The adsorption of urea on a polycrystalline copper electrode from 0.01 M NaClO 4 solution has been studied by impedance spectroscopy and radiometric method. The dependence of the surface concentration of urea on the electrode potential and the bulk concentration was determined. From radiometric data, it follows that the adsorption of urea on the copper electrode takes place in the entire range of studied potentials where no faradaic processes occur. In this range, the process of adsorption is practically reversible with respect to the potential and the bulk concentration of urea. The experimental data were described by the Langmuir and the virial isotherms and the Gibbs energy of adsorption were calculated. The data of the urea adsorption on different electrodes have been compared and the role of the kind of the metal on the adsorption process was discussed.

The adsorption of thiourea (TU) on monocrystalline copper electrodes of basal indices: (111), (100) and (110) was studied by impedance spectroscopy and radiometry. Differential capacity of copper electrodes in 0.01M NaClO4 solution containing TU of concentrations from 10−6M to 5×10−4M has been measured. The obtained electrochemical and radiometric data were analyzed to determine the surface concentration, coverage and adsorption parameters of TU as a function of electrode potential, and the bulk concentration of the adsorbate. The values of limiting surface concentration of adsorbed TU as well as the Gibbs energy of adsorption depend on the plane of Cu electrode and follow the sequence: Cu(111)>Cu(100)>Cu(110) which is in agreement with the surface density of copper atoms.

The potentials of zero charge (pzc) of copper monocrystalline electrodes of three basal indices, namely (111), (110) and (100) and a polycrystalline electrode have been determined by impedance spectroscopy methods, in the frequency range of 1–100 Hz. Experiments were carried out in perchlorate solutions. The values of the pzc for (111), (100) and (110) were equal to −0.70, −0.73 and −0.75 V vs SHE electrode, respectively. This sequence of pzc is the same as for other monocrystalline electrodes of fcc structure, i.e. the pzc is most negative for the open (110) and less negative for the close packed (111) structures, though the differences are much smaller than reported by others. A dependence of the pzc on the pH of the solution was observed.

The adsorption of urea on a polycrystalline silver electrode was studied by radiometry and impedance spectroscopy. The differential capacity of the silver electrode in 0.01 M NaClO4 solution containing urea in concentrations from 10−6 to 5×10−4 M has been determined. The isotherms of urea adsorption, found from the capacitance and radiometric measurements have been compared. The experimental data were described by the Langmuir isotherm, and the Gibbs energy of adsorption was calculated. The urea adsorption takes place in the entire range of the applied potential. The process is reversible with respect to the electrode potential and the bulk urea concentration.

The electrical double layer has been dealt with in countless papers and in a number of reviews, including those published in previous volumes of the Modern Aspects of Electrochemistry series. The experimental double layer data have been reported and commented on in several important works in which various theories of the structure of the double layer have been postulated. Nevertheless, many double layer-related problems have not been solved yet, mainly because certain important parameters describing the interface cannot be measured. This applies to the electric permittivity, dipole moments, surface density, and other physical quantities that are influenced by the electric field at the interface. It is also often difficult to separate the electrostatic and specific interactions of the solvent and the adsorbate with the electrode. To acquire necessary knowledge about the metal/solution interface, different metals, solvents, and adsorbates have been studied. In the earlier concepts of the interface structure, the metal was treated as a reservoir of electrons, uniformly distributed in the bulk of the phase. Spatial distribution of charges was considered mainly on the solution side. No such considerations were made for the solid electrodes, except for the

Electrosorption of thiourea (TU) on single crystal electrodes: Ag(111), Ag(100) and Ag(110) was studied using electrochemical and radiochemical methods. A radiotracer method, with TU labeled with C-14 in 0.1 M HClO4, was used to determine the surface concentration of the adsorbate. From radiometric data it follows that adsorption of TU is reversible with respect to the bulk concentration and the potential in the range of ideal polarizability of electrodes. The maximum surface concentration of TU, determined radiometrically, follows the sequence: Ag(111) > Ag(100) > Ag(110), which is in agreement with the atom surface density of the silver planes. The simple Langmuir adsorption equation describes experimental data up to 80% of surface coverage. The Gibbs energy of adsorption is similar (25.5 +/- 1 kJ/mol) for all planes studied.

