In aqueous media, heptyl viologen (HV2+) is reduced via a one-electron transfer to its cation radical (HV•+), the salt of which forms a two-dimensional (2D) phase on a mercury electrode. In this work, we studied the influence of temperature, the reagent concentration, and the type of anion present in the medium on the formation of this 2D phase. The critical temperature for the phase transition was found to be 80−85 °C. The experimental value obtained for the surface area occupied by the heptyl viologen molecule below 30 °C is about 70 Å2; accordingly, the molecules must arrange themselves at an angle with the electrode surface. The anion concentration in a chloride medium was found to exert no appreciable effect on the 2D phase; on the other hand, increasing bromide ion concentrations in the medium shifted the nucleation peak to increasingly more negative potentials.

In aqueous mixed media containing DMSO in fractions by volume above 0.50 the cation radical of heptyl viologen (HV•+) can be absorbed in two different orientations on a mercury electrode. Thus, the cation radical can take a planar conformation relative to the surface; however, depending on the applied potential, it can reorient itself to adopt a different conformation where the bipyridine group lies normal to or at an angle with the electrode. In this work, the reorientation was found to take place via a two-dimensional phase transition mechanism. The experimental conditions under which the phenomenon occurs are analyzed.

No abstract

On the basis of the electrochemical results obtained for thin films of 7,7,8,8- tetracyanoquinodimethane (TCNQ) on a glassy carbon electrode, the reduction and oxidation of the [TCNQ](0/)(-) couple in KCl aqueous media occurs via a mechanism involving layer-by-layer nucleation and growth. In situ recorded UV-visible spectroelectrochemical data allow two different crystal structures for the oxidized form of TCNQ to be discriminated.

A TTF-TCNQ/PVC composite electrode is proposed as a voltammetric cation and anion sensor. The electrode relies on the principle that, during redox processes involving the TCNQ 0/À couple for cations and the TTF þ/0 couple for anions, electrolyte ions are included into lattice sites in the charge neutralization process. This voltammetric ionsensor provides results that are similar to those of sensors based on two electrodes (viz. one modified with TCNQ for cations and another modified with TTF for anions) but with some practical advantages over them.