Influence of cationic gemini and conventional CTAB on the interaction of [Cr(III)-Gly-Tyr]2+ complex with ninhydrin

Abstract

The kinetics of reaction between ninhydrin and [Cr(III)-Gly-Tyr]2+ complex have been studied spectrophotometrically in aqueous and aqueous–micellar media of CTAB and geminis (16-s-16, s=4, 5, 6) at 353K and pH 5.0. The reaction follows first- and fractional-order kinetics with respect to [Cr(III)-Gly-Tyr]2+ and [ninhydrin], respectively, both in the absence and presence of micelles. The observed rate constant is affected by [CTAB] changes and the maximum rate enhancement is nearly 4-fold. The catalytic role of [CTAB] can be related to the extent of incorporation or association of the [Cr(III)-Gly-Tyr]2+ and ninhydrin into the micelles. Dicationic gemini surfactants provide much better environment for the reaction as compared to the corresponding monocationic counterpart CTAB. Also, whereas typical rate constant (kΨ) increase and leveling-off regions, just like CTAB, are observed with the geminis, the latter produce a third region of increasing kΨ at higher concentrations. The results obtained in micellar media are explained in terms of the pseudo-phase model of the micelles (proposed by Menger and Portnoy and developed by Bunton and Romsted). The kinetic data have been used to evaluate the micellar binding constants KS for [Cr(III)-Gly-Tyr]2+complex and KN for ninhydrin.