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Due to the wide use of plastic bags as containers for almost everything in everyday life, I naturally use them to wrap and hold dirty diapers as a temporary stop before getting to throw them in the garbage outside. However, these bags do not stop the smell from spreading around the bags at all.

I wonder why is that?
These bags seem to be tight enough to hold liquids (when intact), but they do not hold smells at all. I guess the obvious answer is that the plastic\nylon polymers have wide enough gaps between them for the passage of small clusters of molecules in the micro- or nano-level, but I was wondering if there was some other explanation and if anyone could elaborate on the matter.

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Well, you basically answered your own question nicely with your "obvious answer" discussion. On a molecular scale, smallish molecules, like the volatile compounds that evaporate and make way to your nose, are able to permeate the plastic. This happens in part for the reason you said; on such a small scale, the plastic that appears fully "solid" and impermeable on a macro scale is actually a mesh of molecules with paths large enough for these small, smelly, molecules to escape.

There are factors at play other than molecular size, like the chemical composition of the plastic and the escaping compounds. If the compounds have a strong affinity for the plastic, it will take less time for them to permeate as they will more quickly absorb through the plastic.

Regarding your observation that the plastic seems to hold liquids, but not smells; this is probably an illusion. I suspect that the liquids you've placed in the bag actually have little or no smell (i.e. water, food oils). For example, in an analytical laboratory setting, water samples or water-based standards stored in various plastic containers have a limited shelf life because the water slowly permeates the plastic, evaporating into it's surroundings. So the water is escaping the plastic just like a smelly compound might do, but of course water has no smell.

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  • $\begingroup$ Another factor could be surface tension. Water in a plastic bag may not wet well, whereas urine may have enough organics to wet plastic better. And thin plastic bags almost certainly has pinholes, which may be a more certain exit for wetting liquids. Similarly, light duty aluminum foil has about one visible (but tiny) pinhole per square foot. The heavy duty foil is about 10 times less porous. $\endgroup$ – James Gaidis Nov 4 '19 at 14:33

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