Why Doesn't Water Leak Out of Dialysis Tubing?

[Sascha_K]

The way I think about it, the dialysis membrane basically has holes in it of a given size which keep molecules larger than the holes from moving out.

My question is, since the pore size of the tubing is always bigger than the size of water molecules, why doesn't water leak out of the dialysis tubing when you are filling it in air?

 

[Bystander]

What's the surface tension of water?

 

[Sascha_K]

Aha! I see! Thanks for clearing that up for me!

 

[Sascha_K]

So essentially the surface tension is holding the water inside the bag until it is placed in the buffer, upon which the surface tension is no longer applicable (surface tension of water against air is 71.97 dyn/cm at 25 C)

 

[Sascha_K]

I now have another question: At what pore size will the surface tension fail and allow water to leak out of the bag? Or at least, what are the appropriate equations for calculating such a problem.

 

[Bystander]

Assuming a circular x-section for a pore (or a spherical chicken), ηπd (η = surface tension, d = pore dia.) is the force holding liquid in the pore, and force/area (ηπd/{πd²/4}) is pressure required for flow through the pore(s).

 

[Sascha_K]

Using a 10 cm long dialysis tube, I roughly estimate that the diameter of the pore would need to be in the vicinity of 0.2 to 0.3 mm

 

[Bystander]

Looks like a decent ballpark number.

 

[Sascha_K]

You are awesome dude. Not only have you helped me with understanding this (heretofore) seeming contradiction of how dialysis membranes work, but you also helped resolve a friendly argument with a co-worker. Physic forums is awesome, thanks to people like you! :)

 

[Laroxe]

The tubing is a semi porous membrane but remember it runs through a dialysis solution, the content of which can be altered depending on what you want to achieve. The solution exerts an osmotic pull on the waste in the blood removing them. Modern systems can add specific filters to remove certain drugs etc.