B. B. Damaskin

Potentials of zero charge, interaction of metals with water and adsorption of organic substances—III. The role of the water dipoles in the structure of the dense part of the electric double layer

Abstract Two types of solvent particles resident on the electrode surface have been considered: (1) associates of water molecules freely oriented along the electric field and (2) chemisorbed water dipoles oriented with the oxygen atom towards the metal surface. On the basis of this model, a semi-quantitative interpretation has been given to the dependence of the differential capacity of the dense layer on the electrode charge. It has been shown that a change in this dependence, when passing from mercury to gallium, can be described assuming the energy of interaction of chemisorbed water dipoles with the metal surface to increase.

Potentials of Zero Charge

The notion of the potential of zero charge (pzc) and the relevant term were introduced 50 years ago.(1) Later, the pzc was proved to be an important electrochemical characteristic of metal and to play a major role in electrocapillary and electrokinetic phenomena, electric double-layer structure, adsorption of ions and neutral organic molecules on the electrode, wetting phenomena, physico-chemical mechanics of solids, photoemission of electrons from metal into solution, and in electrochemical kinetics. The introduction of the notion of pzc led to solution of the Volta problem and to rigorous interpretation of the attempts to measure or calculate the “absolute” electrode potential. All this testifies to the fundamental nature of the notion of pzc.

The congruence of the adsorption isotherm with respect to the electrode potential or charge and the choice of an independent electric variable

Summary In a strictly thermodynamic approach to the investigation of the adsorption isotherm, the choice of an electric variable is of no fundamental importance and is determined by considerations of expediency or convenience. This is not the case, however, when the thermodynamic approach to the investigation of adsorption is supplemented by the assumption of the congruence of the adsorption isotherm with respect to the electrode potential or charge. These assumptions are only compatible provided the double-layer capacity in the supporting electrolyte solution, C0, does not differ from the capacity, C′, at complete surface coverage with adsorbed substance. If C0 markedly exceeds C′ as, for example, in the case of adsorption of aliphatic compounds on mercury, the assumption of the congruence of the adsorption isotherm with respect to the electrode potential is in better agreement with the experimental data. Moreover, this assumption corresponds to a clear and consistent physical picture of the surface layer in the presence of adsorbed molecules of organic substance (the model of two parallel capacitors).

Potentials of zero charge, interaction of metals with water and adsorption of organic substances—I. Potentials of zero charge and hydrophilicity of metals

The most reliable values of the potentials1 of zero charge for metals not adsorbing hydrogen are obtained from the position of the minimum on the differential capacity-potential curves. These data are confirmed by the scrape method and by electron photoemission measurements. However, as it was first found from the comparison of the behaviour of gallium and mercury, at the same potentials referred to pz the adsorption behaviour of various electrodes with respect to the simplest aliphatic surfactants—aliphatic alcohols—differs. This was explained by preferential water chemisorption at electrodes of the gallium type. The determination of the position of the differential capacity curve of the amyl alcohol desorption peak relative to pzc gives a semi-quantitative estimate of the hydrophilicity of various metals, inreasing in the sequence Hg < Bi < Sn < Pb < Cd < In < Ga. The potential at which the differential capacity of the dense layer starts to increase with decreasing negative charge shifts to more negative values relative to pzc in a similar sequence. The obtained results are compared with Trasattis data. The case of antimony requires further investigation.

The influence of crystallographic inhomogeneity of a polycrystalline electrode surface on the behavior of the electric double-layer

Abstract The specific features of the electric double-layer structure at polycrystalline electrodes in the absence of specific ion adsorption have been examined for two different models: Model I and Model II. In the case of Model I each of the faces showing on the surface of a polycrystalline electrode retains its own Helmholtz and diffuse double layers. In the case of Model II the faces retain only their own Helmholtz layer, whereas the diffuse layer is common to the entire electrode surface. The difference of zero charge potentials of the faces is defined both by their dissimilar hydrophilic properties and by different work functions. The experimental data available at present on the electric double-layer structure at polycrystalline electrodes for which the potentials of zero charge of the faces differ significantly are described by Model I.

