Kinetics of the acid-catalysed hydrolysis of dodecylsulphate and dodecyldiethoxysulphate surfactants in concentrated micellar solutions. Part 1.—Effects of acid and surfactant concentrations and of the nature and concentration of counterions

Abstract

The rates of the acid-catalysed hydrolysis of sodium dodecylsulphate (SDS) and sodium dodecyldiethoxysulphate (SDE2S) have been investigated in concentrated surfactant solutions (0.035–0.6 mol dm–3). The acid concentration dependence of the initial rates shows a ‘saturation’ effect, whereas increasing surfactant concentrations above the c.m.c. lead to a maximum in the k2, obs values, beyond which they decrease sharply. These results are discussed in terms of the ion-exchange pseudophase model of the miceller reaction. The nature of the counterion has an effect on k2, obs, the values following the sequence NH4 > Li > Na Mg. This has been ascribed to differences in the ion-exchange constants, KH/X, of these cations in the Stern layer of the micelles with the hydrogen ion. Maintaining the total counterion concentration and the ratio of concentrations of surfactant counterion to the concentration of hydrogen ion constant (at 30, i.e. 0.6:0.02 mol dm–3) largely eliminates the decrease in the k2, obs values with increasing surfactant concentration, as expected from the pseudo-phase ion-exchange model. Quantitative agreement between theory and experiment is less good, however, owing at least partly to deviations from ideality in solutions of high ionic strength.