Micellar inhibition of the aquation of tris-(3,4,7,8-tetramethyl-1,10-phenanthroline)iron(II) by sodium dodecyl sulphate in aqueous acid medium

Abstract

The inhibition of the aquation of Fe(Me₄phen)²⁺₃ by sodium dodecyl sulphate (SDS) in aqueous acid media has been investigated and a mechanism which explains the pronounced inhibition and pre-micellar activity at low [SDS]_T has been proposed. Inhibition is due to favourable thermodynamic-/hydrophobic/electrostatic binding between the Fe^II complex and SDS monomer aggregates. The bound Fe^II complex is stabilised with respect to dissociation and binding takes place between the ridges of the Stern layer. The partitioning of the substrate between the bulk-water phase and the micellar phase is in favour of the latter at low [H⁺]_T and low [SDS]_T. From the rate law obtained and the observed kinetic data, the micelle–complex binding constant (K₁) and micelle–acid binding constant (K₃) were calculated to be (2.81 ± 0.08)× 10⁵ and (13.80 ± 0.16) dm³ mol^–1, respectively, in acid media. Using the Scatchard method, K₁ values of (3.95 ± 0.08)× 10⁵ and (3.04 ± 0.16)× 10⁵ dm³ mol^–1 were calculated for the binding in neutral medium (distilled water) and 2 × 10^–5 mol dm^–3 H⁺, respectively. The decrease in K₁ in acid media is attributed to competition between H⁺ and the complex ion for the binding sites on the micelle. The k_ψ–[SDS]_T profiles are structured owing to the evolution (size, geometry, aggregation number, etc.) of the micelle. The inhibition of the aquation rate by HSO^–₄ and SO^2–₄ ions which form a negative field around the Fe^II complex is only significant at high acid concentrations. The micelle-bound complex and micelle-bound protons have opposing effects on the aquation rate. The degree of inhibition is therefore sensitive to the ratio of the concentration of these bound species.