Adsorption electrical double-layer structure

In situ Fourier transform infrared and in situ infrared reflection spectroscopies have been used to study the electrical double layer structure and adsorption of various species at low-index single-crystal faces of Au, Pt, and other electrodes.206"210 It has been shown that if the ions in the solution have vibrational bands, it is possible to relate their excess density to the experimentally observed surface. 

As shown in the Figure, electro capillary curves are affected by - specific adsorption of ions (here, anions) at the electrode surface. Additionally, they are influenced by the - space charge region of the electrical double layer. Thus, electrocapillary curves as well as capacitance curves provide useful information on the electrical double-layer structure of electrode surfaces. 

The subsequent three chapters are devoted to the electric double-layer structure at the interface between immiscible electrolytes examined by the electrocapillary curves method (Prof. Senda and coauthors) and by measurement of the electric double-layer capacity (Dr. Samec and Dr. Mare ek) as well as to the investigation of the Galvani and Volta potentials in the above-mentioned systems (Prof. Koczorowski). These chapters will be of interest to many electrochemists since the results obtained here are comparable with the thoroughly studied metal/electrolyte solution interface. An insignificant potential shift in the compact layer at the interface between immiscible electrolytes in the absence of specific ion adsorption - this is the main conclusion arrived at by the authors of Chaps. 4 and 5. Chapter 6 deals with the scale of potentials in a system of immiscible electrolytes and the thermodynamic relation between the distribution coefficients and the Volta potentials. 

Islam, M. M. Alam, M. T. Okajima, T. Oshaka, T. (2009). Electrical double layer structure in ionic liquids an understanding of the xmusual capacitance-potential curve at a nonmetallic electrode. /. Phys. Chem. C, Vol. 113,3386-3389 Johnson, M. Nordholm, S. (1981). Generalized van der Waals theory. VI. Application to adsorption. /. Chem. Phys., Vbl. 75,1953-1957... 

Contaminant adsorption (competitive in mixtures with preferential adsorption of the largest-affinity contaminant), contaminant decomposi-tion/electrochemical reaction intermediates production, O reduction reaction pathway modification (atop Oj adsorption favored rather than bridged Oj, electric double layer structure change induced by cation insertion in iono-mer, Pt oxide modification including kinetics, changes in proton activity) or contaminant deposition reduces the catalyst area, increases the reduction reaction overpotential, decreases faradaic efficiency, and increases product selectivity (increased HjO contaminant production) Pt particle dissolirtion acceleration by adsorbed S on Pt from SOj or other soirrces decreasing iono-mer ionic conductivity... 

There are other components of the charging current which cannot be rigorously quantified. These are related to the changes in the electrical double layer structure that include adsorption and desorption, reorientation, and the changes in the electrode area caused by the electrode reaction. 

Clavilier eta/.196,794-796have studied CO adsorption on electrochemi-cally faceted Pt(lll) and Pt(110) electrodes and from the charge transients, with the provision that the CO dipole has a negligible contribution to the electrical double-layer potential these authors have provided a definite determination of ( =o- However, electrochemically faceted Pt(lll) electrodes have a polycrystalline surface structure, and thus the value of Eq-q for such electrodes lies between fiULO for terraces and forst s.197 786 787... 

Damaskin, B.B.andFrumkffiA. N. (1974) Potentials of zero charge, interaction of metals with water and adsorption of organic substances—111. The role of the water dipoles in the structure of the dense part of the electric double layer. Electrochim. Acta, 19, 173-176. 

Fig. 4.1 Structure of the electric double layer and electric potential distribution at (A) a metal-electrolyte solution interface, (B) a semiconductor-electrolyte solution interface and (C) an interface of two immiscible electrolyte solutions (ITIES) in the absence of specific adsorption. The region between the electrode and the outer Helmholtz plane (OHP, at the distance jc2 from the electrode) contains a layer of oriented solvent molecules while in the Verwey and Niessen model of ITIES (C) this layer is absent...

At present it is impossible to formulate an exact theory of the structure of the electrical double layer, even in the simple case where no specific adsorption occurs. This is partly because of the lack of experimental data (e.g. on the permittivity in electric fields of up to 109 V m"1) and partly because even the largest computers are incapable of carrying out such a task. The analysis of a system where an electrically charged metal in which the positions of the ions in the lattice are known (the situation is more complicated with liquid metals) is in contact with an electrolyte solution should include the effect of the electrical field on the permittivity of the solvent, its structure and electrolyte ion concentrations in the vicinity of the interface, and, at the same time, the effect of varying ion concentrations on the structure and the permittivity of the solvent. Because of the unsolved difficulties in the solution of this problem, simplifying models must be employed the electrical double layer is divided into three regions that interact only electrostatically, i.e. the electrode itself, the compact layer and the diffuse layer. 

If the electrolyte components can react chemically, it often occurs that, in the absence of current flow, they are in chemical equilibrium, while their formation or consumption during the electrode process results in a chemical reaction leading to renewal of equilibrium. Electroactive substances mostly enter the charge transfer reaction when they approach the electrode to a distance roughly equal to that of the outer Helmholtz plane (Section 5.3.1). It is, however, sometimes necessary that they first be adsorbed. Similarly, adsorption of the products of the electrode reaction affects the electrode reaction and often retards it. Sometimes, the electroinactive components of the solution are also adsorbed, leading to a change in the structure of the electrical double layer which makes the approach of the electroactive substances to the electrode easier or more difficult. Electroactive substances can also be formed through surface reactions of the adsorbed substances. Crystallization processes can also play a role in processes connected with the formation of the solid phase, e.g. in the cathodic deposition of metals. 

