A.G. Anastopoulos

Phase transitions in models for adsorption on electrodes: Further examination of the polarization catastrophe

A statistical thermodynamic analysis of phase transitions at charged interfaces is presented. It is shown that the lattice models for adsorption on electrodes predict phase transitions with respect to both charge density σM and potential E. However, it is proved that the transition properties with respect to σM are an artifact, which is due to the continuity of the various thermodynamic functions inside the unstable region, when these functions are calculated from the canonical ensemble. For the same reason, the transition properties, calculated via the canonical ensemble, are characterized by the appearance of S-shaped curves when an extensive or a specific quantity is plotted against an intensive one. The transition loops in the plots of σM vs. E always have portions of negative slope and therefore the differential capacity inevitably has negative values. These negative values are eliminated only when the equilibrium properties of the model are calculated from generalized ensembles, which use the potential as the independent electrical variable. In this case, all the S-shaped curves are replaced by curves with abrupt vertical steps, in agreement with experiment. Moreover, the differential capacitance vs. potential curves form characteristic pits wih properties which are also in qualitative agreement with experiment.

Electrosorption of neutral organic compounds from solvents of high dielectric constant. Triphenylphosphine oxide as a probe molecule

Abstract The adsorption of triphenylphosphine oxide at the mercury electrode, from solvents of high dielectric constant, is studied by means of capacitance measurements against potential and time. The usual interfacial parameters are determined and discussed. The calculated values of the Gibbs energy of maximum adsorption are found to be in satisfactory correlation with donor properties of the solvets used.

Differential capacitance and electrochemical impedance study of surfactant adsorption on polycrystalline Ni electrode

The adsorption of neutral organophosphorous compounds on a charged polycrystalline Ni electrode is studied by differential capacitance and impedance measurements in methanol solutions. Analysis of capacitance measurements reveals that tributylphosphine oxide, tri(n)octylphosphine oxide, and triphenylphosphine follow Langmuir adsorption isotherm. Saturation capacitance Csat, potential of maximum adsorption Emax, limiting surface concentration Γmax, and standard Gibbs energy of adsorption ΔGmaxo at Emax are determined. The cathodic reduction of (NiOCH3)ads film formed on Ni surface at positive potentials, introducing a faradaic contribution in addition to the electrostatic charging of the interface, is supported by linear sweep voltammetric and impedance measurements. High values of electrode coverage and strong depression of faradaic currents are attained, indicating that these adsorbates may be suitable as corrosion inhibitors.

On the fractality of Hg/non-aqueous solution interfaces

The fractal morphology of adsorbed triphenylphosphine oxide layer at the hanging mercury drop electrode is investigated in pure formamide, N-methylformamide and N,N-dimethylformamide solutions. Although no typical indications are detected for the occurrence of a phase transition during the development of the adsorbed film, the increase of fractal dimension from 2.00 to non-integer greater than 2.00 values, by the introduction of triphenylphosphine oxide, is systematically confirmed for the above solvents except N,N-dimethylformamide. These findings are interpreted at a qualitative level in terms of the structure making ability of the interfacial solvent.

Mechanism of retarded amalgam formation and electron transfer reactions in pure formamide solutions

Abstract In solutions of pure formamide, the mechanism of the electroreduction of Cd 2+ , Zn 2+ and azobenzene, retarded by triphenylphosphine oxide (TPO) monolayers, is studied at the dropping Hg electrode. The size parameters of the activated complex and the inhibitor molecule show that the mechanism of retarded azobenzene reduction in formamide is similar to that of azobenzene in methanol and of p-aminoazobenzene in water. On the other hand, the mechanism of the inhibited reduction of Cd 2+ and Zn 2+ in formamide is similar to that in water but differs from methanol. These conclusions are correlated with the interaction parameters between the reactant and the adsorbate determined in this work.

Solvent Effects on the Gibbs Energy of Adsorption of Neutral Organic Compounds

The effect of neat solvents and methanol-water mixtures on the adsorption energy of phosphororganic and arsenicorganic neutral compounds is described by means of empirical polarity indices.

Mechanism of the Zn(II)/Zn(Hg) reduction inhibited by liquid expanded organic films in aprotic solvents of high dielectric constant

The mechanism of the electroreduction of Zn(II) at the dropping Hg electrode, inhibited by adsorbed triphenylphosphine oxide, is studied in solutions of pure acetonitrile and propylene carbonate. The aprotic character of the solvents and the expanded liquid nature of the inhibitor film impose a somewhat different approach to the analysis of the kinetic data obtained from dc polarographic experiments.

On the origin of frequency dispersion at the interface between mercury electrode and aqueous solutions of alkali halides

The origin of frequency dispersion of electrochemical impedance is investigated at the interface of mercury and aqueous solutions of single alkali halides. It is found that in the presence of each one of KI, CsI, CsF and CsBr salts, the interface presents certain potential regions where frequency dispersion effects are detected and others where the ideal capacitor behavior is closely approximated. Frequency dispersion effects are contributed by interfacial processes such as anion and cation adsorption, mercury halide film formation and dissolution and charge transfer reactions. The discrimination between frequency dispersion due to charge transfer processes occurring at the Hg/solution interface and that due to reactant adsorption itself is generally difficult and depends on the reaction mechanism, provided that a discrete adsorption step is anticipated.

On the effect of adsorbed substances undergoing interfacial rearrangements on the kinetics of ion and electron-transfer reactions: The case of n-hexadecyltributylphosphonium bromide

The polarography of inhibited ion-transfer and electron-transfer reactions and tensametric measurements are used for the elucidation of a phenomenon of decreasing inhibitory action with increasing concentration of n-hexadecyltributylphosphonium bromide.

Morphological changes at the Hg/solution interface related to the presence of adsorbed cations combined with solvent properties

Morphological changes are detected at the interface of the negatively polarized Hg electrode in contact with aqueous and nonaqueous solutions of single electrolytes. Those changes are indicated by the increase of the topological dimension of the interface above the value of 2.00, as found by combining capacitance measurements with the size scaling method of the hanging mercury drop electrode and also from impedance spectroscopy measurements. Both methods confirm that in highly structured solvents, the adsorption of a wide size range of univalent cations, viz tetrabutylammonium, caesium, and potassium ones, gives rise to structural changes which are identified with the deviation of the interface from its uniform structure. In weakly structured solvents, those effects are still detected, but they are markedly diminished.