Osmotic pressure as a force

Question

Where does osmotic pressure as a force originate? When salt water and fresh water are put together, they would be comfortable to stay as they were if the concentration difference weren't considered, because they seem to be in a mechanical equilibrium when either alone or put together. Is there any microscopic force driving the macroscopic osmosis? I'm asking from a philosophical perspective regarding the nature of that osmotic force, for example whether it's purely out of mechanical balance. When analyzed from an energetic perspective, it's clear that osmosis reduces free energy, thereby giving a real force. But how to get this force without using an energetic perspective?

Answer 1

I think that you lack a little research, because this information should not be too hard to find after a google search, unless I misunderstood your question.

EDIT: as Aaron Stevens mentionned below, the explanation that I am about to give might not be the one that is commonly accepted today. It seems to still be taught this way in many places though. This answer is not deleted as it gives a potential explanation for what else than the balance of forces may cause the osmosis, but it should be taken with caution. My bad.

I advise you to have a good look at the phenomenon of diffusion first, which is important to understand osmosis. Diffusion (in a gas or in a liquid, cases of solids being a bit different) is a consequence of random thermal motion of molecules. For purely statistical reasons, this random movement will always induce a flux of a given species from regions of higher concentration to regions of lower concentration (because statistically, there are more molecules coming from places of high concentration where there are plenty, than from low concentrations where there is just a bunch of them). If you just put a drop of milk in your coffee, diffusion will do its job of spreading the drop until the milk concentration is the same everywhere in the mug (which might take a while by pure diffusion)

Important point: diffusion is not a "microscopic force". It is only the result of random thermal motion, and basic statistics. It is a pretty inutuitive result when you think about it, and if you are still confused about it after this short introduction, I invite you to google it, you will find plenty of resources to explain it in great details.

Osmosis happens when you put a semi-permeable membrane between 2 solutions having different concentration of whatever (let's say pure wter vs salty water).

The salt can not go through the membrane, so let's ignore its movements and focus on the water. You can see the salty water as having a higher concentration in salt but also as having... a lower concentration in water.

So what will water do ? Start flowing through the membrane because of this concentration gradient: classic diffusion process. Now, if this was the simplest case of diffusion, it would reach equilibrium when all concentrations are homogeneous, but since you will Always have salt on one side only, this won't happen here. Instead, the process will be stopped when the water level starts raising in the salty water, enough to generate a pressure that will stop the flux.

The equilibrium is the result of a balance between two driving processes: diffusion (statistical motion of molecules) and pressure difference (force opposing to the free movement of those molecules)

If you want more quantitative insights, the formula for osmotic pressure can be derived from thermodynamics, following the ideas that we discussed here with the appropriate rigourous notions. but I don't think this is what you are after. Let me know if I am wrong.