Problem:

Assume that we have a semi-permeable membrane with water on both sides. First, straightforward, case: On the right side, we have a concentration of the substance A, and on the left side, we have pure water. Obvious effect: water flows from left to right.

Now, imagine we have a second case. We have the same membrane, and a water solution of A to the right. To the left, we have a water solution of a different substance, let's call it B.

Now, what happens? I can imagine a few possibilities:

• the system behaves exactly as in the first case, because the substances have no influence on each other
• the system behaves as if it was a solution of A on both sides, with water flowing in the direction of higher concentration (because it only matters if there is something dispersed between the water molecules, but it does not matter what)
• some interesting cross-influence effect happens
• either one of the three things above can happen, depending on the particular combination of substances A and B

Question: Which one is the true effect? What happens, and why?

If you need an example of the actual substances, let B be thiamine and A be plain sodium chloride, but I am interested in the general case.

Any help would be greatly appreciated!