Interaction between copper(II) complex of glycylphenylalanine and ninhydrin in aqueous–micellar solutions of gemini surfactants

Abstract

In the present study, we have reported the interaction between copper(II) complex of glycylphenylalanine, [Cu(II)-Gly-Phe]+, and ninhydrin in aqueous–micellar solutions of gemini surfactants (16-s-16, s=4, 5, 6). The reaction showed first- and fractional-order dependencies, respectively, on [complex] and [ninhydrin]. The surfactants used in this reaction are a dicationic gemini and its catalytic behavior was also seen on the reaction rate. It is found that dicationic gemini surfactants are more effective than the respective monomeric surfactant (CTAB) with the same number of carbon atoms in the hydrophobic chain per hydrophilic head group. Moreover, while the typical rate constant (kΨ) increases and leveling-off regions, same as conventional monomeric surfactant CTAB (cetyltrimethylammonium bromide), are observed with geminis, the latter produced a third region of increasing kΨ at higher concentrations. The observed micellar catalysis is interpreted by the means of pseudo-phase model of the micelles (proposed by Menger and Portnoy and developed by Bunton). The values of thermodynamic parameters and binding constants were also evaluated by using the observed kinetic data.