Adsorption of cinnamaldehyde at the mercury-water interface

The adsorption of cinnamaldehyde from aqueous 1 M KCl has been determined by means of differential capacity, zero charge potential and maximum surface tension measurements. A Frumkin isotherm is obeyed with α = 2.4, corresponding to repulsive interaction, and Γ_s= 3.5 × 10^?10 mol cm^?2, which is independent of potential in the range ?350/?750 mV. The standard free energy of adsorption is a quadratic function of potential with maximum adsorption occurring at the potential of zero charge. The interaction of the molecular dipole with the electric field and the partial charge transfer between the electrode and the adsorbate are considered.     

Search for a model to describe the specific adsorption of anions A? in Ga/[N-Methylformamide + mc KCl + (1 ? m)c KClO4] Systems, where KA is KCl, KBr, or KI

The adsorption parameters for systems Ga/[NMF + 0.1m M KCl + 0.1(1 ? m) M KClO₄], Ga/[NMF + 0.1m M KBr + 0.1(1 ? m) M KClO₄], and Ga/[NMF + 0.1m M KI + 0.1(1 ? m) M KClO₄] are calculated by using the regression analysis of the adsorption potential shift vs. electrode charge dependences for the following molar fractions m of the surface-active anion: 0.05, 0.1, 0.2, 0.5, and 1 within the framework of two models. The models are based on the Frumkin isotherm with the free adsorption energy dependent on the electrode charge, of which one model takes into account the diffuse layer and the other ignores it. It is shown that for electrode charges q ?? 16 ??C/cm², both models provide equal accuracy; however, for higher q, preference should be given to the model that takes into account the contribution of the double layer diffuse part.     

Statistical-mechanical treatment of an adsorbed monolayer with local order: Part III. Its use in the interpretation of the adsorption behaviour of aliphatic compounds

The statistical-mechanical treatment of Part II of thi series is applied to a monolayer of water molecules and open-chain polymeric solute molecules consisting of structurally indistinguishable segments. Water molecules are assumed to interact between themselves both via dipole-dipole forces and via H-bonds, whereas water-solute and solute-solute interactions are considered to be of the dipole-dipole type. The resulting adsorption isotherm explains several features of the adsorption behaviour of aliphatic compounds, such as the Frumkin isotherm behaviour with an attractive interaction factor, the parabolic dependence of the standard Gibbs energy of adsorption ΔG°_ads on charge σ_m, the correct order of magnitude for b  ∂ ΔG°_ads/∂(σ_m  σ_max)², and the experimental dependence of b on the charge of maximum adsorption σ_max. The model also accounts satisfactorily for the shape of the experimental curves of the potential drop Δφ across the inner layer against surface coverage at constant charge. An explanation of two-dimensional condensation based on inhibition of H-bond formation between adsorbed and non-adsorbed water molecules is proposed.     

Adsorption behaviour of n-hexylamine at the Hg/water interphase and its comparison with a molecular model accounting for local order

The adsorption of n-hexylamine on mercury from aqueous 0.1 M NaOH was studied by means of electrocapillary measurements. Adsorption at constant charge was found to satisfy a Frumkin isotherm with a charge-dependent attractive interaction factor. Analogously, adsorption at constant potential was found to be characterized by a potential-dependent Frumkin interaction factor. The lack of a sound physical basis for a general monolayer model. The particular significance of the interaction factor under conditions of maximum adsorption was stressed. The charge dependence of the standard Gibbs energy of adsorption and that of the Frumkin interaction factor as well as the coverage dependence of the potential drop across the inner layer at constant charge were interpreted on the basis of a monolayer model of H-bonded water molecules and solute dipoles in which local order is accounted for on the basis of the quasi-chemical approximation.     

Adsorption of succinonitrile at the mercury-aqueous solution interface: Unusual role of a fixed dipole in the adsorbed molecule

The adsorption of succinonitrile (SN) on a polarized Hg electrode has been studied from 0.25 M NaF aqueous solutions by means of both capacity and electrocapillary curves. Adsorption isotherms have been found to be strictly congruent with respect to both charge and potential. The charge of maximum adsorption is at ?4.6 μC cm^?2 and ΔG_ads⁰, the free energy of adsorption at zero coverage, decreases more rapidly as the charge is made positive with respect to O_max^M than on the other side. The adsorption obeys a Frumkin isotherm with a=?1.87 corresponding to attraction and saturation concentration of 4.8×10^?10 mol cm^?2 corresponding to an area per molecule of about 34.5 Ų. Experimental results suggest that SN adsorbs flat on the surface but, unlike acetonitrile and propionitrile, the CN groups are slightly turned towards the solution. This is responsible for the absence of electronic interaction of the CN groups with the metal at positive charges, which is apparent with the other two nitriles. Implications of the particular structure of the adsorbed molecule on the other adsorption parameters are discussed in detail.     

