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

Abstract

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°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 (kΨ) increase and leveling-off regions, just like CTAB, were observed with geminis, the latter produced a third region of increasing kΨ 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 (Ea), enthalpy of activation (ΔH#) and entropy of activation (ΔS#) have been evaluated.