Mechanism of Ni2+ and NiOH+ interaction with hydroxamic acids in SDS: evaluation of the contributions to the equilibrium and rate parameters in the aqueous and micellar phase

Abstract

The equilibria and kinetics (stopped-flow) of the binding of Ni(II) to salicylhydroxamic acid (SHA) and phenylbenzohydroxamic acid (PBHA) have been investigated in aqueous solutions containing SDS micelles. The two ligands are fairly distributed between the two pseudophases present, so the binding reaction occurs in both phases. The contributions to the total reaction from each phase has been evaluated, following a procedure where use is made of the experimentally determined partition coefficients of the reactants involved. The mechanism of the reaction occurring on the micelle surface has been derived and comparison with the mechanism in water shows that the step Ni²⁺ + HL ⇄ NiHL²⁺ is operative in both pseudophases, whereas the step Ni²⁺ + L⁻ ⇄ NiL⁺, which is operative in water, is replaced in SDS by the step NiOH⁺ + HL ⇄ NiL⁺. The analysis of the equilibrium and of the kinetic data enabled the evaluation of the equilibrium and the rate constants of the individual steps taking part in the binding process over the micelle surface. Interestingly, the first hydrolysis constant of the Ni(H₂O)₆²⁺ ion in SDS is more than two orders of magnitude higher than in water. The agreement between the equilibrium constants derived from kinetics and those obtained by static measurements confirms the validity of the proposed mechanism.