The effect of diffuse layer on adsorption potential shifts and differential capacitance in systems (In-Ga)/[N-methylformamide + mc KA + (1 - m)c KClO4], where KA is KCl, KBr and KI

By the regression analysis of dependences of the adsorption potential shift (E _ads) on the electrode charge in systems (In-Ga)/[NMF + 0.1m M KCl + 0.1(1 - m) M KClO₄, (In-Ga)/[NMF + 0.1m M KBr + 0.1(1 ? m) M KClO₄], and (In-Ga)/[NMF + 0.1m M KI + 0.1(1 - m) M KClO₄] with the following m fractions of the surface-active anion: 0.05, 0.1, 0.2, 0.5, and 1, the adsorption parameters are calculated in terms of two models both based on the Frumkin isotherm and considering the free adsorption energy as a function of the electrode charge, where one model takes into account the diffuse layer and the other ignores it. It is shown that for the studied electrode charges q ≤ 10 μC/cm², both models provide equal accuracy in calculating E _ads and the differential capacitance (C) in the systems under study. However, for determination of adsorption parameters, the regression analysis of E _ads vs. q curves has several advantages over the analogous analysis of C vs. q curves.     

Regression analysis of dependences of adsorption potential shifts on the charge in systems (Tl-Ga)/[N-methylformamide + mc KI + (1 − m)c KClO4], (Tl-Ga)/[N-methylformamide + mc KBr + (1 − m)c KClO4] and (Tl-Ga)/[N-methylformamide + mc KCl + (1 − m)c KClO4]

By the regression analysis of dependences of the adsorption potential shift (E _ads) on the electrode charge in systems (Tl-Ga)/[NMF + 0.1m M KI + 0.1(1 ? m) M KClO₄], (Tl-Ga)/[NMF + 0.1m M KBr + 0.1(1 ? m) M KClO₄], and (Tl-Ga)/[NMF + 0.1m M KCl + 0.1(1 ? m) M KClO₄] with the following m fractions of the surface-active anion: 0.05, 0.1, 0.2, 0.5, and 1, the adsorption parameters are calculated in terms of two models based on the Frumkin isotherm both considering the free adsorption energy as a quadratic function of the electrode charge, where one model takes into account the diffuse layer and the other ignores it. It is shown that for the studied electrode charges q ≤ 2 μC/cm², both models provide equal accuracy in calculating E _ads in the systems under study.     

Electrostatic effect of specifically adsorbed electroinactive ions upon electrode processes: Part IV. Carbon Tetrachloride electroreduction in the presence of Cl−, Br−, SCN−, and N3−

The rate constant for CCl₄ electroreduction on mercury, once corrected for diffuselayer effects according to Frumkin, still depends on the charge density q_i due to specifically abdsorbed supporting ions. Thus, in the presence of the adsorbed anions Cl^?, Br^?, SCN^? and N₃^?, the logarithm Φ of the rate constant corrected for diffuse-layer effects decreases linearly with |q_i|. The slopes of the various Φ vs. q_i plots are in fairly good agreement with the theoretical treatment of ref. 9, which accounts for the electrostatic interactions between the activated complex for the electrode reaction and the neighbouring adsorbed electroinactive ions within the compact layer. An analogous behaviour is observed in the reduction of CBr₄.     

Electrostatic effect of specifically adsorbed electroinactive ions upon electrode processes: Part VII. Case of a charged adsorbed reactant. Dibromoacetate electroreduction in the presence of Br−, N3−, SCN− and Tl+ ions

The logarithmof the rate constant for CHBr₂COO^? electroreduction at constant applied potential, corrected for diffuse-layer effects according to Frumkin, varies linearly with the charge density q_i due to specifically adsorbed supporting ions. The rate of decrease ofwith increasing |q_i| as observed in the presence of the adsorbed anions Br^?, N₃^?, and SCN^? is about 5 times less than the rate of increase ofwith increasing the charge q_i due to adsorbed Tl⁺ cations. This behaviour, analogous to that observed in the reduction of the CCl₃COO^? ion [4], has been explained by considering that the time of adsorption of the electroactive anion CHBr₂COO^? is long enough to cause a perturbation in the distribution of the adsorbed supporting ions in the neighbourhood of the adsorbed anionic reactant.     

