Why Doesn't Alcohol Just Evaporate when Cleaning Wipes are Left "Open"?

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kyphysics
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When shopping at grocery stores, they have these boxes of sanitizing wipes containing alcohol on them. You know the type: they are often in the front entrance area for people to wipe carts. The way these are designed is that you have one wipe sticking out of the container and the rest tucked away in the tube. The opening from which the wipes can come out is small.

I understand that the small opening prevents a lot of air probably from getting to the alcohol wipes inside the container and evaporating all the alcohol away. (Feel free to correct my reasoning if I'm wrong - not a science person, but a liberal arts/social sciences dude.)

However, what about that one wipe sticking out? Someone it ALWAYS manages to stay damp/moist and doesn't (unless I'm wrong) lose its alcohol-ness. How is that possible. I'm presuming there are times when the wipes are untouched for a few hours. Is that not enough time to evaporate the alcohol on them? If I spill rubbing alcohol on my counter, it's gone within a few minutes and evaporated.

Thank you for helping me understand.
 

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  • #2
fresh_42
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Most certainly because of capillary action. It does evaporate, so it is only a matter of time. On the other hand, I expect the producers to combine the alcohol with a fluid that doesn't evaporate, e.g. some oil.
 
  • #3
russ_watters
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When shopping at grocery stores, they have these boxes of sanitizing wipes containing alcohol on them. You know the type: they are often in the front entrance area for people to wipe carts. The way these are designed is that you have one wipe sticking out of the container and the rest tucked away in the tube. The opening from which the wipes can come out is small.

I understand that the small opening prevents a lot of air probably from getting to the alcohol wipes inside the container and evaporating all the alcohol away. (Feel free to correct my reasoning if I'm wrong - not a science person, but a liberal arts/social sciences dude.)

However, what about that one wipe sticking out? Someone it ALWAYS manages to stay damp/moist and doesn't (unless I'm wrong) lose its alcohol-ness. How is that possible. I'm presuming there are times when the wipes are untouched for a few hours. Is that not enough time to evaporate the alcohol on them? If I spill rubbing alcohol on my counter, it's gone within a few minutes and evaporated.
You're instinct isn't wrong, it's just that it apparently doesn't evaporate as fast as you think and the traffic through the grocery store is enough to keep them fresh.

From a study on lab ventilation safety I did a few years ago and a wet paper towel I just weighed, a well saturated 10x10cm square wipe may contain up to about 10mg of fluid (usually 70% or 60% IPA) and will evaporate at a rate of around 0.3 g/min. So based on that, I wouldn't expect them to last more than about 10 min exposed. Obviously, that will depend on how much is sticking out and how well exposed it is -- e.g., if it is crumpled-up it will evaporate much slower.

A moderate traffic grocery store may have 10 people a minute go through the door, so as long as more than a few percent take a wipe, they should be fine.

But if you take a wipe and notice it isn't significantly wet, discard it and take a second. The second should be perfectly fine.
 
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chemisttree
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They’re using alcohol wipes at the entrance to the store? Very unusual! I’ve only seen water/quaternary ammonium sanitizing wipes being used.
 
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  • #5
russ_watters
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They’re using alcohol wipes at the entrance to the store? Very unusual! I’ve only seen water/quaternary ammonium sanitizing wipes being used.
Heh - good point. I don't actually know what they use, but if that's more common then it's probably right. Prior to this crisis I never used the ones at the grocery store, and now I use my own IPA wipes that I bring with me. The ones provided at the gym, which I do use, are definitely not IPA.
 
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phinds
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A moderate traffic grocery store may have 10 people a minute go through the door ...
I don't disagree w/ your conclusion, BUT ... 10 people a minute? Suppose they had 10 cash registers (a large assumption for most stores). They would need to process one person every MINUTE through the checkout station to keep the store from filling up. Does your time through a checkout average under a minute? Mine sure never has.
 
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  • #7
kyphysics
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Most certainly because of capillary action. It does evaporate, so it is only a matter of time. On the other hand, I expect the producers to combine the alcohol with a fluid that doesn't evaporate, e.g. some oil.
Oil, really? Interesting. :smile: But, does oil not evaporate at all or is it just slow to? I see black oil marks all over Walmart parking lots (or any parking lot). They seem to dry (even though there can be "damp" areas). I guess some parts look dried (unless my eyes are fooling me).

So, combining the alcohol with a less and/or non-evaporative fluid makes the alcohol evaporate slow?
 
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TeethWhitener
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Oil, really? Interesting. :smile: But, does oil not evaporate at all or is it just slow to? I see black oil marks all over Walmart parking lots (or any parking lot). They seem to dry (even though there can be "damp" areas). I guess some parts look dried (unless my eyes are fooling me).
The oil is disappearing because it is sinking into the asphalt, not evaporating into the air.
So, combining the alcohol with a less and/or non-evaporative fluid makes the alcohol evaporate slow?
Yes, basically. Mixing two liquids together will affect the vapor pressures of the liquids. The simplest treatment (of ideal liquid mixtures) leads to Raoult's law, but most mixtures are far from ideal.
 
  • #9
fresh_42
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Oil, really? Interesting. :smile: But, does oil not evaporate at all or is it just slow to? I see black oil marks all over Walmart parking lots (or any parking lot). They seem to dry (even though there can be "damp" areas). I guess some parts look dried (unless my eyes are fooling me).

So, combining the alcohol with a less and/or non-evaporative fluid makes the alcohol evaporate slow?
I don't know for sure, but that is what I would do. They will probably need a kind of emulsifier. E.g. I have an alcohol based mouthwash which doesn't evaporate either, at least not fast. However, besides alcohol, water and the typical odors, there are quite a few additional substances. E.g. sorbitol, which is a carrier substance for humectants, and a poloxamer which Wikipedia describes as
Poloxamers are low-foaming and foam-suppressing nonionic surfactants that are used for dispersion and emulsification in the chemical-technical industry.
So it seems plausible that there are various possibilities to bind the alcohols and avoid (a fast) evaporation. I wondered that there was no glycerol in it, but this might be disguised by another ingredient or is left out because it dries out the oral mucosa.

Another example are perfumes or after shaves. It takes quite a while until they lose their alcohol.
 
  • #10
kyphysics
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The oil is disappearing because it is sinking into the asphalt, not evaporating into the air.

If true, that would blow my mind. Oil does not evaporate AT ALL?

If it "sinks" into the asphalt, why doesn't water just get "absorbed" into as well? I'm using absorb and sink in the same sense, by the way.

Apologies if my questions are simplistic.
 
  • #11
TeethWhitener
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Oil does not evaporate AT ALL?
The vapor pressure of most of the components of oil is very low, meaning that it would take an extremely long time for oil to evaporate at ordinary temperatures and pressures. You can leave a dish of oil (or a drop of oil, for that matter) out in the open and basically nothing will happen.
If it "sinks" into the asphalt, why doesn't water just get "absorbed" into as well?
Water does get absorbed. Not to the same extent, since oil and asphalt are much more similar chemically than either oil and water or asphalt and water. That’s one reason why asphalt is used: it’s relatively water-repellent.
 
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  • #12
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I don't disagree w/ your conclusion, BUT ... 10 people a minute? Suppose they had 10 cash registers (a large assumption for most stores). They would need to process one person every MINUTE through the checkout station to keep the store from filling up. Does your time through a checkout average under a minute? Mine sure never has.
shoplifters? 🤔
 

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