If it makes you feel better, Size does not matter?
szicree said:
If this were true, we'd all have pure nitrogen eventually: We fill up with air, the oxygen leaks out, we top up with air, that oxygen leaks out, etc. Proportion of oxygen gets closer to zero each time we fill up.
Well I guess what airguy said. On first blush it seems that your theory that tire/tube would distill the O out of the air might be correct. In theory that is how nitrogen generators work, a membrane.
Yes,
I grant you molecule size is probably not the reason, as I said, my bad, but it sounded good to me at the time. However for fun, the size (Atomic radius) of AIR's top three elements are: Oxygen 60 pm; Nitrogen: 65 pm; Argon: 71 pm. I confess total ignorance and apathy.
In the case of the tire filtering elements, the Helium, Neon and Hydrogen would be the first to pass out of the tire, if your theory or premise where true.
There may be a small truth to it. Clearly Hydrogen and Helium are much smaller molecules, but how they are bonded to the 'air' compound and react with rubber may be another issue. I know
one tire company sells "low leak" tubes. People report it keeps pressure better. So the 'AIR' DOES get through the rubber. What elements I don't know, probably as the complete compound minus the water and may be the CO2.
However it's a fact 'N' does lose less pressure over time by about a 30% to 40%, compared to "Air". Why, I'll leave that to the chemist but its a fact. I'll buy airguy's explanation, although when it comes to words like osmosis, my eyes glaze over.
The distilling property of rubber to turn "Air" compound into constituent gases is a mystery or probably non-existent.
However commercial Nitrogen generators use a filter/membrane, taking regular AIR and extracting the N out, partly based on element size. An example membrane:
(Note: If you read how it works, it uses the size of very small micro tubes and inertia to separate air into its elements. Its a flow through process, unlike a tire where the gas is held under pressure in rubber envelope or membrane if you will. So size does matter? Isn't chemistry fun! - not)
Does the air pressure in your tire change over time? Yes, but do different elements of the gas compound "Air" escape at different rates, changing the compound? Probably not or not in any significant ratio.
Not withstanding airguy's explanation which I have no reason to disagree with, lets say a tire did leak out by elements, a little; you'll never get to the point it becomes 99% nitrogen. If it where to work like that, you would likely end up with CO2 and water left, not Nitrogen. People do report their mounted tires end up full of water, not withstanding
"Tractor-man" (with tires that have 3 inches of rubber). There is something there. Clearly the water is getting in but not getting out, and clearly air is leaking out.
BTW I am not saying Nitrogen or BUST, I use air. But the properties clearly have benefits. I guess a good second place would be DRY air.