. Even though you may think the higher concentration is more effective, experts say 70% is actually better for disinfecting. It has more water, which helps it to dissolve more slowly, penetrate cells, and kill bacteria.
a) 70% ethanol has been found to be most effective in killing microbes. Higher or lower concentrations are not so effective in killing microbes.
Alcohols are not recommended for sterilizing medical and surgical materials principally because they lack sporicidal action and they cannot penetrate protein-rich materials.
70% Ethanol
Aqueous alcohol solutions are not appropriate for surface decontamination because of the evaporative nature of the solution; a contact time of ten minutes or more is necessary and not achievable using a 70% (v/v) aqueous solution of ethanol.
Because that carbon chain is not very long (only 2 carbons), ethanol is still miscible with water, and can also dissolve a wide range of molecules – both slightly polar and slightly nonpolar. Hence it is an effective solvent for many flavor compounds, coloring agents, and bioactive or medicinal compounds.
The most prevalent obstacle currently in the way of E100 in the U.S. is the fact that engines simply aren't designed to burn pure ethanol. “You have to change the engine to optimize it to get [competitive] mileage on ethanol,” Siefkes says.
The most effective concentration of ethanol as a bactericide is typically 70 percent. It's effective in killing bacteria by damaging their proteins and dissolving their lipids. The 100% concentration is less effective as it only dehydrates the cells rather than destroying them.
Disinfection and sterilization are both decontamination processes. While disinfection is the process of eliminating or reducing harmful microorganisms from inanimate objects and surfaces, sterilization is the process of killing all microorganisms.