What would happen if you had a rigid structure that helium could permeate, but nothing larger could, and then filled it up with helium and waited?
Would most of the helium exit, until it was balanced with just the partial pressure of helium in the atmosphere? That would be nearly a vacuum, wouldn't it?
> Pretty sure it would simply equalize to the external pressure.
External partial pressure of helium which is extremely low.
> Consider it another way. If you have such a device with a vacuum inside, would it not pull in the external helium over time to reduce the vacuum?
It would, but only until the partial pressure of helium inside is equal to the partial pressure of helium outside (assuming the membrane is permeable only to helium). After that point the same amount of helium will traverse both ways, establishing the equilibrium.
Internal pressure would equilibrate to the partial pressure of helium in the atmosphere, presuming all other species can be assumed to have zero permeability. Osmotic pressure/semipermeable membranes is the analogous liquid system.
It would be a vacuum I guess, but generally not in the normal sense because this is the case with most metals. You don't really consider the interstitial space between the atoms in the crystal lattice a vacuum even though there is space for small atoms (He/H) to diffuse through.
Ignoring the fact that what you're describing sounds suspiciously like a Maxwell's demon, I think the equilibrium would be at a higher pressure because helium escaping against an overall pressure gradient would be doing work.
In essence, at the boundary I think the rate at which helium escapes would not simply be proportional to the gradient created by the internal pressure and the exterior partial pressure, but I think would include a term involving the whole exterior pressure.
It's not a Maxwell daemon, it's just a device for which nothing but helium exists. In the ideal gas model it would indeed create almost vacuum but the second law of thermodynamics is not violated since the process is symmetrical. Pressure of helium outside is also almost zero.
This relates to the difference between how a real gas and how an ideal gas behave in this scenario.
The difference in this situation will be very small in my opinion, broadly because the gas molecules in the atmosphere still have comparatively high mean free paths and therefore won’t interact with the “escaping” helium molecules.
I don't know the answer, but this does make me think of atomic sieves like the ones used in oxygen concentrators.
This is an armchair scientist explanation of them, but they "concentrate" oxygen by first having atmospheric air pumped in and then pressurized. The microsieves have holes so small that mostly only oxygen can fit through, the larger CO2 and Nitrogen atoms simply won't fit.
Then, pressure is let off and fresh air brought in. The fresh air scrubs out the oxygen depleted air and refreshes it with standard air.
Then, the pressure decrease allows the oxygen to leak back out of the sieves, leaving you with oxygen enriched air.
I don't know if there are any atomic helium sieves, but if you can find one it might be a start to testing the question.
It would create an almost perfect vacuum. You could then extract work by collapsing the vessel. The free energy for the work would come from putting the trapped helium (state 1) in a higher entropy state (state 2).
For what it's worth, this is done industrially to separate gasses using porous membranes.
Would most of the helium exit, until it was balanced with just the partial pressure of helium in the atmosphere? That would be nearly a vacuum, wouldn't it?