ELECTROCHEMICAL INVESTIGATION OF CETYLPYRIDINIUM CATION MICELLE ADSORPTION AT THE HG AQUEOUS-SOLUTION INTERFACE

Differential capacitance measurements at a mercury electrode in contac t with 0.1 M Na2SO4 aqueous solutions containing cetylpyridinium catio ns (CP+) are carried out to examine the formation of micelles on the e lectrode surface and their phase transitions in the various polarizati on regions. Analysing the experimental data by means of theoretical tr eatments presented in J. Phys. Chem., 96 (1992) 8453 and J. Electroana l. Chem., 348 (1993) 59, it is shown that, below the critical micelle concentration (CMC), a monolayer film of surface micelles is formed at positive potentials, in the region from about + 0.10 to 0 V with resp ect to the saturated calomel electrode. At concentrations above the CM C and up to 1 x 10(-3) M, the micellar film is formed in the region be tween +0.20 and +0.05 V, and extends to three dimensions, forming a bi layer on the mercury surface. Finally, at concentrations above 1 x 10( -3) M, the formation of a polylayer micellar film is observed. This fi lm, either monolayer or polylayer, covers the electrode surface up to -0.95 V, where it is destroyed as a result of the onset of the electro chemical reduction of CP+. Thus, the micellar film of CP+ cations is t ransformed into a film of uncharged monomer species of the reduction p roduct. This film is not particularly stable and, in the region betwee n -0.95 and -1.4 V, it collapses to a compact layer. At potentials mor e negative than about -1.4 V, the compact layer is destroyed and the r eduction product is desorbed from the electrode. The substance, in the form of CP+, is also desorbed at potentials more positive than +0.2 V , except at concentrations above 1 x 10(-2) M, where a compact, possib ly polylayer is formed in this polarization region.