How does osmosis work?

Question

I have (very) basic knowledge in chemistry.

I am curious as to why osmosis works. I read a couple of introductory chemistry books and also searched online but most explanations I found only explained the phenomena (i.e. solvent molecules move through semipermeable membrane from low solute concentration area to high concentration area), but not the reason behind it at the molecular level (i.e. what makes more solvent molecules move towards the high solute concentration area than in the reverse direction).

Wikipedia seems to go a step further:

... there is an interaction between the solute and water that counteracts the pressure that otherwise free solute molecules would exert.

And a bit later:

The virial theorem demonstrates that attraction between the molecules (water and solute) reduces the pressure, and thus the pressure exerted by water molecules on each other in solution is less than in pure water, allowing pure water to "force" the solution until the pressure reaches equilibrium.

I think I understand what it means, but what I don't understand is this: osmotic pressure is said to be a colligative property, i.e. only affected by the concentration of the solute and not by its type. If the reason for osmotic pressure is the attraction between water and solute molecules - wouldn't we expect different osmotic pressures for different kinds of solutes?

Answer 1

According to this document (https://arxiv.org/pdf/physics/0305011v1.pdf)

"Water molecules can pass through the membranes in either direction, and they do. But because the concentration of water molecules is greater in the pure water than in the solution, there is a net flow from the pure water into the solution." Hydraulic equilibrium is achieved when the lower concentration of water in the high pressure side is balanced by the higher pressure and therefore higher energy pV on that side

This is the diffusion model of osmosis which is easy to understand at your level.

Answer 2

To first order the solute doesn't matter, though it probably has a small influence. The driving force for osmosis is entropic. The osmotic pressure then is not due to the forces between the solute and solvent, but mostly from the fact that you can mix the two together.

Indeed, osmosis works even when there's less interaction between the solvent and solute, but at some point, the solute is no longer soluble in solvent, so there is no mixing.

Answer 3

Two factors are involved. First: the Gibbs free energy or chemical potential of the molecules. This is a function of the temperature which results in their random velocities and the entropy of the molecules, their concentrations. With no physical constraints and ignoring solvation effects and chemical reactions diffusion occurs until the solution is homogeneous. Entropy is maximized at constant energy; the free energy is minimized.

Second: the semipermeable membrane. These can be natural or synthetic and work on two general principles: The membrane either has holes small enough to sort molecules or a chemical composition that is selectively soluble to specialized chemicals, with possibly a combination of both. An example is one's skin. Skin is hydrophobic with sweat glands to pass water and can be very permeable to small organic solvents.

Osmosis involves two solutions with different chemical potentials and a membrane selective to one or more components of the solutions and impermeable to others. [The selectivity can be very high but is not absolute there is an efficiency.] The solutions are separated by the membrane. The permeable molecules diffuse thru the membrane at rates determined by their chemical potentials until the chemical potentials are equal and the free energy change is zero. The equality can be equal concentrations or equal collision frequencies and can be influenced or hastened by gravity or a centrifuge or by temperature or electrical forces.

If the solute reacts minimally with the solvent, ie. a reasonably ideal solution, osmosis is dependent on the chemical potential, activity, of the permeable substance and reasonably independent of the nonpermeable substances.

Answer 4

I think you can find what you are looking for by googling "entropy" http://www.mpcfaculty.net/mark_bishop/solubility_entropy.htm

To summarize the article : "In general, particles of matter tend to become more dispersed (spread out)."

My guess is that it has something to do with the leonard jones force. Here is a video I made where the "text" is like a positive force and all the particle are a negative force. So the particles are slightly attracted to the text but when they get too close to the text, they get launched away (because I made the repulsive force to be huge).

- But in reality every single particle would have a positive and a negative so they all are attracted to each other but push away from each other once they get close enough. https://vimeo.com/182775330

Answer 5

As far I as I understood from my professor Osmosis is the flow of water from higher solute concertation to lower through a semipermeable membrane That solely because of increased collision between water and solute in the high solute concentration area Now the question arises why water and not the solute movement. Solute being less in size than water technically should move. But here comes thdy(thermodynamics) in play The let's say the solute here is salt. That when added in water dissociates into Na+ and Cl- now in order to nullify the charge and satisfy the entropy loss the water arranges it self around Na+ in hydration shells this hydration shell helps in dispersal of the +ive charge carried by the Na+ and similar hydration shells are made with Cl- to it's only the orientation of H and OH differs Na+ makes hydration shell with OH oriented outwards while Cl- with H of water oriented outward.

So the hydrated Na+ and hydrated Cl- are to large to pass through the semi permeable membrane and that's is why it's the water.. Hope it helps. Feel free the correct if sth is wrong