Specific adsorption double-layer structure

The double layer structure is totally different in the two liquids and hence the surface potential will differ, meaning that specific adsorption of organics will differ,... 

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. 

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... 

Equation (a), with set equal to ( >, is surprisingly successful in describing the effect of varying the double-layer structure upon the kinetics of electrochemical reactions at Hg electrodes, at least in the absence of specific adsorption of the supporting electrolyte (i.e., when the inner-layer region adjacent to the electrode contains only solvent molecules). However, this does not necessarily imply that average electrostatic interactions provide the sole contribution to the work terms, because contributions may arise from other sources that remain constant under these conditions. In particular, inner-sphere pathways commonly involve reaction sites within the outer Helmholtz plane. Consequently, the overall work terms consist of separate contributions from transporting the reactant from the bulk solution to this outer plane and from this plane to the reaction site within the inner layer. The latter will then be independent of and, therefore, influence only k j.. in Eq. (a). 

The structure of the double layer can affect the rates of electrode processes. Consider an electroactive species that is not specifically adsorbed. This species can approach the electrode only to the OHP, and the total potential it experiences is less than the potential between the electrode and the solution by an amount 2 — which is the potential drop across the diffuse layer. For example, in 0.1 M NaF, 2 - (f> is -0.021 V at = -0.55 V vs. SCE, but it has somewhat larger magnitudes at more negative and more positive potentials. Sometimes one can neglect double-layer effects in considering electrode reaction kinetics. At other times they must be taken into account. The importance of adsorption and double-layer structure is considered in greater detail in Chapter 13. 

Typically, a supercapacitor is composed of two electrodes dipped in an electrolyte solution with a suitable separator. It is generally accepted that the energy storage mechanism of supercapacitors can be classified into electrical double layer capacitors (EDLCs) and pseudocapacitors (Fig. 6.1A) (Jost et al., 2014). In EDLCs, the charge storage is based on a reversible ion adsorption from an electrolyte onto electrodes with high specific surface areas to form a double layer structure. The capacitance comes from the pure electrostatic... 

Fig. 9 Schematic representation of the double-layer structure of the interface between nitrobenzene and aqueous solutions in the presence of the specific adsorption of hexadecyltrimethylammonium ions [21]. (Reproduced by permission of the Chemical Society of Japan.)...

Although many models for the double layer have been published in the literature, there is no general model that can be used in all experimental situations. This is because the double-layer structure and its capacity depend on several parameters such as electrode material (metals, types of carbon, semiconductors, material porosity, the presence of layers of either oxides or polymeric films or other solid materials at the surface), type of solvent, type of supporting eleetrolyte, extent of specific adsorption of ions and molecules, and temperature. 

The use of in situ FTIR spectroscopy to study ions adsorption is related to the need of experimental tools to access the doublelayer structure with molecular specificity. This is especially true for the solid metals, where the study of the double layer using the classical capacitance studies is not easy. Spectroscopy combined with in situ scanning tunneling microscopy (STM) [42, 43] constitutes a very powerful approach, allowing the determination of the molecular identity and organization of the doublelayer components. However, it is not the purpose of this chapter to review in detail the double-layer structure, but rather, to show how in situ infrared spectroscopy can contribute to the understanding of the double layer at molecular specificity. 

The structure of the interfacial region is not considered in the derivation of the butler-Volmer equation, and it is implicit in that treatment that the whole potential drop between the metal and the solution is effective in driving the charge transfer reaction. A more exact treatment takes into account the effect of the double layer structure. The simplest model neglects specific adsorption effects and assumes for theoretical convenience that the electroactive species is situated at the Outer Helmholtz Plane when the charge transfer occurs. The corrections are twofold ... 

When the electrolyte is changed, it will not only change the double-layer structure of the electrochemical interface but also influence the electrochemical reaction rates, and even the potential window. For example, the double layer will be compressed or expanded with the increase or decrease in the concentration of the electrolyte, respectively. Depending on the electrolyte, nonspecific or specific adsorption may occur on the electrode surface. The specifically adsorbed ions, due to a strong interaction with the metal surface, will possibly induce a shift of surface plasmon resonance. The electrolyte ion may also be coadsorbed with the adsorbates in a competitive or induced way for example, thiourea is coadsorbed with CIO4 and/or 804 on the Ag electrode [38]. 

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. 

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. 

The fundamental electrochemical event, that is, electron transfer, occurs at the electrode surface. Peculiarities of electrochemical reactions include an electrical field, which in a special way complicates the phenomena of adsorption and desorption at the surface. The first layer of the solution, which is in contact with the electrode, possesses a specific structure. It is important for charged particles that the orientation of medium molecules in the vicinity of the electrode produces a decrease in dielectric permeability in the compact part of the double layer (Damaskin and Kryshtalik 1984). 

Specific adsorption may involve short-range, strong interactions due to the overlapping of the electronic orbitals of the adsorbate and the electrode and ionic species or dipoles in the electrolyte. These will be considered in Sect. 6.1 together with the effect of changes of the structure of the interfacial region on electrode kinetics (double layer effects [3,5]). 

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. 

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