Rate Enhancement of Nickel(II)-pada Complex-Formation in Mixed Sodium Perfluorooctanoate/Octanesulfonate Micellar Solutions

Abstract

Kinetic studies of the Ni_aq²⁺ ion complexation with the bidentate ligand pyridine-2-azo-p-dimethylaniline ( pada ) were conducted by the stopped-flow technique in the presence of mixed micelles of sodium perfluorooctanoate ( spfo ) and sodium octanesulfonate ( sos ). Binding constants for Ni_aq²⁺ and pada partitioning between bulk solution and micelles were determined through solubility measurements and the murexide techniques respectively. The molar reaction volume for the surfactant was obtained through the fitting of the kinetic data and partitioning constants to the Robinson equation. It was found that spfo micelles promoted the complexation reaction 400-fold just beyond the critical micelle concentration but this maximum value decreased with sos , NaCl or further spfo addition. The unexpected effectiveness of spfo in rate enhancement is due to its compact micellar reaction volume of 0.16 dm³ mol^-1. The kinetic results are consistent with spfo and sos forming separate micelles of pure or almost pure composition.