# The partition coefficient for a chemical species across a membrane

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.

• 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 $D$. – Loong Jul 16 '15 at 9:50
• @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. – Freeman Jul 16 '15 at 9:53
• 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. – iad22agp Jul 16 '15 at 13:06
• 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? – Freeman Jul 16 '15 at 13:22