5.5.9 Partition Coefficients

• The partition coefficient (K_pc) is the ratio of the concentrations of a solute in two different immiscible solvents in contact with each other when equilibrium has been established (at a particular temperature)
• For example, methylamine (CH₃NH₂) is dissolved in two immiscible solvents:
  • Water
  • An organic solvent

• A separating funnel is shaken with the organic solvent and aqueous methylamine
• The methylamine is soluble in both solvents, so when the mixture is left to settle an equilibrium is established
  • The rate of methylamine molecules moving from the organic layer into the aqueous layer is equal to the rate of molecules moving from the aqueous layer to the organic layer

CH₃NH₂(aq) ⇌ CH₃NH₂(organic solvent)

• The value of its equilibrium constant is also called the partition coefficient

K_pc = 

The partition coefficient is the ratio of methylamine molecules in the organic and aqueous layer once equilibrium has been established

• The partition coefficient (K_pc) for a system in which the solute is in the same physical state in the two solvents can be calculated using the equilibrium expression

• The partition coefficient (K_pc) depends on the solubilities of the solute in the two solvents
• The degree of solubility of a solute is determined by how strong the intermolecular bonds between solute and solvent  are
• The strength of these intermolecular bonds, in turn, depends on the polarity of the solute and solvent molecules
• For example, ammonia is more soluble in water than in an organic solvent such as carbon tetrachloride (CCl₄)
  • Ammonia and water are both polar molecules that form hydrogen bonds with each other
  • Ammonia forms permanent dipole-induced dipole forces with the non-polar CCl₄ molecules
  • Since these forces are much weaker than hydrogen bonding, ammonia is less soluble in CCl₄

• When K_pc is < 1 the solute is more soluble in water than the organic solvent
• When K_pc is > 1 the solute is more soluble in the organic solvent than the water