Kinetics involving divalent metal ions and ligands in surfactant self-assembly systems: Applications to metal-ion extraction

This paper describes kinetic studies of metal complexation in the presence of micelles and vesicles of different charge type. The results are interpreted in terms of the effects of the surfactant self-assembly systems on the extraction of metal ions from an aqueous medium. It is found, in the case of anionic micelles, that the extracting ligand is preferentially located close to the surface of the surfactant aggregate, where it is held by hydrophobic interactions. In this location, it is accessible to the metal ion and so is readily complexed; there is no apparent tendency for the ligand to hide inside the micelle. The same situation is found for vesicles that are negatively charged to a similar surface potential. In contrast, when positively charged surfactants are used to form micelles, the metal ion is strongly repelled from the like-charged surface into the aqueous medium. Motion across a vesicle bilayer is found to be slow; furthermore, in our systems it was difficult to maintain a pH gradient for the times that are needed for the operation of an effective extraction procedure. The kinetic and thermodynamic behavior of ligands inside vesicles was further investigated for the dye pyridine-2-azo-p-dimethylaniline (used as the ligand in our model extraction studies), and some surprising results were obtained. Below the melting temperature of vesicles composed of the long-chain cationic surfactant dioctadecyldimethylammonium bromide; the dye is released from the vesicle into the aqueous solution. However, this is not always the case. The fluorescent dye probe 8-anilino-naphthalene sulfonate behaves very differently and shows complex kinetic behavior for insertion into a range of vesicles both above and below the melting temperature. The results demonstrate the importance in extraction of surface-charge effects and a possible control role for the bilayer melting transition, in the specific case of vesicular systems.