The inhibition of electrode processes as a result of the adsorption of electroinactive surfactants has been studied in detail at catalytically inactive mercury electrodes. In contrast to solid metal electrodes where knowledge of the structure of the electrical double layer is small, it is often possible to determine whether the effect of adsorption on the electrode process at mercury electrodes is solely due to electrostatics (a change in potential 02)... 

The processes classified in the third group are of primary importance in elucidating the significance of electric variables in electrosorption and in the double layer structure at solid electrodes. These processes encompass interactions of ionic components of supporting electrolytes with electrode surfaces and adsorption of some organic molecules such as saturated carboxylic acids and their derivatives (except for formic acid). The species that are concerned here are weakly adsorbed on platinum and rhodium electrodes and their heat of adsorption is well below 20 kcal/mole (25). Due to the reversibility and significant mobility of such weakly adsorbed ions or molecules, the application of the i n situ methods for the surface concentration measurements is more appropriate than that of the vacuum... 

The mechanism of interaction of amino acids at solid/ aqueous solution interfaces has been investigated through adsorption and electrokinetic measurements. Isotherms for the adsorption of glutamic acid, proline and lysine from aqueous solutions at the surface of rutile are quite different from those on hydroxyapatite. To delineate the role of the electrical double layer in adsorption behavior, electrophoretic mobilities were measured as a function of pH and amino acid concentrations. Mechanisms for interaction of these surfactants with rutile and hydroxyapatite are proposed, taking into consideration the structure of the amino acid ions, solution chemistry and the electrical aspects of adsorption. 

The ssDNA was immobilized stronger and faster on the GC surface in the presence of the lipid membrane than on a bare GC surface and using milder conditions [61]. The lipid membrane enhanced the stabihty of ssDNA towards desorption from the GC surface [61,62]. Moreover, the adsorption of ssDNA on BLM induced a conductance enhancement due to (1) structural changes (i.e., defect sites) within the membrane and (2) the increase in negative surface charge density of the membrane. The charge of the phosphate groups of ssDNA induced an increase of cation concentration in the electrical double layer [63]. 

The electrical double-layer (edl) properties pose a fundamental problem for electrochemistry because the rate and mechanism of electrochemical reactions depend on the structure of the metal-electrolyte interface. The theoretical analysis of edl structures of the solid metal electrodes is more complicated in comparison with that of liquid metal and alloys. One of the reasons is the difference in the properties of the individual faces of the metal and the influence of various defects of the surface [1]. Electrical doublelayer properties of solid polycrystalline cadmium (pc-Cd) electrodes have been studied for several decades. The dependence of these properties on temperature and electrode roughness, and the adsorption of ions and organic molecules on Cd, which were studied in aqueous and organic solvents and described in many works, were reviewed by Trasatti and Lust [2]. 

Fig. 2.2 Structure of the electric double layer under different conditions of electrode polarization (a) metal positively charged, anions present at the inner Helmholtz plane (chemically interacting with metal) and in the diffuse double layer beyond the outer Helmholtz plane (b) metal negatively charged, inner Helmholtz plane empty, cations in diffuse layer (c) metal positively charged, strong adsorption of anions in inner Helmholtz plane, balancing cations in the diffuse layer...

At the next level we also take specific adsorption of ions into account (Fig. 4.6). Specifically adsorbed ions bind tightly at a short distance. This distance characterizes the inner Helmholtz plane. In reality all models can only describe certain aspects of the electric double layer. A good model for the structure of many metallic surfaces in an aqueous medium is shown in Fig. 4.6. The metal itself is negatively charged. This can be due to an applied potential or due to the dissolution of metal cations. Often anions bind relatively strongly, and with a certain specificity, to metal surfaces. Water molecules show a distinct preferential orientation and thus a strongly reduced permittivity. They determine the inner Helmholtz plane. 

As Schmickler states [3], Electrochemistiy is the study of structures and processes at the interface between an electronic conductor (the electrode) and an ionic conductor (the electrolyte) or at the interface between two electrolytes . The electrode/electrolyte or electrolyte/electrolyte interface is the region whose properties differ from the two adjoining phases, and/or the place where reactant adsorption and electrochemical reactions occur. Commonly, it is recognized as the interface between an electronic conductor (e.g., metals and semiconductors) and an ionic conductor (e.g., electrolyte solutions, molten salts, and solid electrolytes), known as an electrochemical interface. In a narrow region of an electrode/electrolyte interface, an electrical double layer (EDL) exists. The EDL is believed to be extremely thin, and is an important component of the interface. 

At the interface between O and W, the presence of the electrical double layers on both sides of the interface also causes the variation of y with Aq<. In the absence of the specific adsorption of ions at the interface, the Gouy-Chapman theory satisfactorily describes the double-layer structure at the interface between two immiscible electrolyte soultions [20,21]. For the diffuse part of the double layer for a z z electrolyte of concentration c in the phase W whose permittivity is e, the Gouy-Chapman theory [22,23] gives an expression... 

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