Mixed micellar systems of cleavable surfactants

Previous studies have shown that the alkaline hydrolysis of cleavable ester surfactants is strongly affected by aggregation. The alkaline hydrolysis of the cationic species decyl betainate (DB) is strongly enhanced by micellization, whereas the nonionic species tetra(ethylene glycol)mono-n-octanoate (TEO) is virtually protected when residing in aggregates. In the present work, mixtures of DB and TEO were studied at concentrations above the critical micelle concentration, and the rate of hydrolysis of each surfactant in the presence of the other was assessed. The micellar interaction parameter (beta) was determined from the critical micelle concentrations of various mixtures of the two surfactants. The result (beta = -2.4) indicates a moderate net attraction. The hydrolysis of the surfactants was monitored using 1H NMR. It was shown that the hydrolysis of DB exhibits the main characteristics of the pseudophase ion-exchange model and that the reaction rate decreases with an increasing molar ratio of TEO. There are indications that the hydrolysis rate parallels the expected total counterion binding to the mixed micelles. The hydrolysis of TEO was not affected by the presence of DB. However, complementary experiments showed that it is possible to accelerate or retard the hydrolysis of TEO by coaggregation with stable cationic or anionic surfactants, respectively.