Effect of Cationic Micelles on the Kinetics of Interaction of [Cr(III)-Gly-Gly]2+ with Ninhydrin

The effect of cationic micelles of cetyltrimethylammonium bromide (CTAB) on the interaction of chromium dipeptide complex ([Cr(III)-Gly-Gly]2+) with ninhydrin under varying conditions has been investigated. The rates of the reaction were determined in both water and surfactant micelles in the absence and presence of various organic and inorganic salts at 70 °C and pH 5.0. The reaction followed first- and fractional-order kinetics with respect to [Cr(III)-Gly-Gly2+] and [ninhydrin]. Increase in the total concentration of CTAB from 0 to 40×10−3 mol·dm−3 resulted in an increase in the pseudo-first-order rate constant (kψ) by a factor of ca 3. Quantitative kinetic analysis of kψ−[CTAB] data was performed on the basis of the pseudo-phase model of the micelles. As added salts induce structural changes in micellar systems that may modify the substrate-surfactant interactions, the effect of some inorganic (NaBr, NaCl, Na2 SO4) and organic (NaBenz, NaSal, NaTos) salts on the rate was also explored. It was found that the tightly bound counterions (derived from organic salts) were the most effective.