KINETICS OF THE DISSOCIATION OF NICKEL-2-METHYL-8-HYDROXYQUINOLINE INWATER AND SURFACTANT MICELLES

The kinetics of the dissociation of the Ni complex (NiL(2)) of 2-methy l-8-hydroxyquinoline (HL) by H+ was studied in water and in micelles f ormed by the surfactants Triton X-100 and Brij 35 (neutral), sodium do decyl sulfate (SDS; anionic) and cetyltrimethylammonium nitrate (CTAN; cationic). The rate limiting step in all the media is the dissociatio n of the NiL(+) complex, which in micelles occurs predominantly at the aqueous-micellar interface. The dissociation rate constant shows litt le variation between H2O, Triton X-100, BriJ 35, and CTAN but is about 100 times larger in SDS. The activation parameters are identical in H 2O and in the neutral micelles, with the entropies of activation being positive in line with the proposed rate limiting step. These paramete rs are quite different in SDS and CTAN. The enthalpy of activation is 5 times smaller in SDS compared to H2O and neutral micelles with a neg ative entropy of activation. Surprisingly, the enthalpy of activation is also smaller in CTAN, being about 1.6 times smaller than the value in H2O with a much smaller entropy of activation as well. The activati on 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 H2O may stem from the st abilization of the activated complex through association with CTA(+). The offsetting contributions from the enthalpy and entropies of activa tion in CTAN lead to an apparent similarity in the dissociation rate c onstant in CTAN and aqueous media.