Micelle-catalyzed reaction between ninhydrin and nickel dipeptide complex [Ni(II)-Gly-Tyr]+

The interaction of nickel dipeptide complex [Ni(II)-Gly-Tyr](+) with ninhydrin has been investigated in the absence and presence of cationic cetyltrimethylammonium bromide (CTAB) and gemini (16-s-16, s = 4, 5.6) surfactants spectrophotometrically at 80 degrees C and pH 5.0. The product formed was the same and the reaction followed first- and fractional-order kinetics with respect to [Ni(II)-Gly-Tyr](+) and [ninhydrin], respectively, in both aqueous as well as micellar media. In the presence of CTAB, rate increased and reached up to a maximum, then decreased. Also, whereas typical rate constant (k(Psi)) increase and leveling-off regions, just like CTAB, were observed with geminis, the latter produced a third region of increasing k(Psi), at higher concentrations. This unusual third-region effect of the gemini micelles is assigned to changes in their micellar morphologies. The micellar catalysis is explained in terms of pseudo-phase model. The binding constants and the values of activation parameters such as activation energy (E-a), enthalpy of activation (Delta H-#) and entropy of activation (Delta S-#) have been evaluated. (C) 2012 Elsevier B.V. All rights reserved.