What factors influence the partition factors across a membrane for a chemical species? Wikipedia defines the partition coefficient as:

In the physical sciences, a partition-coefficient (P) or distribution-coefficient (D) is the ratio of concentrations of a compound in a mixture of two immiscible phases at equilibrium.

If we let these "two immiscible phases" be either side of the membrane, In this situation the equilibrium concentrations would be determined by the solution either side of the membrane? If there is water either side of the membrane would they have a P of 1?

I'm not a chemist (or biologist for that matter) so apologies for any misuse of terminology or ambiguity.

  • $\begingroup$ Please note that the term partition coefficient is not recommended and should not be used as a synonym for partition constant $K_\mathrm{D}^ \circ$, partition ratio $K_\mathrm{D}$, or distribution ratio $D$. $\endgroup$
    – user7951
    Commented Jul 16, 2015 at 9:50
  • $\begingroup$ @Loong Ok. For clarity I am referencing pharmpress.com/files/docs/MolBiopharmaceutics_sample.pdf page 6 (140 numbered), eq 3.2.6, partition coefficient defined as alpha. $\endgroup$
    – Freeman
    Commented Jul 16, 2015 at 9:53
  • $\begingroup$ For partitioning across a membrane as you have defined it, you need a system of a solute, two solvents, and a membrane where the solvents do not pass through the membrane and the solute does. $\endgroup$
    – iad22agp
    Commented Jul 16, 2015 at 13:06
  • $\begingroup$ Ok, so either side of the membrane there exists 98% water, so we shall say both solvents are water, then my solute is a large molecule which can pass through the barrier? As it is defined in 3.2.6 in the attached book chapter how would this relate to what you describe? $\endgroup$
    – Freeman
    Commented Jul 16, 2015 at 13:22

1 Answer 1


Partitioning across a membrane is better known as Osmotic pressure. Think of a tube with a "bag" made out of the membrane attached to it. The more concentrated solution goes inside the bag, the less concentrated solution suspends the bag. The liquid from the suspending liquid then diffuses into the bag causes the fluid level to rise against gravity. The height of the rise yields the Osmotic pressure. So it is the solvent that diffuses not the solute.

If the solute diffuses through the membrane then it is just a diffusion problem such that the rate of diffusion is related to pore size. Bigger pores give faster diffusion.

To a chemist a "partition coefficient" would also really seem to apply best to a system at equilibrium. If you're considering drugs in the body then an equilibrium probably isn't obtained. The blood sweeps the drug away to react somewhere else in the body. So the drug is continually transported into the blood.

The other side of this is something like urea in urine and blood. The kidneys actually concentrate urea from the blood plasma into the urine. However this biological process consumes energy. So this sort of process is different than a simple equilibrium.


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