I am a cyclist, and not a chemist. When I or my cycling buddies get a flat tire out on the road, after removing the offending sharp object (if any) remaining in the tire, we will typically replace the punctured butyl rubber inner tube with a new one, and then inflate the tire using a $\ce{CO2}$ cartridge.

Inflating the tire in this way takes less than a second to bring the pressure up to 120 psi. At home, naturally, we fill our tires with air rather than a $\ce{CO2}$ cartridge, using a floor pump.

When we fill our tire with an air pump the tire remains inflated for several days. But after filling it with $\ce{CO2}$, the tire is severely flat again by the next morning. $\ce{CO2}$ clearly diffuses through the butyl membrane very much more rapidly than air, even though $\ce{CO2}$ is a larger molecule than either $\ce{N2}$ or $\ce{O2}$. Is it the shape of the molecule? Or its chemical properties?


1 Answer 1


The permeability of a gas through rubber depends mainly on its diffusivity and solubility in rubber.

$\ce{CO2}$ has a significantly higher solubility in rubber than $\ce{O2}$ and $\ce{N2}$, whereas the diffusion coefficients differ not that much.

enter image description here Source: [1].

The result is that carbon dioxide passes ordinary rubber about 5 times faster than oxygen and about 15 times faster than nitrogen[1, 2].

  • $\begingroup$ Thank you for your excellent answer and references. Too bad nitrogen cartridges aren't available for cyclists! $\endgroup$
    – R. Gold
    Jul 13, 2016 at 1:57
  • $\begingroup$ @R.Gold please mark aventurins answer as "accepted" - that is the way to pay tribute for good answers here ;-) $\endgroup$
    – datenheim
    Nov 29, 2023 at 21:56
  • 2
    $\begingroup$ @datenheim R Gold has biked on, last seen 2016. It’s a great answer, I agree (as I upvote…). $\endgroup$
    – Karsten
    Nov 29, 2023 at 22:15
  • $\begingroup$ Carry an attachable bike pump or buy one of the new battery powered minipumps. $\endgroup$
    – jimchmst
    Nov 30, 2023 at 8:43

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