Electrostatic effect of specifically adsorbed electroinactive ions upon electrode processes: Part V. Case of a charged adsorbed reactant. Trichloroacetate electroreduction in the presence of Br−, l−, SCN−, N3−, Ti+ and C(NH2)3+

The rate constant k_f for CCl₃COO^? electroreduction at constant applied potential, once corrected for diffuse-layer effects according to Frumkin, still depends on the charge density q_i due to specifically adsorbed supporting ions. Thus, in the presence of the adsorbed anions Br^?, I^?, SCN^? and N₃^?, the logarithm Φ of the rate constant corrected for diffuse-layer effects decreases linearly with |q_i|, albeit slightly. In the presence of the adsorbed cations Tl⁺ and C(NH₂)₃⁺, Φ increases with the positive charge density q_i due to the cation at a rate which is 3 to 8 times greater than the corresponding rate of decrease in the presence of the adsorbed anions. This behaviour has been interpreted on the basis of the theoretical treatment of ref. 22 by considering that the time of adsorption of the electroactive anion CCl₃COO^? is long enough to permit a perturbation in the distribution of the adsorbed supporting ions in the immediate vicinity of the adsorbed reactant.     

Structure de la couche double sur les électrodes d'or: Détermination des composants de charge

The components of the charge q_±^Au at the interface polycrystalline gold electrode—NaF, KCl or KBr solutions and the charge due to specifically adsorbed Cl^? or Br^? anions have been determined by thermodynamical analysis of differential capacity—potential curves, using the two sets of variables q^M, μ (Grahame and Soderberg's method) and E_?, μ (Esin—Markov effect). In the absence of specific adsorption (NaF), variations of charges q_±^Au with potential are in good agreement with those provided by the diffuse layer theory in the negative charge region of the metal. With specific adsorption of Cl^? or Br^? anions, both q_±^Au(q^Au), (q_?¹)^Au(q^Au) curves obtained by the two methods fit well. Determination of components of charge was made in the whole negative charge region and in part of the positive charge region of the electrode.     

Estimating of surface activity of Cl− and Br− ions in the (In-Ga)/[N-methylformamide + mc KCl + (1 − m)c KClO4] and (In-Ga)/[N-methylformamide + mc KBr + (1 − m)c KClO4] systems by different methods

Differential capacitance curves in the (In-Ga)/[N-methylformamide + mc KCl + (1 ? m)c KClO₄] and (In-Ga)/[N-methylformamide + mc KBr + (1 ? m)c KClO₄] systems are measured using an ac bridge for the following molar portions m of the surface-active anion: 0, 0.01, 0.02, 0.05, 0.1, 0.2, 0.5, and 1. The Cl^? and Br^? anions specific adsorption in the systems can be described quantitatively by the Frumkin isotherm. The principal parameters of Cl^? and Br^? anions adsorption at the (In-Ga)/N-methylformamide interface are determined by different methods. Unlike Ga/N-methylformamide interface, where the adsorption energy increased in the sequence I^? ≈ Br^? < Cl^?, at the (In-Ga)/N-methylformamide interface it increased in the reverse sequence: Cl^? < Br^? < I^?. The adsorption parameters at the charge density q = 0, obtained by three different methods, are close to each other. However, the parameters α₁ and α₂, which characterize the charge effect on the adsorption energy, when determined by the analyzing of dependences of adsorption potential drop E _ads on ln(mc), differ from those determined by two other methods. The error may be caused by the assuming that the adsorption potential drop is proportional to the coverage of dense layer with the specifically adsorbed ions.     

Parameters of the double electric layer and n-butanol adsorption on lead in methanesulfonate solutions

A differential capacity measurement showed that the zero charge potential of a lead electrode in a sodium methanesulfonate solution was ?0.69 V. Like sulfate anions, methanesulfonate anions had low specific adsorbability on lead. The adsorption of n-butanol from a methanesulfonate electrolyte on a lead electrode was approximated by the Frumkin isotherm equation. The parameters of n-butanol adsorption in a methanesulfonate solution were close to those reported for fluoride and sulfate solutions in the literature.     

