Studies on cobalt(III) metallosurfactants. Kinetics and mechanism of reduction of cobalt(III) by iron(II) in aqueous acid medium

The kinetics and mechanism of reduction of the surfactant complex ions, cis-chloro/bromo(dodecylamine)(triethylenetetramine) cobalt(III) by iron(II) in aqueous solution were studied at 303, 308 and 313 K by spectrophotometry under pseudo-first-order conditions using an excess of the reductant. The second-order rate constant increases with cobalt( III) concentration and the presence of aggregation of the complex itself alters the reaction rate. The reductions are acid-independent in the range [H+] 0.05-0.25 mol dm(-3). Variation of ionic strength (mu) influences the reaction rate. Activation and thermodynamic parameters have been computed. It is suggested that the reaction of Fe2+(aq) with the cobalt(III) complex proceeds by an inner-sphere mechanism. The critical micelle concentration (CMC) values of these surfactant metal complexes in aqueous solution were obtained from conductance measurements. Specific conductivity data (at 303, 308 and 313 K) served for the evaluation of the temperature-dependent CMC and the standard Gibbs energy of micellization (Delta G(m)(0)).