Ligand-exchange kinetics in ternary and quaternary surfactant systems

Abstract

Results are presented for the rates of a metal–ligand substitution reaction [that between Ni²⁺(aq) and the hydrophobic azo dye ligand PADA] in the presence of sodium dodecyl sulphate micelles and various alcohols of different chain lengths. The micelle-accelerated reaction rate is decreased on addition of alcohols; a more pronounced rate decrease is observed for the longer-chain alcohols. The results can be quantitatively interpreted in terms of a pseudo-phase model. The alcohols are found to swell the micelle/water interface region where the ligand substitution reaction occurs.

A four-component microemulsion system consisting of sodium dodecyl sulphate, pentan-1-ol, dodecane and water was also investigated using the probe reaction. A laser flash-photolysis technique was used. Reaction rates were relatively insensitive to the microemulsion composition and were close to those found in bulk water. The results can be qualitatively interpreted in terms of partitioning between the pseudo-phase volumes of the four-component system. The results demonstrate that this type of surfactant solution is unlikely to provide the large rate enhancements or retardations that are observed in normal micellar solutions.