Model of the dense part of the double layer in the absence of specific adsorption

Abstract A model is suggested for the dense part of the double layer in the absence of specific adsorption, which is based on the following assumptions: (1) There are two kinds of adsorbed solvent particles on the electrode surface: associates freely oriented in the double layer field and separate chemisorbed water dipoles; 92) the total number of solvent molecules on the surface is determined by the geometric dimensions of associates and chemisorbed dipoles; (3) account is taken of the electrostatic interaction of associates and chemisorbed dipoles in the surface layer; (4) a discreteness coefficient is introduced, accounting for the degree of influence of the field due to all the other dipoles and associates in the surface layer on the dipole or associate being considered; (5) the dipole moment of a chemisorbed water molecule exceeds the value of μH 2 O in the bulk of the solution; (6) account is taken of the decrease with increasing temperature of the average number of water molecules in surface associate. The calculations on the basis of this model describe satisfactorily the shape of the differential capacity curves of the dense layer and their temperature dependence for the system Hg-aqueous NaF solution.

The notion of the electrode charge and the Lippmann equation

Summary The notion of the charge has been considered for the case of an ideally polarized electrode. It has been proved necessary to distinguish between the total charge, which figures in thermodynamic relations, and the free charge, which can be determined only in terms of a certain electric double-layer model. A definition of the total charge is given, equally applicable to ideally polarized and reversible electrodes, as the amount of electricity to be supplied to the electrode to keep the electrode potential constant when its surface is increased by unity and the composition of the bulk phases of the system is maintained constant. The total charge thus determined satisfies in all cases the Lippmann equation. Expressions are given for the Lippmann equation for reversible redox systems, as exemplified by platinum-hydrogen and amalgam-thallium electrodes. It has been shown that in such systems two kinds of electrocapillary curves can be obtained, depending on which chemical potential is held constant: that of the oxidised or that of the reduced component. The number of characteristic electrocapillary curves in the general case of a reversible redox system has been shown to be equal to that of the independent variables in the Nernst equation expressing the conditions of electrochemical equilibrium of the system. The results obtained have been used for the interpretation of the electrocapillary dependences observed under polarographic conditions.

Investigation of adsorption of organic substances by differential capacitance measurements

Abstract By using the frumkin isotherm and taking into consideration the dependence of the attraction constant on the electrode potential (ϕ), it is possible to interpret quantitatively the C/ϕ curves (C is the differential capacitance) of the mercury electrode in the presence of aliphatic organic compounds (t-C5 H11OH; n-C5H11OH; i-C5H11OH; n-C4H9COOH). The constants necessary for the calculation characterize the adsorption state (the adsorption energy, the orientation of the molecules on the surface, and the adsorption potential) and allow the calculation of the dependence of the coverage on the potential and on the concentration of the organic substance. In the case of adsorption of pyridine and aniline on mercury, the C/ϕ curves are also characterized by the presence of two peaks. But, as shown by the comparison of the calculated values with the experimental ones, the anodic peaks observed are due to a change in the orientation of adsorbed molecules upon the transition to positive mercury charge, rather than to their desorption.

Model of mercury/solution interface in the presence of organic compounds adsorbed in two different positions

Summary The capacity of the mercury-solution interface is calculated on the basis of a model of three capacitors in parallel representing the surface with no adsorbed species and the surface with adsorbed species oriented in two ways. The calculated curves are compared with experimental data on the adsorption of p -phenylenediamine and coumarin.

On the role of polarity of adsorbate and solvent molecules in adsorption of organic substances on electrodes

Summary The role of the orientation of adsorbed dipoles of water and organic substance at the electrode/solution interface is discussed. It is shown that the charge corresponding to maximum adsorption on mercury of different aliphatic compounds does not remain constant but varies within rather wide limits depending on the polarity of adsorbate molecules. This proves the importance of the interaction of adsorbed organic dipoles with the double layer field in the adsorption of organic substances on electrodes. The displacement of adsorbed organic molecules by water molecules at sufficient large electrode charges is determined by the ratio of the capacities in the absence of adsorbate and at complete coverage of the electrode surface with it, rather than by the ratio of the effective dipole moments of these molecules per unit surface.