EXTRACTION COEFFICIENTS AND FACILITATED TRANSPORT - THE EFFECT OF ABSORPTION ENHANCERS

A theoretical model of the extraction coefficients of weak acids has been developed to account for the variation in partition dependent upon pK(a) using the partition coefficients of the ionised and unionised species (P(i), P(u)). The effect of these variables on the plots of both the extraction ratios and the relative changes, as indicated by the ratio of these coefficients, is illustrated. The model has been applied to the distribution of salicylic acid, as a function of pH, between an aqueous phase and isopropyl myristate containing tertiary amide enhancers of percutaneous absorption. The extraction coefficients determined are consistent with the model and show that ion-pairing between the ion and enhancer need not be invoked to explain the behaviour of the system.