Dissociation kinetics at polyacrylic latex-coated electrodes

Polystyrene/poly(acrylic acid) latex particles 0.26 μm in diameter, which were synthesized by emulsion copolymerization of styrene and acrylic acid, were immobilized on the platinum electrode in a uniform film. The film did not block the diffusion current of a ferrocene derivative in the solution. The voltammograms showed the reduction of hydrogen ion, which was supplied not only from the solution but also from the dissociation of carboxyl acid of the adsorbed latex. The peak currents at pH less than 4.5 were proportional to the thickness of the latex film, and were supplied by dissociation from the latex. In contrast, those at pH more than 5.5 were independent of the film thickness, and were controlled by the dissociation kinetics of the carboxyl acid within the first particle layer. Expressions for the voltammetric currents with the CE mechanism were derived theoretically. The theory predicts that the reaction rate constant can be evaluated from the lower deviation from the proportionality of the peak currents to scan rates, as in consistent with the experimental results. The rate constant was by one order smaller than the homogeneous rate constant of acrylic acid. pH-Titration demonstrated the electrostatic interaction of –COO−, like in self-assembled carboxylate films. The average concentration of carboxylate in the latex was ca. 2.7 M. The dissociation was limited to the surface domain of the latex.