The adsorption of thiourea on polycrystalline and monocrystalline copper electrodes of basal indices: (111), (100) and (110) was studied by radiometric and electrochemical methods. The measurements were carried out in perchloric acid solution at ambient temperature. Thiourea labeled with C-14 was used in the experiments. The dependence of the surface concentration of thiourea on the electrode potential and on the bulk concentration was determined for each studied surface. Based upon the obtained radiometric data it can be stated that adsorption of thiourea on investigated copper electrodes takes place in the entire range of the applied potential range. The adsorption process is mainly reversible at constant potential which allowed us to determine adsorption isotherms and also to calculate values of the Gibbs energy of adsorption for the copper surfaces studied. The calculated values of surface concentration of the adsorbed species were different and dependent on a copper plane whereas the Gibbs energy of adsorption was practically the same. This means that the energy of interaction of TU molecules with adsorption sites on copper electrodes is similar and the differences in surface concentrations Γmax, are mainly due to the different number of adsorption sites, i.e. the surface atom density of individual planes of Cu electrodes.

A radiotracer technique combined with cyclic voltammetry has been used for the investigation of underpotential deposition of thallium onto (111), (110) and (100) single crystal silver electrodes from perchloric acid solutions (0.1 M). Two modes of thallium adsorption have been found: the weak adsorption without charge transfer and the strong adsorption with the charge transfer, at potentials less negative and more negative, respectively, than those at which the upd peaks occur. The electrosorption valency scans the range from zero to one. A dynamic equilibrium between the adsorption and the desorption processes exists at all potentials studied. Radiometrically determined monolayer surface concentration of thallium does not depend on the crystallographic orientation of silver electrodes. Apparently, a nonepitaxial thallium monolayer is formed, but it is less dense than the close packed structure and leaves some surface active sites on silver electrode free for hydrogen evolution. (C) 1999 Elsevier Science Ltd: All rights reserved.

Adsorption of sulfate ions was studied on three basal planes and a polycrystalline surface of silver in 0.1 M HClO4 by radiometric and electrochemical methods. On all four surfaces, adsorption was found to be reversible with respect to the potential and bulk solution concentration of sulfate. Radiometrically determined adsorption isotherms show a clear difference in sulfate adsorption on individual planes. Adsorption of the anion on the basal planes of silver decreases in the sequence: Ag(111)≥Ag(110)>Ag(100). As expected, the polycrystalline silver electrode exhibits intermediate adsorption properties between the most and the least active basal planes. Thermodynamic parameters of adsorption were calculated by fitting the Frumkin isotherm equation to the experimental data points. High values of surface concentration for adsorbed sulfate on the (111) and (110) planes of silver can be accounted for by a match between a tetrahedral structure of the anion and a trigonal distribution pattern of the surface atoms of silver on these planes.

Adsorption of benzoic acid was studied on polycrystalline silver electrodes in a 0.1M perchloric acid solution. The electrodes were obtained by electroplating of Ag-black onto a vacuum-deposited silver substrate. The real surface area of such prepared electrodes was calculated from a total capacitance of the electric double layer, determined from the plot of the voltammetric double-layer charging current as a function of the scan rate. Quantitative adsorption data for benzoic acid, obtained using the thin-layer radiometric method, indicate that benzoic acid is surface active in the entire range of potentials available to electrochemical studies on silver in aqueous solutions. The Γ vs E plots show that adsorption increases as potential becomes more positive. An overlap of the positive- and negative-going Γ–E plots points to a full reversibility of the surface process, regardless of the potential range. In turn, surface/bulk exchange experiments attest to a reversibility of adsorption with respect to solution concentration. The apparent value of the Gibbs energy of benzoic acid adsorption on an Ag electrode, as estimated from the Langmuir isotherm, is equal to −31kJmol−1. A kinetic analysis of the radiometric data suggests that the surface process rather than the diffusion is a rate-determining step of adsorption. The adsorption of benzoic acid on polycrystalline silver is compared with the adsorption of benzoic acid on other transition metals.

The phenomenon of sulfate ion adsorption on monocrystalline silver surfaces was investigated quantitatively. The adsoprtion of sulfate from 0.1 M HClO4 was found reversible with respect to the potential and also to the bulk concentration of sulfate. The adsorption isotherms as well as the surface concentration/potential dependences were determined radiometrically. The surface concentration sequence was found to be Ag(111)2Ag(110) >Ag(100). The thermodynamic parameters of adsorption were calculated by fitting experimental data to virial, Frumkin and Henry adsorption models. The sequence of sulfate adsorption on three basal silver planes from 0.1 M NaClO4 was found to be quite different to that from an acidic supporting electrolyte, in agreement with the sequence of zero-charge potentials for the silver surfaces investigated.

