Correlation and prediction of mass transport across membranes I: Influence of alkyl chain length on flux‐determining properties of barrier and diffusant

The effect of alkyl chain length on the physicochemical properties of a series of p‐aminobenzoate esters, determinant of the rate at which they penetrate inert barriers, was investigated. Relationships are drawn for the influence of chain length on partition coefficient and solubility; and these, in conjunction with a diffusion layer membrane model, have been incorporated into a general, theoretical expression for maximum steady‐state flux as a function of homolog size. This expression predicts a parabolic dependency as the homologous series is ascended for certain reasonably normal conditions. This model was tested and found valid for an uninterrupted ester series containing from one to seven carbons, utilizing dimethylpolysiloxane membranes. An analysis of the effect of each parameter in the equation was performed with computer assistance. This analysis shows the extent to which the optimum chain length for absorption (barrier penetration) is dependent on controllable and uncontrollable experimental factors. The broad applications of the diffusion layer membrane model with superimposed solubility constraints for the interpretation of membrane transport phenomena, including bioavailability, are detailed.