Reaction Kinetics in Aqueous Micellar Systems: The Mechanism of the Reaction Between Ni2+(Aq) and Neutral Bidentate Ligands of Differing Hydrophobicities in the Presence of Sodium Dodecyl-Sulphate Micelles
The kinetics of the reaction between Ni2+(aq) and the ligands 2,2′-bipyridyl and 4,4′-dimethyl-2,2′-bipyridyl have been investigated in micellar SDS solutions. A maximum rate enhancement is found close to the critical micelle concentration. Since the ligand partitions significantly between the micellar pseudo-phase and the aqueous region, the increase in reaction rate observed is related to the hydrophobicity of the ligand. A quantitative expression for the variation in rate with micelle concentration is derived, which includes terms allowing for partitioning of both Ni2+ and the ligand between the micellar and the aqueous phases.
A detailed analysis of the results suggests that when the ligands are associated with the micelle they are entirely available for reaction at the micelle surface. The kinetic results also indicate that the ligand is not involved in the rate-limiting step for reaction on the micelle surface. The rate-limiting step is, in fact, associated with release of a water molecule from the solvation shell of the metal ion prior to complexation with the ligand. The rate constant for this process is similar to that measured in aqueous solution in the absence of micelles.
The number of papers dealing with the kinetics of chemical reactions in the presence of micelles has greatly increased since 1960. For most reactions, only modest rate changes are observed in the presence of micelles but for some reactions rates are increased by factors approaching 106. Results reported in the literature up to 1974 have been extensively reviewed (1).
Until recently, a sound theoretical basis for the mechanistic interpretation of these kinetic measurements has been lacking. It is now recognised that several distinct factors can influence reactivity in micellar solutions. These include (i) partitioning coefficients for the reactants between the micelle and aqueous solution, (ii) kinetic effects due to a change in the environment in which the reaction takes place (either through a medium effect or direct involvement of the surfactant head-groups in the reaction) and (iii) the influence of the micelle surface potential on the pKa’s etc. of the reactants. These factors have recently been incorporated into a general kinetic theory by Berezin et al, (2).
Where does the reaction predominantly occur?
How rapidly do the reactants (which are an ion and a molecule in our experiments) diffuse on/in micelles?
How rapidly do the reactants exchange between micelles?
Are the reactants in the correct configuration for reaction when associated with the micelle?
Can any change in rate which is observed be explained solely in terms of the concentrative effect of the micelles on the reactants (i.e. in terms of partitioning coefficient effects) or is there some additional specific influence of the micellar environment on the kinetics?
The main aim of the present study is, therefore, to investigate the effect of varying the hydrophobicity of the ligand on the kinetics and mechanism of the reaction.
KeywordsCritical Micelle Concentration Micelle Surface Sound Theoretical Basis Correct Configuration Micellar Environment
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