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

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 (k _ψ) increase and leveling-off regions are observed with CTAB and geminis, the latter produce a third region of increasing k _ψ 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.