O. A. Baturina

Coadsorption of ions and organic molecules

Abstract Within the framework of the model of a dense part of an electrical double layer as three parallel capacitors coupled with a set of Frumkin adsorption isotherms, equations are derived for simulating coadsorption of two solution components (neutral and ionic species). The adsorption behavior of model systems n-C4H9OH+NaCl (A) and 1-AdOH+NaCl (B) is analyzed and the results of estimation are compared with experimental data. It is concluded that, from the height of anodic capacitance peak (for the system A) and the shape of capacitance curve near the anodic phase transition potential (for the system B), certain conclusions can be drawn regarding the value of lateral interaction between surface-active ions and organic molecules, if the parameters of their individual adsorption are known. A general agreement is found between experimental results and simulated ones.

Coadsorption of two organic substances within the framework of a system of two mixed Frumkin isotherms

AbstractWithin the framework of a system of two mixed Frumkin isotherms, the shape of the Gibbs energy surface for an adsorption layerformed by two coadsorbed organic substances is analyzed as a function of their degrees of surface coverage u 1 and u 2 . It is shownthat, for displacement coefficients n 1 / n 2 1, an increase in the parameter of intermolecular interaction a 12 results in a decrease inthe number of possible solutions of the aforementioned system of equations. At the same time, for n 1 / n 2 2, there exists a regionof a 12 parameters, in which the system of equations can haveseven solutions, so that the number of possible solutions passesthrough a maximum with an increase in a 12 . The results obtained are important for simulating unstable states, which can be realizedon the electrode j solution interface during coadsorption of two different organic substances.# 2003 Elsevier Science B.V. All rights reserved. Keywords: Adsorption isotherms; Gibbs energy of a surface layer; Intermolecular interaction in an adsorbed layer

Parameters of n-hexanol adsorption on Au (111). Comparison between differential capacity and chronocoulometry results

The adsorption behaviour of n-hexanol on a Au (111) single-crystal electrode from aqueous 0.2 mol dm -3 NaClO 4 was studied on the basis of dynamic capacity measurements and of capacitive charge measurements by chronocoulometry. The charge density, σ M , as a function of the applied potential, E, and of the n-hexanol concentration was analysed thermodynamically, yielding the Gibbs surface excess, Γ, and the Gibbs energy of adsorption, Δ ads G o , of n-hexanol as a function of potential and concentration. The results of the thermodynamic analysis of the adsorption data at the potential of maximum adsorption are discussed in comparison with the literature data for the adsorption of this compound on Hg [1,2] and on silver single-crystals [3,4]. The results obtained show that the adsorption process could be described by a Frumkin isotherm, the parameters of which were determined and discussed. The values of the standard Gibbs energy of adsorption suggest physical adsorption of the alcohol, without the existence of specific interactions of the adsorbed molecules with gold. Differences and inconsistencies for the results obtained from the analysis of charge and capacity data are discussed in terms of the experimental technique.

Effect of Potential on Coadsorption of Two Organic Substances in the Frumkin Model at a Maximum Possible Number of Solutions for the Relevant Set of Isotherms

Effect of the electrode potential on the coadsorption of two organic substances is studied using a set of two mixed Frumkin isotherms and a model of three parallel capacitors in conditions where in the absence of an electric field this system has seven solutions. It is shown that only four of these solutions may correspond to realizable adsorption states. Effect of the electrode potential on the number of possible solutions of the isotherm set, the number of realizable adsorption states, as well as on the composition and properties of a surface layer is considered.

Adsorption of Complexes of Cations of Alkali Metals with a Macrocyclic Ligand (Kryptofix® 222) on a Mercury Electrode

Adsorption of complexes of alkali metal cations with a macrocyclic ligand (C18H36N2O6, Kryptofix® 222) is studied as a function of their concentration by measuring the differential capacitance on a stationary mercury drop. The complexes (kryptates) possess a high adsorption capability. The adsorption parameters of sodium kryptate are found within a model of two parallel capacitors complemented by a Frumkin adsorption isotherm. Curves of the differential capacitance and the electrocapillary curves, calculated from these parameters, nicely conform to the experimental data obtained in this work and to the literature data.

