So, I've been looking over data on permeabilities and there's something that bothers me. Permeability data on the noble gases is somewhat hard to find, but the Parker O-Ring Handbook has them for a number of materials:
(as an example material to choose in the list, say, PTFE)
What I don't get is... why is the permeability rate for krypton usually higher than that of argon for most materials? ? Their chemical properties are basically identical, krypton is just a larger atom. By all standards one would expect it to permeate slower. Xenon permeates slower than both of them, as expected. So what's up with krypton?