Kinetic complexation mechanisms of Ni(II) and Zn(II) with a pyridylazo-ligand at liquid-liquid interfaces

Kinetic mechanisms of interfacial complexation of nickel(II) or zinc(II) with 2-(5-bromo-2-pyridylazo)-5-diethylaminophenol (5-Br-PADAP or HL) in heptane/water and toluene/water systems have been investigated by means of a centrifugal liquid membrane (CLM) method. The complexes of Ni(II) and Zn(II) formed in heptane/water were not extracted into the heptane phase but completely adsorbed at the interface, though they were extracted in the toluene/water system. The initial complexation rates in the extraction systems were first-order in both ligand and metal(II) concentrations and increased with increase in pH. However, there was no pH dependence in the apparent complexation rate in the aqueous phase observed by the stopped-flow method. From the experimental results, the difference in the reaction mechanisms between the aqueous phase and the interface were elucidated. It was also found that the interfacial complexation rate constant in the heptane system was higher than that in the toluene system.