Catalytic role of gemini surfactant micelles in the ninhydrin-L-isoleucine reaction

Abstract

Effect of dicationic gemini surfactants C16H33(CH3)2N+-(CH2)s-N+(CH3)2C16H33, 2Br− (where s = 4, 5, 6) on the reaction of ninhydrin with L-isoleucine has been investigated spectrophotometrically as a function of [gemini], [L-isoleucine], [ninhydrin], and pH. The reaction follows first- and fractional-order kinetics, respectively, in [L-isoleucine] and [ninhydrin]. The gemini surfactant micellar media are found more effective for the reaction than their conventional monomeric counterpart CTAB. Furthermore, whereas typical rate constant (kψ) increase and leveling-off regions are observed with CTAB and geminis, the latter produce a third region of increasing kψ at concentrations ≥ 60 cmc’s. 1H NMR studies reveal that this unusual third-region effect of the geminis is due to changes in their micellar morphologies. Quantitative kinetic analysis has been performed on the basis of modified pseudo-phase model.