# Why does CO2 diffuse through a butyl rubber membrane more readily than air?

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?

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