Solute transfer across liquid-liquid interfaces. Kinetic and thermodynamic evaluation

The transfer kinetics of methyl nicotinate (methyl 3-pyridinecarboxylate) across water-liquid hydrocarbon interfaces have been followed at various temperatures under different aqueous-phase conditions. A rotating diffusion cell, which enables accurate delineation of the interfacial area and close control of the hydrodynamics transporting solute to and from the phase boundary regions, was used in the study. Transfer rate constants for movement across three simple water-alkane interfaces suggested a free energy barrier to transport that was the direct result of a significant endothermic enthalpy contribution. The entropy associated with interfacial transfer, on the other hand, was favorably positive for both water → organic and organic → water processes. The basic experimental procedure was subsequently performed with a single hydrocarbon and aqueous phases perturbed by large concentrations of (i) urea and (ii) polyethylene glycol 400. In the presence of urea, the barrier to solute interfacial transfer, over the temperature range studied, was absent. Polyethylene glycol in the aqueous phase led to a decrease in the rate of phase transfer; the temperature dependence of the kinetics, however, indicated significant reduction of the favorable entropic change observed when simply water was used as the aqueous phase. It is suggested that these results can be understood in terms of the molecular organization, and alterations thereof, at the interfacial region.