Kinetics of complexation of metal ions by extractants solubilized in aqueous micellar solutions of cetyltrimethylammonium salts: Effect of the surfactant counterion

Abstract

Several experiments have demonstrated that the complexation of metal ions by lipophilic extractants solubilized in cationic micelles does occur, although the electrostatic repulsions between the positively charged surface of the micelles and the metal ions are not favourable for the reaction to proceed. Owing to the very high concentration of anion X − which exists close to the micelle surface one may wonder about the possible contribution in this process of species of the type [MX 4] 2−. This problem is addressed here by comparing the kinetics of complexation of four metal extractant couples in different CTA +X − micelles where X − = Br −, Cl − or NO − 3. The existence of anionic metal species is assumed to be less probable with nitrate than with chloride or bromide. The results definitely show a different behaviour in the presence of nitrate when the extractant is known to be strongly partitioned in the micellar pseudophase, but no counterion effect is detected when less hydrophobic complexing agents are considered. These observations further substantiate the distinction previously made between interfacial complex formation reaction and bulk reaction, this being a function of the extractant hydrophobicity.