On the Origin of Electrochemical Oscillations in the Picric Acid/CTAB Two-Phase System

Abstract

The oscillatory picric acid/CTAB two-phase systemas introduced by Yoshikawa and Matsubara [J. Am. Chem. Soc. 1984, 106, 4423−4427]has been revisited. UV−vis spectroscopic studies were presented that provide a clearer and new insight into the possible kinetic mechanism for the oscillatory behavior. It was shown that the key process in the system is the formation of 1:1 ion pairs between picrate and CTA+ at the liquid/liquid interface that desorb and move into the organic phase. Kinetic UV−vis experiments also showed that the presence of alcoholwhich was essential to observe oscillatory behaviorinhibits the formation of ion pairs. In the course of this process, this inhibition is released presumably due to the transfer of the alcohol from the interface into the organic phase. The corresponding inhibition/acceleration dynamics were observed in a subsystem experiment as S-shaped, autocatalytic-like kinetics with respect to the ion-pair formation. A new mechanism based on Langmuir−Hinshelwood kinetics was proposed that includes the competitive adsorption of CTA+, picrate, and the alcohol at the liquid/liquid interface. The release of inhibition, which depends on the concentration of vacant interface sites and which occurs autocatalytically, expresses the key process for the nonlinear behavior. Computer simulations were performed which confirm that the proposed kinetic mechanism is dynamically reasonable.