Electrical double layer and adsorption of iodide ions at the Bi|ethylene carbonate interface

The adsorption of I^? anions on the Bi(111) single-crystal plane from solutions in ethylene carbonate has been investigated by impedance measurement method. The ionic charge due to the specific adsorption has been obtained by integration of differential capacitance curves, and the Gibbs excess has been calculated using the mixed-electrolyte method applied for both electrode charge and electrode potential as the independent electrical variables. The Gibbs energy of I^? anion adsorption has been calculated using the virial adsorption isotherm. It was found that under comparable conditions, the results obtained at constant electrode potential and at constant electrode charge were coincident. The Gibbs energy of I^? anion adsorption and the calculated electrosorption valency value were found to be very close to these values obtained in propylene carbonate.     

Charge dependence of adsorption energy of ions at electrodes

Analysis of the energetic and geometric characteristics of the inner part of the electrical double layer has been carried out in the presence of the specific adsorption of ions accompanied by change in the dimensions and dielectric properties of the inner layer. On the basis of the formulae for the electrochemical potential of the adsorbed ion on the electrode surface and for the components of the inner layer capacity, the equations of the Frumkin isotherm and of its parameters have been derived. It has been shown that, in general, the Frumkin isotherm is non-linear at the given electrode charge and the charge dependence of the adsorption equilibrium constant has a parabolic form. The results of the corresponding theoretical calculations have been compared with the experimental data obtained by the study of the specific adsorption of tetra-alkylammonium cations on a bismuth electrode in ethanolic and aqueous solutions. Good agreement of the experimental results with those calculated theoretically confirm the validity of the equations derived.     

The influence of activation conditions on the performance of platinum/ruthenium methanol electro-oxidation catalysts surface enrichment phenomena

Starting from the electrocapillary equation for the ideally reversible electrode a relation has been derived between the parameters q^*-q^M+nFΛ₀ (q^M=electrode charge density, Λ₀=surface excess of oxidant) and the derivatives of the free energies of adsorption of the Ox and Red component with respect to potential. In this way a model is obtained for the explicit interpretation of the low frequency capacity C_LF and the high frequency capacity C_HF, occurring as parameters in the expression for the electrode admittance in the case of reactant and/or product adsorption. The experimental results obtained by Timmer et al. for the Pb(II) reduction in 1 M KCl appear to fit to this model if a linear isotherm with a potential-dependent adsorption coefficient is assumed. It is concluded that most probably the neutral PbCl₂ species is adsorbed.     

Adsorption of tert-butanol at the electrode from concentrated NaClO4 solutions

The parameters of the double layer for tert-butanol adsorption determined in the supporting electrolytes: 2 mol dm^?3, 3 mol dm^?3 and 4 mol dm^?3 NaClO₄, show an increase of tert-butanol adsorption on the mercury electrode together with the increase of NaClO₄ concentration. The adsorption of tert-butanol on the electrode takes place via the ?CH₃ group which is shown by the changes in the values of zero charge potentials, E _z. On the basis of the analysis of the changes of relative surface excess values, Γ′, and the parameters determining the maximum adsorption, the process of adsorption in the discussed systems can be recognized as a physical process. Besides, it can be said that a major drawback in the process of adsorption of the organic substance on the electrode is to remove water from the electrode surface.     

Studies on membrane concentration potentials across heavy metal soap discs

The electrostatic effect of specifically adsorbed electroinactive ions upon electrode processes involving adsorbed reactants with a time of adsorption greater than the characteristic time of surface-diffusion jumps is considered. The validity of the most general form of the Frumkin correction term for double-layer effects, with ψ₁ equal to the local potential ₃ at the position occupied by the reactant in the transition state, is first examined on the basis of current molecular theories of electrode kinetics. Subsequently, approximate expressions of ₃ are derived for the case in which adsorbed reactant and adsorbed supporting ion are charged both of equal and of opposite sign. In both cases the logarithm Φ of the rate constant for the electrode reaction at constant applied potential, as corrected for diffuse-layer effects only, is expected to vary linearly with the charge density q_i due to the adsorbed supporting ion. |ΔΦ/Δq_i| is, however, much greater in the case of electrostatic attraction between reactant and adsorbed supporting ion than in the case of electrostatic repulsion. The influence of reactant adsorption with partial charge transfer upon the magnitude of ΔΦ/Δq_i is considered.