Colloids and Surfaces A: Physicochem. Eng. Aspects 243 (2004) 127–132 Kinetics and the effect of electrostatic surface potential on nickel(II) extraction by 2-hydroxy-5-nonylacetophenone oxime (LIX 84) in a micellar phase Dallas B. Warren a,∗,1 , Franz Grieser b , Jilska M. Perera a , Geoff W. Stevens a a Particulate Fluids Processing Centre, Department of Chemical and Biomolecular Engineering, The University of Melbourne, Melbourne, Vic. 3010, Australia b Particulate Fluids Processing Centre, School of Chemistry, The University of Melbourne, Melbourne, Vic. 3010, Australia Received 3 February 2004; accepted 14 May 2004 Available online 28 July 2004 Abstract The kinetics of extraction and the effect of electrostatic surface potential on the nickel(II)/2-hydroxy-5-nonylacetophenone oxime (HNAPO) extraction system were characterised in a non-ionic micellar system. A two step reaction mechanism between nickel(II) and HNAPO was found to satisfactorily explain the observed kinetics. Simulation of the kinetics of the complexation reaction over a wide range of concentrations provided the rate constants for the proposed mechanism. The addition of a cationic surfactant, dodecyl trimethyl ammonium chloride (DTAC), slowed the reaction kinetics, while the inclusion of an anionic surfactant, sodium dodecyl sulphate (SDS), enhanced the reaction kinetics relative to the non-ionic micellar system. The charge effects on the reaction rate could be successfully modelled based on the electrostatic surface potential generated by the anionic and cationic surfactants. © 2004 Elsevier B.V. All rights reserved. Keywords: Micellar phase; Reaction kinetics; Electrostatic surface potential; Solvent extraction 1. Introduction The process of solvent extraction involves mass transfer and chemical reactions in a heterogeneous system. Species to be extracted have to move from the bulk phase to near the phase boundary or interface, react with a different species (soluble in the second phase) to form a complex that is sol- uble in the second phase, and then move into the second bulk phase. Therefore, the interfacial zone plays an impor- tant role in the overall process. Many of the extractants used industrially are interfacially active. Consequently, any alter- ation to the physicochemical properties of the interface can have an effect on the extraction process. In general, such al- terations to the interface affect the kinetics of the extraction 1 Present address: Department of Pharmaceutical Biology and Pharma- cology, Victorian College of Pharmacy, Monash University, Parkville, Vic. 3052, Australia. ∗ Corresponding author. Tel.: +61 3 9903 9083; fax: +61 3 9903 9638. E-mail address: dallas.warren@vcp.monash.edu.au (D.B. Warren). process only, as a consequence of the interface being an in- termediate location for reaction species and not involved in the final system equilibrium. The complexing agent 2-hydroxy-5-nonylacetophenone oxime (HNAPO) is commonly used to extract nickel(II) [1] and is the active ingredient of the industrial extractant LIX 84i. It can also extract copper(II) [2], cobalt(II) [3], palla- dium(II) [4] and zinc(II) [5] under the appropriate solution conditions. Previous studies have shown that the complex- ation of nickel(II) with HNAPO is an interfacial reaction [6–9] and is therefore suitable for observing the effect of in- terfacial physicochemical properties on extraction reaction rates. The aim of this study was to study the effects of the elec- trostatic surface potential on metal ion extraction kinetics in a micellar system. This was achieved by examining the extraction of nickel(II) by 2-hydroxy-5-nonylacetophenone oxime (HNAPO) in non-ionic and charged micellar sys- tems. The equilibrium properties of this system have been previously reported [10]. Non-ionic micellar phases have 0927-7757/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2004.05.011