Micellar effects on reaction kinetics: Part II. Study of the action of dodecyltrimethylammonium bromide on the reactions between Pyrogallol Red and chromium(VI), vanadium(V) and titanium(IV)

Abstract

The complexation kinetics and equilibrium of the reactions of vanadium(V) and titanium(IV) with Pyrogaliol Red (PGR), and the kinetics of oxidation of chromium(VI) by PGR were studied in micellar solutions of dodecyltrimethylammonium bromide (DTAB). The 1:1 PGR-V(V) and 2:1 PGR-Ti(IV) complexes formed in an aqueous medium associate with DTAB molecules of the micellar system to yield the new complexes 1:1:2 PGR-V(WDTAB and 2:1:1 PGR-Ti(IV)-DTAB. The stability constants of the ternary complexes were calculated by considering the volume fraction of micelles and were compared with those determined from the overall solution volume. The binding constants for the associates of PGR, chromium(VI), vanadium(V), titanium(IV), PGR-V(V) and PGR-Ti(IV) complexes with DTAB micelles were spectrophotometrtcally determined. The observed rate of formation of the PGR-Ti(IV) complex was increased by the presence of DTAB micelles, but the second-order rate constant in the micellar pseudo-phase was slightly lower than in water. Therefore, the whole rate increase observed was due to the concentration of reactants in the micellar pseudo-phase. The kinetics of formation of the PGR-V(V) complex could not be investigated by using the stopped-flow technique because the reaction was too fast in both the aqueous and the micellar medium. The second-order rate constant for the oxidation reaction between PGR and Cr(VI) was found to exceed that in aqueous solutions by a factor of ca. 55. Therefore, the increased rate observed for this reaction in the DTAB micellar medium can be ascribed to intrinsic effects of the micellar pseudo-phase on reactivity.