A partition coefficient is the ratio of concentrations of a compound in a mixture of two immiscible solvents at equilibrium.
If a solute is added to a mixture of two immiscible liquids, it will distribute between the two phases and reach an equilibrium at a constant temperature. The distribution of the solute (unaggregated and undissociated) between the two immiscible layers can be described thus:
K = Cu/Cl
Where,
- K is the distribution constant or partition constant,
- Cu is the concentration of the drug in the upper phase, and
- Cl is the concentration of the drug in the lower phase.
The octanol–water partition coefficient is commonly used in formulation development. Following the illustrations provided earlier, it is defined as –
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