Originally Posted by tdrace
"My gut feeling is that it's a myth (the molecular weight of N2 is lower than O2, so everything else being equal the N2 molecule would be slightly smaller than O2) but there could be some subtle way the molecule reacts with the rubber that makes it effectively larger. At any rate the difference would be in the mud"
As you'd expect, diffusion rate of O2 and N2 through various membranes has been studied for over a century, and is well understood.
Diffusion rate is defined by Fick's Law, and is dependent on the frictional coefficient of the molecule, not just its physical size. The frictional coefficient is based on (a) Stokes Radius (not physical diameter) of the molecule, and (b) The viscosity coefficient of the gas at the given pressure and temperature. I'm guessing that N2 has a greater frictional coefficient than O2, due to either greater viscosity and/or larger Stokes Radius than O2, and that probably explains the lower diffusion rate.
The equations to calculate these are complex, but it's not a myth. If any physics major is reading this, they could probably give more details.
http://en.wikipedia.org/wiki/Fick%27s_law_of_diffusion
http://en.wikipedia.org/wiki/Stokes_radius