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The oncotic pressure or colloidal osmotic pressure is the osmotic pressure developed due to the presence of colloids in a solution. But since the colloids are not true solution, why should the colloids be termed as solutes soluble in the solvent and consequently capable of producing any osmotic pressure? Should they not behave like insoluble substances who do not affect the osmolarity of the solution? And theses colloidal osmotic pressure plays a very important role in trans-capillary transfer dynamics.

If the colloids do affect the osmotic properties, is the expression to measure the colloidal osmotic pressure same as that for true solutions(vant-hoff equation)?

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  • $\begingroup$ For a real world example of colloidal osmotic pressure, think of the albumin in human blood. This certainly exerts an osmotic pressure and among other things prevents most of the water in blood from entering the extracellular fluid compartment outside the circulation and causing oedema. $\endgroup$ – Tomcat Aug 10 '13 at 0:27
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I think, first I should clarify what causes the osmotic pressure: Osmosis occurs when two solutions of different concentrations are separated by a membrane which will selectively allow some species, e.g. the solvent, through it but not others, e.g. the solute. So, there is a concentration gradient between the two solutions which would lead to a diffusion from the side with the higher to the side with the lower concentration. But since the solute cannot pass through the membrane the diffusive flow cannot even the concentration differences out and thus constantly presses against the membrane exerting a force on it which leads to a pressure. The pressure originating from this tendency for osmotic flow to occur is called the osmotic pressure $\Pi$ of the solution.

So, the question is: Is the theory behind diffusion applicable to colloidal solutions? To reason about this, I will first address the matter of whether colloids can be considered "real" solutes or not. Colloidal particles are usually some polymer particle coated with a tenside layer. So, it is the molecules of those tenside layers that interact with the solvent molecules and they are chosen in such a way that the colloidal particles neither lump together (coalesce) nor sink to the ground or float to the top - that keeps the colloidal solution stable. So, a colloidal particle is surrounded by solvent molecules which solvate the tenside layer. Of course, the situation is a bit different from a "normal" solution since the colloidal particle is a lot bigger than, say, an alcohol molecule and thus the the ratio between solute particle size and the size of its solvation sphere is much smaller. But, in essence, a colloidal particle and an alcohol molecule are solvated in pretty much the same way. Furthermore it can also be assumed that interactions (e.g. electrostatic forces) between the colloidal particles are weak. This means that colloidal particles are thus not subject to external forces which influence their movement. They simply move through the solution by Brownian motion. This is important because then their motion can mathematically be described by a stochastic process which is the basis for the thermodynamic description of diffusion. And, as I described above, the reason for diffusion to happen is the same as the origin of the osmotic pressure.

So, to answer your last question: Yes, I think, the description for osmotic pressure caused by colloidal particles should be the same as for "normal" solutes.

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  • $\begingroup$ So, according to this reasoning, the process of surface film formation on the colloidal particles is absolutely required for them to exhibit osmotic pressure. That means, hydrophobic colloids and colloids forming a layer on top or bottom cannot be described by the same equations, right? $\endgroup$ – Satwik Pasani Aug 10 '13 at 14:10
  • $\begingroup$ @SatwikPasani It depends on the colloidal particle whether you need the tenside coating or not. The key feature they must posess is solubility in the solvent. Because only if they are in solution there can be a concentration gradient leading to the osmotic flow. If a colloid simply sediments, then their is no appreciable of it present in solution, thus you have no gradient. $\endgroup$ – Philipp Aug 10 '13 at 14:53

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