Studies on synthesis, determination of CMC values, kinetics and the mechanism of iron(II) reduction of surfactant–Co(III)–ethylenediamine complexes in aqueous acid medium

Abstract

The surfactant–Co(III) complexes of the type cis-[Co(en)₂AX]²⁺ (A = Tetradecylamine, X = Cl⁻, Br⁻) were synthesised from corresponding dihalogeno complexes by the ligand substitution method. The critical micelle concentration (CMC) values of these surfactant complexes in aqueous solution were obtained from conductance measurements. The kinetics and mechanism of iron(II) reduction of surfactant–Co(III) complexes, cis-[Co(en)₂(C₁₄H₂₉NH₂)Cl](ClO₄)₂ and cis-[Co(en)₂(C₁₄H₂₉NH₂)Br] (ClO₄)₂ ions were studied spectrophotometrically in an aqueous acid medium by following the disappearance of Co(III) using an excess of the reductant under pseudo-first-order conditions: [Fe(II)] = 0.25 mol dm⁻³, [H⁺] = 0.1 mol dm⁻³, [μ] = 1.0 mol dm⁻³ ionic strength in a nitrogen atmosphere at 303, 308 and 313 K. The reaction was found to be of second order and showed acid independence in the range [H⁺] = 0.05–0.25 mol dm⁻³. The second-order rate constant increased with surfactant–Co(III) concentration and the presence of aggregation of the complex itself altered the reaction rate. The effects of [Fe(II)], [H⁺] and [μ] on the rate were determined. Activation and thermodynamic parameters were computed. It is suggested that the reaction of [Fe(II)] with Co(III) complex proceeds by an inner-sphere mechanism.

Keywords:

• surfactant–cobalt(III)
• micelle
• reduction
• thermodynamic parameters
• kinetics