Catalytic Behavior of a Series of Cationic Gemini (16-s-16 Type, s = 4, 5, 6) and CTAB Surfactants on the Reaction of Ninhydrin with [Ni(II)–Gly-Phe

Abstract

The influence of cationic CTAB/gemini surfactants on the reaction of ninhydrin with the nickel–glycylphenylalanine ([Ni(II)–Gly-Phe]+) complex was investigated spectrophotometrically, at 70 °C and pH = 5.0. The kinetics were found to follow a pseudo-first- and fractional-order reaction path with respect to concentrations of [Ni(II)–Gly-Phe]+ complex and ninhydrin, respectively. The values of rate constant (k ψ) increase with increase in CTAB concentration and reach a maximum value, and then, with further increase in the CTAB concentration, a slow decreasing effect is observed. In the case of geminis, k ψ increase and leveling-off regions (just like the monocationic CTAB) are obtained with [geminis], in addition to a third region of increasing k ψ at higher concentrations. On the basis of the observed data a probable mechanism has been proposed.