Osmosis in a U-shaped tube with selective permeable membrane

Starting from the classical osmosis experiment, a U-shaped tube with a semi-permeable membrane, I would like to consider the case when the solute added to one of the compartments (labelled A) is composed of two species—or rather, the same species, e.g. dextran, but two very different molecular weights : the high-molecular weight dextran cannot cross the membrane, but the low-molecular weight can. Assume that low-molecular one is in large excess.

What will be the final state:

• equal concentration of low-weight dextran in A and B with a column height difference so that hydrostatic and osmotic pressures equilibrate,
• or, equal total concentration in A and B, that is, low-weight dextran has a higher concentration in B than in A so that the osmotic pressure, to which both low- and high-weight solute contribute, is the same on both sides. Column heights remain equal.

Equilibrating chemical potential across the membrane leads me to the second final state. Is that correct? Another question with only a comment as an answer seems to point to the first answer, and seems to make sense when considering the diffusion of the low-molecular weight solute.

I am interested also to read about interesting additional effects if the solutes differ in other ways in addition to their different permeation of the membrane.

posted 9 months ago

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Joce‭

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