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?