No theory - just fact.
O2 "permeates" approximately 3-4 times faster than does N2 through a typical rubber, as is used in tires, primarily
because O2 has a slightly smaller effective molecular size than does N2.
Why?
The following examples expressed in Angstrom units demonstrate this (one Angstrom unit is 10 to the -10th power meters,
i.e., one-ten-billionth of a meter): from gas viscosity data, O2 2.96 and N2 3.16 (difference 0.20); from van der Waal's
interaction data, O2 2.90 and N2 3.14 (difference 0.24); from molecular refraction data, O2 2.34 and N2 2.40
(difference 0.06).
If you would like to know precisely why Nitrogen will not leak out of a tire as fast (still does though) as Oxygen then you must read and understand Graham's Law as it pertains to effusion and permutation. Oh, you might need to read Fick's law of diffusion as well.
Knowing the facts, I think it's unnecessary to fill your tires with nitrogen unless you are an airplane or a race car driver. You need to ckeck your tire pressure weekly though no matter what it's filled with.
