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 interpre ted 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 o f anionic micelles, that the extracting ligand is preferentially located cl ose to the surface of the surfactant aggregate, where it is held by hydroph obic 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 hi de inside the micelle. The same situation is found for vesicles that are ne gatively charged to a similar surface potential. In contrast, when positive ly charged surfactants are used to form micelles, the metal ion is strongly repelled from the like-charged surface into the aqueous medium. Motion acr oss a vesicle bilayer is found to be slow; furthermore, in our systems it w as difficult to maintain a pH gradient for the times that are needed for th e operation of an effective extraction procedure. The kinetic and thermodyn amic behavior of ligands inside vesicles was further investigated for the d ye pyridine-2-azo-p-dimethylaniline (used as the ligand in our model extrac tion studies), and some surprising results were obtained. Below the melting temperature of vesicles composed of the long-chain cationic surfactant dio ctadecyldimethylammonium bromide; the dye is released from the vesicle into the aqueous solution. However, this is not always the case. The fluorescen t dye probe 8-anilino-naphthalene sulfonate behaves very differently and sh ows complex kinetic behavior for insertion into a range of vesicles both ab ove and below the melting temperature. The results demonstrate the importan ce in extraction of surface-charge effects and a possible control role for the bilayer melting transition, in the specific case of vesicular systems.