Comment by varenc

7 years ago

They do have a strong hermetical seal on them, but the older ones used by Apple are still susceptible to “small gas molecules”.

They must have some seal in place and fortunately our air is Nitrogen, O2, and a little bit of Argon. These can’t get through the seal! Helium is only found in minuscule amounts naturally and apparently this concentration is so low it doesn’t matter.

Helium is special because is it has the smallest kinetic diameter of all the noble gases. You can compare its diameter to other gases here: https://en.wikipedia.org/wiki/Kinetic_diameter#List_of_diame...

Helium is frequently used in leak tests since it has a way of working through even the tiniest of leaks. Elemental Hydrogen is of course smaller, but I believe you only find Hydrogen as H2 which is a larger molecule than Helium.

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varenc

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Klathmon  7 years ago

According to that table, Helium is 260, and water is 265, and there are a few more under 300.

Does that mean that water (in some form) also acts like helium and would be able to get through that seal?

  • varenc  7 years ago

    I think the answer is that water has polarity! Though we’re definitely hitting my chemistry knowledge limit...

    While water’s overall charge is neutral, its charge isn’t distributed symmetrically ans tbis makes it a rather polar molecule. This means part of molecule have different charges which leads to attraction to other water molecules thanks to van der waals forces. Basically water naturally clumps up and won’t squeeze through tiny holes even if it’s kinetic diameter would technically allow it. (Though I believe very high pressure can force it through)

    Water’s polarity is what causes surface tension and droplets! For comparison, gasoline is relatively nonpolar, though not as nonpolar as He. If you’ve ever spilled some you can see it spread out immediately and avoid the droplet formation and cohesion you’d see with water. Similarly, water tight containers might not be gasoline tight. (but don’t test this out...)

    To correct my comment from earlier: Helium is special because of its small kinetic diameter AND because it’s naturally found in a monoatomic and nonpolar form, as are all noble gases. This lack of polarity leads to minimal intermolecular attraction that would cause clumping and limit flow.

    (I found a reference that helium can be measured flowing through a 1.0nm hole, albeit at a very low rate. I wonder how close to the 0.260nm kinetic diameter a hole can get for He to still flow)