Rate-retarding effects of mixed anionic/non-ionic micelles on the alkaline hydrolysis of the chloropentamminocobalt(III) complex - Role of the anionic surfactant chain nature

Rate data for the alkaline hydrolysis of the chloropentaamminecobalt(III) cation in the presence of mixed micelles composed of (i) anionic sodium decylsulphate (SDeS) and non-ionic dodecylpenta(oxyethylene glycol) monoether (C12E5) surfactants and (ii) anionic sodium perfluorooctanoate (SPFO) and non-ionic C12E5 surfactants has been obtained at T 298K and constant electrolyte concentration 0.08 mol dm−3 ([NaOH] = 0.01 mol dm−3, [NaClO4] = 0.07 mol dm−3) over a wide range of total surfactant concentration (Ct) and anionic mole fraction (χ). The critical micelle concentrations (c.m.c.s) of the mixed micelles have been determined over the entire χ range by means of surface tension measurements. The experimental data have been analyzed by the regular solution model for non-ideal mixing. The estimated negative values of the interaction parameter β indicate that attractive interactions between the two types of surfactants in the mixed anionic/non-ionic micelles take place. The kinetic data have shown that, at all mole fraction examined, at surfactant concentrations below the critical micelle concentration the rate constant values are not significantly influenced by the presence of surfactants while beyond the c.m.c. value, in all cases examined, the rate constant is conspicuously inhibited. The reaction rates decrease with increasing Ct (at constant χ) and χ (at constant Ct) to an extent which depends on the nature of the anionic surfactant used, i.e., the hydrogenated anionic surfactant is much more effective in inhibiting the hydrolysis process than the fluorinated one. The kinetic data, interpreted quantitatively by applying the pseudo-phase ion exchange model, indicate that two reaction paths contribute to the overall hydrolysis rate and that the binding constant of the complex (KM) decreases markedly as χ decreases. These findings have been attributed to the reduction of the charge density at the mixed micellar surface. Moreover, the KM values indicate that the capacity of the hydrogenated anionic micelles to bind the cobalt complex is higher than that of the fluorinated anionic micelle-forming surfactant. The present findings have been compared with those previously obtained concerning the effects of other aggregated systems upon the rate of the same hydrolysis reaction.