Prediction of interfacial transfer kinetics

Abstract

Interfacial transfer kinetics were determined for a number of homologous series comprising non-ionised model solutes across a variety of aqueous: organic interfaces in the two-phase transfer cell. Experimentally determined transport rate constants were compared to those estimated from a theoretical equation. The importance of aqueous and organic diffusional resistances to solute transfer was examined. A method was described to calculate a theoretical solute and solvent dependent ratio that enabled the estimation of the dominant diffusional resistance to which a given solute was subject during interfacial transfer in a particular solvent system. Variation of solute and solvent systems allowed the predictive theories to be tested under conditions where aqueous, organic and mixed diffusional control predominated. Successful prediction of the non-ionised transfer kinetics of homologues in a series was possible from a knowledge of the partition coefficient and transfer kinetics of the parent compound, the partition coefficient of the homologue and some easily determined system-dependent variables. Octanol: aqueous interfacial transfer kinetics of a variety of model solutes were investigated as a function of aqueous phase pH and ionic strength in the two-phase transfer cell. Transfer rate constants varied unpredictably with pH and fell significantly with increasing aqueous phase ionic strength. Results could not be explained by ionic strength induced variations in partition coefficient, pKₐ or kinematic viscosity of the phases. When the same solutes were studied in the rotating diffusion cell, transfer rate constants fell with increasing pH in agreement with theoretical predictions. In the rotating diffusion cell, which has a mechanically stabilised interface, aqueous diffusivities were unaffected by pH or ionic strength. Interfacial resistance to transfer was negligible for each solute under study. Interfacial instability, which was affected by the presence of ions in the aqueous phase, appeared to be the reason for the unpredictable results in the two-phase transfer cell.