Kinetics of the Dissociation of Nickel-2-Methyl-8-hydroxyquinoline in Water and Surfactant Micelles

Abstract

The kinetics of the dissociation of the Ni complex (NiL 2 ) of 2-methyl-8-hydroxyquinoline (HL) by H + was studied in water and in micelles formed by the surfactants Triton X-100 and Brij 35 (neutral), sodium dodecyl sulfate (SDS ; anionic) and cetyltrimethylammonium nitrate (CTAN ; cationic). The rate limiting step in all the media is the dissociation of the NiL + complex, which in micelles occurs predominantly at the aqueous-micellar interface. The dissociation rate constant shows little variation between H 2 O, Triton X-100, Brij 35, and CTAN but is about 100 times larger in SDS. The activation parameters are identical in H 2 O and in the neutral micelles, with the entropies of activation being positive in line with the proposed rate limiting step. These parameters are quite different in SDS and CTAN. The enthalpy of activation is 5 times smaller in SDS compared to H 2 O and neutral micelles with a negative entropy of activation. Surprisingly, the enthalpy of activation is also smaller in CTAN, being about 1.6 times smaller than the value in H 2 O with a much smaller entropy of activation as well. The activation parameters in SDS strongly suggest the adsorption of NiL + on the negatively charged micellar surface. The smaller enthalpy and entropy of activation in the case of CTAN compared to H 2 O may stem from the stabilization of the activated complex through association with CTA + . The offsetting contributions from the enthalpy and entropies of activation in CTAN lead to an apparent similarity in the dissociation rate constant in CTAN and aqueous media.