MICELLAR EFFECTS ON REACTION-KINETICS .2. STUDY OF THE ACTION OF DODECYLTRIMETHYLAMMONIUM BROMIDE ON THE REACTIONS BETWEEN PYROGALLOL-RED AND CHROMIUM(VI), VANADIUM(V) AND TITANIUM(IV)

The complexation kinetics and equilibrium of the reactions of vanadium (V) and titanium(IV) with Pyrogallol Red (PGR), and the kinetics of ox idation of chromium(VI) by PGR were studied in micellar solutions of d odecyltrimethylammonium bromide (DTAB). The 1:1 PGR-V(V) and 2:1 PGR-T i(IV) complexes formed in an aqueous medium associate with DTAB molecu les of the micellar system to yield the new complexes 1:1:2 PGR-V(V)-D TAB and 2: 1: 1 PGR-Ti(IV)-DTAB, The stability constants of the ternar y complexes were calculated by considering the volume fraction of mice lles 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 DT AB micelles were spectrophotometrically determined. The observed rate of formation of the PGR-Ti(IV) complex was increased by the presence o f DTAB micelles, but the second-order rate constant in the micellar ps eudo-phase was slightly lower than in water. Therefore, the whole rate increase observed was due to the concentration of reactants in the mi cellar 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 PG R 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 i n the DTAB micellar medium can be ascribed to intrinsic effects of the micellar pseudo-phase on reactivity.