What exactly causes osmotic pressure?

[scilearner]

Hello everyone,

If there is a semi permeable membrane that allows only water to pass through. Also this membrane separates the sides into A and B.

There are big molecules like colloids in side A along with water. Side B has only water . What mechanism causes side A to have more osmotic pressure, and thus allow water to flow from side B to A.

1) Is it because the particles are so big , that they physically reduce the interactions of water molecules in side A with the semi permeable membrane.

2) Is it because the colloid create a chemical interaction with water molecules and prevent them from moving away.

I just want to know if this osmotic pressure created by a pure physical block (like big guy covering small guy) or an actual chemical interaction (Eg big guy holding the small guy)

3) Is osmotic and oncotic pressure mean same thing.

Thanks a lot

[swansont]

Having dissolved a solute represents a difference in energy, meaning there is a potential difference between the two systems. A gradient in a potential is a force, or over an area, a pressure. This is why systems tend to equilibrate; a force of some sort will be present when not in equilibrium.

[studiot]

Like many physical phenomena, osmotic pressure has its origins in humble simple processes.

 

To understand these let us build up one step at a time.

 

Start with a container of liquid solvent. No membrane at this stage.

 

Now add some solute or disperse material to one side only of the container. Coloured ink or Sugar is good.

 

What happens?

 

Well the solute spreads out throughout the whole of the container and pretty soon the concentration is even throught.

 

How come?

 

Well think about the initial stage where there are two regions.

 

A zone of high concentration where you add the solute (left hand side say) and a low of low concentration at the other side where you didn't (right hand side)

 

This means that the % of solute particles on the LHS is greater than the % on the RHS.

 

Since there is no reason to suppose otherwise we assume the particles move equally in all directions.

 

But there are more on the LHS than the RHS

 

So more are travelling from left to right than right to left.

 

This continues until there are the same on both sides when the concentration is even.

 

This applies in reverse to the solvent particles.

 

What we have shown is this evening out of concentration is a powerful tendency in Nature.

 

Now introduce a semi permeable membrane, like a fishing net, across the middle of the container an repeat the experiment.

 

Semi permeable means that the holes in the net will only let the small fish through, but stop the big ones.

Usually the solvent particles are small compared to the solute ones, so it is the solute particles that are held back.

 

So the solute cannot spread out, but the system still wants to even out the concentration.

So more solvent particles initially pass from right to left than from left to right as before, which reduces the concentration on the LHS.

This is the same as if there was a pressure pushing solvent this way, which is why we call it osmotic pressure.

This will raise the level of the surface on the LHS relative to the RHS, or increase the pressure within the LHS v the RHS if the container is sealed.

 

Eventually the atmosphere and gravity push back so the net solvent flow from right to left ceases.

This happens when the pressures balance.

 

Yes I believe oncotic pressure is an example of this phenomenon with a sealed container (the tumour or whatever)

 

does this help?

[Enthalpy]

It's because the water molecules replaced by the colloid don't meet the membrane, hence the water flow from that side is smaller. It can be compensated by a pressure difference.

 

Though, some solute have a strong effect on water molecules, and then the quantitative result differs.

[scilearner]

@Studiot: Thanks for taking your time for that lengthy response. I appreciate it Yes this is also how I understand osmotic pressure, but my question is in addition is there a chemical bonding occuring here.

 

@Enthalpy: Thanks for the response I think you understood my question. So there are no interactions eg polar bonds between colloid and water molecules, which make it harder for water molecules to pass through membrane, because their structure is changed now because they are bound to colloids. So their is no chemistry involved here just physics

 

@Swansont: Thanks for the response But I don't understand your technical terms. The way I understand is when ever one side is more concentrated than the other, concentrated side would naturally seek empty space and even everything out.

Edited January 30, 2014 by scilearner