Monday, April 21, 2008

Mysterious osmosis

In college I took an intro biology course with a lab component. There was an osmosis demo that I found memorable due to the fact that nobody has been able to explain it to my satisfaction. The problem is that it was a bio lab, but I think the answer involves thermodynamics and biologists are generally not that up to speed on such things (a generalization, I know). But I didn't get along with my thermo prof and so can't remember enough to answers my own question. Here's the demo:

A U-shaped tube has a semi-permeable membrane at the bottom which allows water (the solven) to diffuse across it, but larger solute molecules are blocked (like salt). One side of the U contains fresh water while the other has some salt dissolved in it. The water levels start even on both sides. Leave it for a while (overnight) and the water will diffuse from the fresh side to the salt solution side diluting the solution in the process. The level of the fresh water will drop and the salt water level will rise. The rule is that the water will diffuse in order to "try to" equalize the solutions.

My question is this: A certain amount of work has been done in raising up the water level on the salt solution side. That risen water now has greater gravitational potential energy. I could run a little water wheel and use that energy for something. Where did that energy come from? What provides the energy for the work done raising the water level on the salt water side?

My lab TA in college didn't know. The PhD bio teacher here at my school doesn't know. Our chem teacher doesn't know. The internet is doesn't know (or is at least holding out). This Hyper Physics page suggests that the "internal" or "thermal energy" is responsible. I can sort of see how there might be a change in temperature to explain the gain in GPE.

Can anybody quantify this in a better way? I'm looking at you, PhD physics people!


Matt said...
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Matt said...
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Matt said...

This is not a branch of physics that I work with or think much about nowadays, but you raise a very interesting question. I think it's pretty safe to say that the energy comes from chemical bonds between molecules. I think it is because water-water bonds are stronger than water-solute bonds, and decreasing the solution concentration allows there to be a greater fraction of water-water bonds, which releases energy that can be used to raise the water level against gravity. But that answer comes more from intuition than a thorough understanding.

Dan said...

Thanks, Matt.

Apparently, the solvent (water) has little to due with it. It is the solute collisions with the membrane that provide the osmotic pressure, and somehow this guy says that "the process of elastic collisions with a moving wall is the mechanism by which the microscopic kinetic energy of the particles is transformed into macroscopic mechanical work." I'm not quite sure how yet.

Interestingly, the osmotic pressure exerted by the solute molecules colliding with the membrane is the same as if they were in the gaseous state at the same temperature!

Anonymous said...

Your hunch is correct. The engine driving the whole osmotic exchange is the thermal energy involved, more of which gets "trapped" by the solute side of the membrane. But in terms of work done, consider that the level of the water on the other side has also dropped. So the energy in the whole system has not changed as much as it appears. Yes, you could drive a water wheel by dumping in more salt, but only if you kept up with your heating bill ;)