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]+

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 A degrees 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 (psi)) 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 (psi) increase and leveling-off regions (just like the monocationic CTAB) are obtained with [geminis], in addition to a third region of increasing k (psi) at higher concentrations. On the basis of the observed data a probable mechanism has been proposed.