# Why would permeability for krypton be higher than argon?

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:

http://www.parker.com/literature/O-Ring%20Division%20Literature/ORD%205700.pdf

(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?

• The tables are a little hard to compare - the temperatures aren't always the same between, say, the argon entries and the krypton entries for the same elastomer. However, for those that are directly comparable it does appear that Kr can be a little faster. I think, sadly, that one might have to see if there is a paper from one group directly comparing measurements on the same system - I could see there being large differences for measurements done in different ways on different systems. Oh, and Kr should of course be stronger since it comes from the planet Krypton just like Superman... – Jon Custer Jul 14 '16 at 16:11
• Could possibly be a solubility thing and not just about straight diffusion. – matt_black Jul 14 '16 at 19:19
• – aventurin Jul 14 '16 at 21:28
• @Jon: But as you note, in the cases that are directly comparable... yeah, krypton seems faster than argon, and significantly faster than xenon... the latter is expected, while the former is not. And it's not like there's just one entry... there's a lot of them, for a variety of materials. And given that the measured temperatures, etc are identical and not some overly generic thing (for example, "149C" for PTFE), it sure looks like they're all from the same testing run. – KarenRei Jul 15 '16 at 2:18
• @KarenRei - actually Google pulled up an article by Schowalter et al in Nuclear Instruments and methods A - they have data for all the noble gases through 4 polymers. The diffusivity decreases monotonically from He to Xe. – Jon Custer Jul 15 '16 at 2:27

As noted in some of the comments above, a paper that directly compares noble gas permeation through several materials is S.J. Schowalter et al., Permeability of noble gases through Kapton, butyl, nylon, and "Silver Shield", in Nucl. Instr. Methods A 615 267-271 (2010) . They note that $K = Db$ where $K$ is the permeability, $D$ is the diffusivity, and $b$ is the solubility of the noble gas in the material. For the noble gases, the diffusivity monotonically decreases from He to Xe in all materials. However, the solubility has an odder behavior, with Ne being the oddball with lower solubilities than He or Ar on either side of it.
The net result is that the permeation can end up not being monotonic in nature across the noble gases. In particular, for butyl, $K$ is highest for He, drops for Ne, increases for Ar, drops a bit for Kr, then continues lower for Xe.