There is an effect called diffusio-osmosis, by which a flow is induced close to walls, and is ultimately caused by the repulsion of solute molecules by the solid wall around the pore. See this older article, and more recent ones like this one.

First question: Is this effect, just a more technical term for what is normally called osmosis?

Second question:

As is explained in the literature on diffusio-osmosis, the force can be attracting or repelling. It seems to be that an attracting force would cause a flow in the opposite direction than in the first article, i.e., osmotic flow would occur from the region of high solute concentration, to the region of low solute concentration. This will stop when the pressure in the latter region matches the (negative) osmotic pressure. Osmosis would go the other way.

Is this possible? If yes, do you know of any actual experimental realization? If no, why not?

There is this recent article that seems to suggest that osmosis works by a mechanism, that seems to me the same as diffusio-phoresis. However, as pointed out in the comments, the paper may contain some flaws.

  • $\begingroup$ To be very honest with you... the first paper is total nonsense as its proposed mechanism violates energy conservation. A solute molecule can not leave the region of a pore with higher momentum than it came in with, unless the pore has an active transport mechanism... oh, wait, cells do have active transport mechanisms called ion channels! Why do they need these? So that they don't lose their content to the surrounding medium trough osmosis all the time! Duh! $\endgroup$ – CuriousOne Jul 1 '16 at 22:22
  • $\begingroup$ Well his wording isn't very good nor very precise. The literature on diffusio-osmosis makes the concepts much clearer. Intuitively though, it is certainly possible that the wall around the pore repels the solute molecules right? Say via electric charges, or perhaps some effective entropic force. $\endgroup$ – guillefix Jul 1 '16 at 23:02
  • $\begingroup$ The wording is precise enough to tell that the authors have some very strange ideas about energy conservation. In order to repel something harder than it comes at you, you have perform work on it. We have a couple of sports that work along this principle like tennis. As you can clearly see, there is quite a bit of sweating going on on a court to repel those balls towards the other player's half. Same here... unless the pore has a power source, it can't give molecules hitting it a larger velocity than they already had. $\endgroup$ – CuriousOne Jul 1 '16 at 23:07
  • $\begingroup$ I agree about what you say about energy conservation. However, there can still be repulsion between pore and solute molecule; that is enough for diffusio-osmosis. $\endgroup$ – guillefix Jul 1 '16 at 23:13
  • $\begingroup$ What the authors are suggesting is simply impossible without an energy source, which invalidates the paper completely. $\endgroup$ – CuriousOne Jul 1 '16 at 23:18

After researching more, I think I can answer my own questions:

Is diffusio-osmosis the same as osmosis?

No, although the word "osmosis" seems to be used with slightly varying definitions in the literature. Wikipedia's definition seems to agree with most of them: "Osmosis is the spontaneous net movement of solvent molecules through a semi-permeable membrane into a region of higher solute concentration, in the direction that tends to equalize the solute concentrations on the two sides."

On the other hand, diffusio-osmosis is a more general phenomenon defined as a "flow induced by a solute gradient" (see here for instance).

As a matter of fact, osmosis can often be considered a kind of diffusio-osmosis, taking place in a semi-permeable membrane.

Can osmosis (spontaneously) go the other way?

I.e. solvent flow from the high-solute-concentration region to the low-solute-concentration region, a.k.a. negative osmosis?

Well, if we use Wikipedia's definition, it can't by definition...

However, if we are a bit more open and define osmosis as spontaneous flow through a semi-impermeable membrane, then, as pointed out in the comments to the question, we can get negative osmosis if we make the high-solute-concentration region have higher chemical potential. This can be done by an oil/water mixture, for instance. Here the negative osmosis is just driven by diffusion!

However, osmosis does not only proceed by diffusion. As hinted at above, diffusio-osmotic forces also plays a role in osmosis. This effect can actually be the dominant effect, and among other things determines the solute/membrane-dependent reflection coefficient (see here, here, here, here, here, here for the theory. See here for some applications). Some classic models of osmotic flow don't include this effect (see here). In fact, from what I read in those, as long as the membrane is purely semi-impermeable, then not much interesting can happen.

However, if we are even less restrictive, and allow the membrane to be selectively permeable (so that it is just more permeable to solvent than solute), then we get more interesting phenomena! In particular, it is possible to get negative osmosis, i.e. osmosis going the other way, as mentioned above. This is explicit in the papers I linked above. See here for another example, and here for an older experimental study. There is also this one which is not exactly osmosis, but is very similar.

In fact, although negative osmosis is possible for neutral solutes, it has been much more extensively studied for electrolytes. See here, here, here. You can even get osmotic flow through fully permeable nanochannels! (here).

So in short, if you are a bit more liberal about the definition of osmosis, and allow non-perfect semi-impermeability, osmosis can go the other way, thanks mainly to non-equilibrium diffusio-osmotic forces! This is called negative osmosis.

Even in the more trivial case of diffusion-dominated processes, I think this can happen in some cases, as in the oil/water mixture example in the comments.


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