Interaction of ninhydrin with zinc(II) complex of tryptophan in the three dicationic gemini surfactants

Present work concerns with the interaction of ninhydrin with zinc(II) complex of tryptophan ([Zn(II)-Trp]⁺) in the three dicationic gemini surfactant systems. To record critical micellar concentration (CMC) and absorbance, we have used Systronics conductivity meter and UV-visible spectrophotometer, respectively. Experiment opens up the fractional- and first-order paths in ninhydrin and complex, respectively. Gemini micellar medium is found more superior over aqueous medium. Rate constant (k_ψ) vs. [gemini] plot shows the unusual role of geminis on k_ψ. k_ψ increases with gemini concentration (at concentrations lower than the CMC, part I) and leveling-off regions obtain (concentration up to 400 × 10⁻⁵ mol dm⁻³, part II). Characteristics of part I and part II are just the same as that of conventional surfactant. Later, gemini produces a third region of increasing k_ψ at higher concentrations ([gemini] > 400 × 10⁻⁵ mol dm⁻³, part III). Detail and systematic elucidation about the effect of surfactants are mentioned and discussed in the text. Binding constants (K_S for [Zn(II)-Trp]⁺ and K_N for ninhydrin) and rate constant (k_m in geminis) were determined by nonlinear least squares regression technique. The kinetic results acquired can reasonably be interpreted by pseudo-phase model of surfactant micelles.