3 replies to this topic

[DanSandbergUCONN]

Hi All - I am a graduate student in physical chemistry at (you guessed it) the University of Connecticut.  I'm working on project right now, the details of which I'll spare you, but suffice it to say I am using a cellulose membrane to keep the solutions in two different 1mL cuvettes separated.  The MWCO of the membrane is ~20KDa.  I've drilled holes into two different cuvettes, fixed rubber O-rings to the cuvettes and I then place the membrane between the two O-rings and hold the whole thing together with a tension clamp.  

I noticed that when I fill one side with water it does not permeate the membrane and fill the second cuvette, as I expected it would.  I then took another fresh piece of membrane, which was in the shape of a square pocket (sealed on 3 sides, open on the fourth), and i was able to fill the pocket with DI water through the one open side.  No water leaked out.  

I am quite confused by these results.  The MW of water is 2 orders of magnitude below the cut off for the membrane.  Why isn't the water permeating?  Any insight is greatly appreciated.

[bob1]

Surface tension is the answer!  The MWCO is only for diffusion gradients.

[DanSandbergUCONN]

bob1, on Apr 26 2010, 05:41 PM, said:

Surface tension is the answer!  The MWCO is only for diffusion gradients.

Okay that was my thinking as well.  I tried filling both cuvettes with water and adding an organic dye to one side but i did not see migration to the other side.  Maybe this is because the diffusion was just really slow.  I'll reattempt this today.  Thanks for clarifying that.

[mdfenko]

did you hydrate the membrane before using it? the pores may be blocked by air.

also, are you expecting the water or color to pour through the membrane? they will diffuse slowly through the membrane.