Partitioning and the Effect of Aggregation

When two immiscible solvents are placed in contact with each other and a non-ionizable compound is dissolved in one of the solvents, the compound distributes itself between the two solvents. This distribution is referred to as partitioning. The ratio of the concentrations of the compound in each phase is a constant for a specific set of solvents, pH, buffers, buffer concentrations, ionic strength and temperature. This ratio is referred to as a partition coefficient or distribution coefficient and is equal to the ratio of the solubilities in the two solvents. When the compound is a weak acid or base, the distribution of the compound can be shown to be given by the following equation for a monoprotic compound  

Where ko, is the distribution coefficient kg and kj are the intrinsic partition coefficient for species 0 and 1 where species 0 is the most protonated. 

If aggregation occurs in one of the phases. Equation (4.32) needs to be modified. Let the aggregation be represented by  

Where nis the number of monomers in the aggregate Ci is the concentration of the monomer is the concentration of the aggregate. 

For the condition in which there is only one primary monomeric species present in the aqueous phase and in which the aggregation occurs in the organic phase the following equation can be derived  

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