Negative retention for chloride in hydrolysate filtration

Question

I tried to fractionate proteins as a function of their size with an ultrafiltration membrane. I observed a higher concentration of chloride in my permeate than in my feed or retentate. The proteins do not pass through the membrane and the filtration has been done at pH = 7. I asked, how is it possible to have a higher concentration in the permeate than in the feed or retentate?

I found this explanation but I'm not sure about it :

It may be explained by the Donnan effect. Negatively charged proteins are retained in the retentate. To compensate the positive charges from the cations that are on the other side of the membrane, they attract the chloride.

Is it a possible way to explain this issue?

Answer 1

A combination of osmosis and Gibbs-Donnan effect is the correct explanation.

With a semi-permeable membrane such as the one you are using, or a cell membrane, or in dialysis, water can move freely as can chloride and other small ions, however large molecules such as proteins cannot.

A simplified explanation is this: in osmosis, the tendency is for the osmotic pressure and charge to equalise on either side of the membrane. Proteins usually have a negative charge and exert a large osmotic pressure. Thus, to balance this out, small anions will diffuse to the opposite side to where the proteins are. Water will diffuse until the osmolality is equal on either side of the membrane unless there is a hydrostatic pressure forcing it to the other side. In the absence of hydrostatic pressure, you will end up with a higher concentration chloride solution on one side and a higher concentration protein solution on the other side.

Here is a diagram:

If you assume that the small particles you can see are chloride and the big particles are your proteins, you can see that the chloride concentration in the permeate is higher than the retentate and higher even than the feed.