Simulating unstable states during the coumarin adsorption on a mercury electrode

Dependences of differential capacitance C on potential E of a stationary electrode (hanging mercury drop) in aqueous 0.1 M NaF solutions containing 4.6 × 10–4 to 3 × 10–3 M C9H6O2 are obtained using an automatic impedancemeter. At coumarin concentrations below 0.001 M and potential slowly scanned near –1.1 V (SCE) the capacitance is unstable, which results in differently-shaped C vs. E curves in this potential range. The obtained results are attributed to nonequilibrium phase transitions in the adsorption layer, during which the orientation of coumarin molecules at the electrode surface alters. These phenomena are explained semiquantitatively on the basis of a developed theory.

Some anomalies in coadsorption of two organic substances due to differences in molecular interaction of species undergoing coadsorption

Effect of molecular interaction between species undergoing coadsorption on the potential dependences of coverages θ1and θ2, surface tension, and equilibrium and nonequilibrium differential capacitances is analyzed on the basis of a set of isotherms for coadsorption of components 1 and 2 within a common monolayer when the EDL inner part obeys a model of three parallel capacitors. It is shown that, in a certain potential interval, at a certain ratio between adsorption parameters, the set of isotherms may have solutions that correspond to a minimum in a free energy surface, which is a function of θ1and θ2.

Coadsorption of Halide Anions and 1-Adamantanol Molecules on a Mercury Electrode

Coadsorption of 1-adamantanol (AdOH) and halide anions (F–, Cl–, Br–) at the Hg/H2O interface is studied systematically. Experimental results are compared with calculations of these systems within a modified Alekseev–Popov–Kolotyrkin model complemented by a set of two Frumkin isotherms. A satisfactory agreement between experimental data and theoretical conclusions concerning dependences of the phase transition potentials and the adsorption-desorption potentials on the concentrations of supporting electrolyte and organic substances is established. The experimental and calculated potential dependences of the differential capacitance in the phase transition region in the case of the AdOH adsorption from electrolytes containing halide anions are compared. It is shown that the shape of experimental capacitance curves corresponds to either a weak attraction of species of different nature during concurrent adsorption or the absence of lateral interaction between them. Accounting for the contribution made by the energy of the diffuse double layer to the overall energy gain during the adsorption removes seeming contradictions in the interpretation of dependences studied.

Adsorption parameters in the Alekseev-Popov-Kolotyrkin model complemented by the Frumkin isotherm

The method of a nonlinear regressive analysis of nonequilibrium differential capacitance curves in terms of the Alekseev-Popov-Kolotyrkin (APK) model complemented by the Frumkin isotherm is used to determine adsorption parameters of an organic cation of tetrabutylammonium, an inorganic nitrate anion, and a butanol molecule at an interface with a mercury electrode. The adsorption behavior of the nitrate anion is quantitatively described by the APK model, whereas the Frumkin model is preferable in the case of butanol. With the [C4H9]4N+ cation adsorbed on a mercury electrode, the EDL structure is probably strongly affected by the formation of ionic pairs.

Effect of Differences in the Orientation of Adsorbed Molecules and in the Magnitudes of Limiting Capacitances of the Inner Part on the Regularities of Coadsorption of Two Organic Substances

The Alekseev–Popov–Kolotyrkin model, complemented with a set of mixed Frumkin isotherms, is used to perform an analysis of joint adsorption of two neutral organic substances when there are substantial differences in (1) limiting potential drops ψN1 and ψN2 and (2) capacitances of the inner part at θ1 = 1 and θ2 = 1. It is shown that in the absence of a lateral interaction between coadsorbed species and at a certain ratio between their concentrations, in either case there exists a region of potentials where the set of isotherms has three solutions. The stability of these solutions is characterized and the curves of the differential capacitance (equilibrium and nonequilibrium) and the interfacial tension are calculated. A general analysis of the set of isotherms in the region of physically reasonable values of adsorption parameters is conducted.