Gas in a semi permeable membrane - one for the gas experts

[mtrip]

I believe I know the answer to this question; I need an expert to verify it for me.

I work for a company that makes latex balloons. The people that have been here for a long time insist that if one is putting a small (5" balloon) filled with helium inside a larger (30") balloon then one must fill the larger balloon with helium as well. I've asked why (repeatedly) and get specious arguments that the small balloons don't "leak" as much with helium on "both sides" of the membrane; some go so far as to say that (somehow) the helium in the larger balloon is going into the smaller balloons (I walk away from that one). I think this to be nonsense and in fact a tremendous waste of helium. I would go further and say that in fact all that you are doing is making sure that the larger balloon "leaks" as well, and will deflate at a much faster rate than if it was filled with air. I want to be certain that in fact the larger balloon should be inflated with air.

A postulation that I make, which may in fact not be very accurate, is that the pores in the smaller balloon, with helium "pushing outwards" would have larger molecules (mostly nitrogen) "pushing back" and blocking the pores. In my minds eye I can see this, but have no basis for it being close to fact.

Thanks for any input.

 

[Bystander]

the small balloons don't "leak" as much with helium on "both sides" of the membrane

Yes.

larger molecules (mostly nitrogen) "pushing back" and blocking the pores

No. Helium is going to move across any membrane from higher chemical potential (pressure in this case) to the side with lower chemical potential; the nitrogen is going to likewise move from higher partial pressure of nitrogen to the lower partial pressure side of the membrane, but it's a larger molecule and has a lower diffusion coefficient, so you don't notice it moving except over a long time period (inflate a balloon with nitrogen and leave it in a closet for a month).

 

[mtrip]

Thanks for the reply. I understand the second paragraph, but not clear on the first. Assuming for the moment that the pressure inside the small balloon is the same as the pressure on the outside, it shouldn't matter what kind of gas it is, correct? It's really only the pressure difference that matters?

 

[Bystander]

shouldn't matter what kind of gas it is, correct

No. Chemical activities (approximately the partial pressures) of each gas involved will be the same on each side of the membrane at equilibrium, and when far from equilibrium (just after filling), will be moving from the side of high activity (partial pressure) to the side with lower partial pressure.

 

[Chestermiller]

To expand on what Bystander said, the physical mechanism for helium leaving the smaller balloon is by diffusion through the membrane. Helium diffuses from the higher concentration region within the small balloon to the lower concentration region outside the small balloon, just as heat flows through a wall from the higher temperature region in the room to the lower temperature region outside the wall. If the temperatures on both sides of the wall are the same, no heat will flow.

Chet