Hand sanitizer chemistry

[Hawkmoon]

I just read an article that said hand sanitizer loses potency after about three years due to the alcohol evaporating. If that happens, considering that the alcohol is supposed to be 60+ percent of the composition of the stuff, wouldn't there be a visible loss of volume if the alcohol evaporated to any significant degree?

If you suspect that a bottle of hand sanitizer has lost potency, can it be restored to efficacy by adding in a bit of 91% or 99% rubbing alcohol? (or even 70% rubbing alcohol, if that's all you have.)

[AZRedhawk44]

No idea.  I'm wondering if the glycerine in commercial sanitizer has anything to do with it?

But I did happen to make a batch of hand sanitizer, just yesterday.

Aloe grows like a sumbitch here in AZ and I have a couple of plants in the backyard.  Took some aloe, fileted it to get the goo out and blended it up to chop up any plant matter, added 99% iso alcohol at a 3 parts alcohol to 1 part aloe ratio to wind up around 75% alcohol.  Poured the thinned aloe/iso solution through a strainer to remove the chopped up plant cellular matter, and put the resulting liquid in bottles.

Supposed to have a shelf life of several months before the aloe goes bad.  Runnier since there's no glycerine.  But sterilizes, and deals with the alcohol drying your hands by also providing aloe.

[230RN]

Ar least with respect to ethyl alcohol, the weird thing is that when a liter of water is mixed with a liter of alcohol, you don't get two liters of mixture, you get less than two liters.

Seems like the molecules kind of "cuddle" with each other so they don't take up as much room as you'd expect.  (Well known to folks who took organic chemistry and is a frequent joke puzzler if you hang around with organic chemistry dweebs.)

So one might guess that if the alcohol evaporates from the mixture, not much change in volume might occur.

Challenges and corrections welcome.

Terry, 230RN

[HankB]

I read an article by a microbiologist claiming 70% alky is better than higher concentrations, because the water lets it penetrate the bacteria's cell wall and kill the entire germ, whereas higher strengths fry the outside. Either should do for virii.

Below 70% the germicidal properties go down, so I wouldn't "fortify" hand sanitizer with anything less than 91%. In fact, I frequently add a dollop of 91% or 99% to older sanitizer, figuring it won't hurt.

[brimic]

At work, someone has set up spray bottles everywhere filled with 190 proof ethanol (95%) for disinfecting...
I’ve also heard 70% would be better, but here is nothing quite like the smell of nearly pure ethanol 

[Hawkmoon]

I read an article by a microbiologist claiming 70% alky is better than higher concentrations, because the water lets it penetrate the bacteria's cell wall and kill the entire germ, whereas higher strengths fry the outside. Either should do for virii.

Below 70% the germicidal properties go down, so I wouldn't "fortify" hand sanitizer with anything less than 91%. In fact, I frequently add a dollop of 91% or 99% to older sanitizer, figuring it won't hurt.

I’ve also heard 70% would be better, but here is nothing quite like the smell of nearly pure ethanol

I read up on this recently when the subject arose in a different thread. Apparently isopropyl alcohol is most effective for sanitizing purposes at concentrations from 70% up to 95%, but no higher. The reason is what HankB noted: the water in the dilution helps break down the cell walls to allow the alcohol to work more effectively.

[Silver Bullet]

I read up on this recently when the subject arose in a different thread. Apparently isopropyl alcohol is most effective for sanitizing purposes at concentrations from 70% up to 95%, but no higher. The reason is what HankB noted: the water in the dilution helps break down the cell walls to allow the alcohol to work more effectively.

So we're only talking about using sanitizer for the skin, correct?  I bought a gallon bottle of 99% alcohol, but I use that from a spray bottle to wipe down objects.

[Silver Bullet]

Since I bought a gallon of 99% alcohol, to bring it to 91% or 70%, is it as simple as just adding water?

[Hawkmoon]

Since I bought a gallon of 99% alcohol, to bring it to 91% or 70%, is it as simple as just adding water?

Yes. Preferably distilled water.

[230RN]

Yes. Preferably distilled water.

Why?