The adsorption of benzoic acid on platinum, gold, copper, and silver electrodes has been studied by radiometric and voltammetric methods. The adsorption of benzoic acid is a reversible process on Au, Cu, and Ag electrodes. On Pt electrode the process is partly irreversible. The adsorption can be described by the Frumkin-type isotherm. The influence of water adsorption on the Gibbs energy of adsorption has been discussed. The rate of adsorption follows the Roginsky-Zeldovitsh equation and is controlled by the rate of surface processes.

Adsorption of benzoic acid at copper electrodes, obtained by electroplating copper on gold, has been studied in 0.1 M HClO4 using a radiotracer technique and cyclic voltammetry. Impedance spectroscopy has been applied to verify the real surface area of copper electrodes also determined from voltammetric data in the double-layer region.Radiotracer measurements of adsorption have indicated that benzoic acid is surface active on copper in the entire range of potentials available to electrochemical investigation in the perchloric acid solution, ie, from -0.30 to 0.30 V (vs. rhe). Adsorption increases with the increase in the electrode potential and bulk concentration of benzoic acid, and reaches saturation at the surface coverage not exceeding 0.30. Exchange of the adsorbate with the bulk benzoic acid molecules is slow but is considerable enough to confirm reversibility of the surface process.Principal thermodynamic parameters of adsorption have been calculated from the experimental surface concentration data using the Frumkin isotherm, found applicable in this case. Benzoic acid molecules are most likely oriented parallel to the surface, with radiotracer data providing no indication of change of that orientation at any potential used in this work. Despite apparent lack of reorientation, benzoic acid adsorption on copper is similar to that on polycrystalline gold, particularly if the rates of the surface process, the Gibbs energies of adsorption and the degrees of the surface/bulk exchange are compared for both systems.

The paper reviews two principal variants of the radiometric method of adsorption investigation on solid electrodes. One variant is based upon the thin electrode arrangement that allows beta- particles to reach radiation detector placed directly under the metal layer. The other operates on the thin gap principle according to which particles emitted by the labeled adsorbate penetrate through a very thin solution layer before being detected by the radiation detector, located on the opposite side of the gap. Both methods can be used to determine quantitatively the adsorption as a function of potential, temperature, solution concentration and other variables. The thin electrode method is suitable for measuring adsorption on rough, usually electroplated, surfaces while the thin gap method is capable of providing information from both rough and smooth surfaces, monocrystalline in particular. Several example of adsorption data obtained by using both methods are given in the paper. Discussed are advantages and drawbacks of both methods and their complementary character.

Adsorption of benzoic acid on a polycrystalline Au electrode, obtained by electroplating of gold, has been studied in 0.1 M HClO4 using cyclic voltammetry and radiotracer technique.Adsorption has been found to take place in the entire range of studied potentials, from 0.05 to 1.75 V (rhe), with the surface concentration of the adsorbate exceeding 5 x 10(14) molecules cm-2 at saturation. Desorption into a clean supporting electrolyte is small and extremely slow. On the other hand, surface/bulk exchange of benzoic acid is much faster, attesting to the dynamic equilibrium between the adsorbed and solution species.Adsorption data and model calculations strongly indicate that two different orientations of the adsorbed molecules are present on the surface. Flat (parallel to the surface) orientation dominates at less positive potentials while the vertical (perpendicular to the surface) orientation dominates at more positive potentials.Regardless of orientation, benzoic acid adsorption on gold falls into the chemisorption category. General behavior of the system bears close resemblance to the adsorption of benzoic acid on platinum that was reported earlier in Zelenay and Sobkowski, Electrochim. Acta 29, 1715 (1984).

The application of radiotracer method in the in situ studies of adsorption on semiconductor electrodes has been described. Using this technique the adsorption of HCOOH on n-Si(100) electrode has been studied and the surface concentration of the adsorbate determined. The adsorption takes place in the potential range of −0.5−1.0 V. It has been found that adsorption of HCOOH is an irreversible process. The dependence of the surface concentration of adsorbed species on the bulk concentration of HCOOH has been determined, too. The epm and eps values at the surface concentration close to saturation are equal to 0.4 and 0.9, respectively. It has been found that adsorbed species are arranged in two layers. It is proposed that the first layer is formed by COOH radicals and the second layer is formed by HCOOH molecules. Both layers are most likely linked by hydrogen bonds.

The problem of interaction of solvent molecules with metallic electrode, is reviewed, The orientation of solvent molecules on the electrode surface, the influence of solvent-solvent interactions in the bulk on the metal-solvent interaction, the role of electrolytes, the kind of the metal and structural effects are discussed.The results of the study of metal-solvent interaction by nonelectrochemical methods (spectroscopic, radiometric) are also reported.