Double layer structure Diffuse

The electrical double-layer structure at Ga/DMF, In(Ga)/DMF, and Tl(Ga)/DMF interfaces upon the addition of various amounts of NaC104 as a surface-inactive electrolyte has been investigated by differential capacitance, as well as by the streaming electrode method.358 The capacitance of all the systems was found to be independent of the ac frequency, v. The potential of the diffuse layer minimum was independent of... 

The electrical double-layer structure of a Pt/DMSO interface has been investigated using the potentiostatic pulse method.805 The value of C at E = const, as well as the potential of the diffuse layer minimum, have been found to depend on time, and this has been explained by the chemisorption of DMSO dipoles on the Pt surface, whose strength depends on time. Eg=Q has been found11 at E = -0.64 V (SCE in H2O). 

The presence of the diffuse layer determines the shape of the capacitance-potential curves. For a majority of systems, models describing the double-layer structure are oversimplified because of taking into account only the charge of ions and neglecting their specific nature. Recently, these problems have been analyzed using new theories such as the modified Poisson-Boltzmann equation, later developed by Lamper-ski. The double-layer capacitanties calculated from these equations are... 

In considering the effect of the double-layer structure on electrode kinetics (Section 7.3.1), it was pointed out that the existence of a diffuse chaige region causes the concentration at the outer Helmholtz plane to differ from the bulk concentration (Fig. 7.19). The consumption of electron acceptors by the electronation reaction and... 

The structure of the double layer can be altered if there is interaction of concentration gradients, due to chemical reactions or diffusion processes, and the diffuse ionic double layer. These effects may be important in very fast reactions where relaxation techniques are used and high current densities flow through the interface. From the work of Levich, only in very dilute solutions and at electrode potentials far from the pzc are superposition of concentration gradients due to diffuse double layer and diffusion expected [25]. It has been found that, even at high current densities, no difficulties arise in the use of the equilibrium double layer conditions in the analysis of electrode kinetics, as will be discussed in Sect. 3.5. 

A significant number of studies have characterized the physical properties of eutectic-based ionic liquids but these have tended to focus on bulk properties such as viscosity, conductivity, density and phase behavior. These are all covered in Chapter 2.3. Some data are now emerging on speciation but little information is available on local properties such as double layer structure or adsorption. Deposition mechanisms are also relatively rare as are studies on diffusion. Hence the differences between metal deposition in aqueous and ionic liquids are difficult to analyse because of our lack of understanding about processes occurring close to the electrode/liquid 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... 

Now, although the diffuse double layer is very important to our understanding of double-layer structure, we must ask ourselves how important it is for electrode kinetics. 

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. 

Any modification in the double layer potential affects the concentration profile of ions that reside within the double layer structure. In the case of an enzyme-substrate reaction, in which a nonequilibrium contribution to the potential arises due to differences in substrate and product diffusion coefficient, as seen from Eq. (16), variation in the equilibrium concentration profiles of not only charged reactant and charged product but also variations in the concentration profiles of nonreacting ionic species present is possible.The concentration profiles of all ionic species adjust according to changes in the reaction rate, In order to... 

The physics of ILs at surfaces are important for a deeper understanding of the resulting properties and enables the design of appHcations. Each combination of cation and anion can lead to a different behavior on surfaces of sohds, because the molecular structure of each IL has a strong influence of the formation of layers at the interfaces. In aqueous electrolytes the Hehnholtz-model and its further developments are describing the physics in a sufficient way The Gouy-Chapman-model takes the diffusion into account, and the Stem-model combines the formation of a double layer with diffusion. Compared to aqueous solutions of salts, the situation in ILs is different The ions have no solvent environment Their next neighbors are also ions. As a consequence the physics at the interfaces between sohds and ILs cannot be described by the common models. 

The double-layer structure at ITIES shows different features than that formed at the metal/electrolyte interface. The charge distribution in the both phases preserves its diffuse property even in rather concentrated solutions [7]. The potential difference in the compact double-layer is much smaller than the potential differences in the adjacent diffuse layers. Thus, practically, the overall potential difference only consists of the potential difference in the diffuse layers 02(w) and (f) o)... 

Investigations of the properties of polarizable interfaces of two immiscible electrolyte solutions, as well as of their double-layer structures and zero-charge potentials were undertaken in numerous works [61,145,148]. Samec et al. [149] on the basis of the pioneer works by Verwey and Niessen [96], as well as by Gross et al. [61], formulated a model of the double layer. In this model a layer of oriented solvent molecules (the inner layer) separates two diffuse layers of the Gouy-Chapman type. 

Oil/water interfaces are classified into the ideal-polarized interface and the nonpolarized interface. The interface between a nitrobenzene solution of tetrabutylam-monium tetraphenylborate and an aqueous solution of lithium chloride behaves as an ideal-polarized interface in a certain potential range. Electrocapillary curves of the interface were measured. The results are analyzed using the electrocapillary equation of the ideal-polarized interface and the Gouy-Chapman theory of diffuse double layers. The electric double layer structure consisting of the inner layer and the two diffuse double layers on each side of the interface is discussed. Electrocapillary curves of the nonpolarized oil/water interface are discussed for two cases of a nonpolarized nitrobenzene/water interface. 

Nonaqueous Solvents for Electrochemical Use, Charles K. Mann Use of the Radioactive-Tracer Method for the Investigation of the Electric Double-Layer Structure, N.A. Balashova and V.E. Kazarinov Digital Simulation A General Method for Solving Electrochemical Diffusion-Kinetic Problems, Stephen W. Feldberg... 

As we have discussed in Chapter 5, the electrical double layer, as depicted in Figure 7.1, plays a critical role in the distribution of electrical potential at the electrode-electrolyte interfaces and to the ion transport through the electrolyte from the anode side to the cathode side. Figure 7.1 shows the comprehensive details of the electrical double layer structure, which is composed of an inner Helmholtz plane (IHP), an outer Helmholtz plane (OHP), and the diffusion layer. 

The double-layer capacitance is composed of several contributions. In a geometrical sense the double layer in "supported" systems is represented by the compact "Helmholtz" or "Stem" layer. The electrostatically attracted solvated species reside in the "outer Helmholtz plane" (OHP), and specifically adsorbed species reside closer to the electrode in the "inner Helmholtz plane" (IHP). The double-layer structure is completed by a "diffuse" layer, composed of electrostatically attracted species at some distance from the electrode surface. The fuU thickness of the double layer can be defined as the external boundary of the diffuse layer separating it from the bulk solution, where the measured potential becomes equal to that of the bulk solution and no local potential gradient driven by the difference between the electrode potential ( )j and the solution potential can be determined (Figure 5-4). 

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