Adsorbed electrolyte layers

Adsorbed electrolyte layers In this case the water molecules are bound to the metal surface by Van der Waals forces. It is estimated that at 55% r.h. the film on polished iron is about 15 molecular layers thick, increasing to 90 molecular layers at just below 100% r.h.. Such films are capable of... 

M. Stratmann, The investigation of the corrosion properties of metal surfaces covered by condensed or adsorbed electrolyte layers, in Proceedings of 1st International Symposium on Corrosion of Electronic Materials and Devices 0. D. Sinclair, ed.). The Electrochemical Society, Pennington, NJ,1991,p.l. 

While the pzc of Hg in F solution has not changed by more than 1 mV for over 70 years, marginal variations are visible for Ga, Tl, In, Cd, Bi, Sn, and Sb that are related to electrolyte effects (weak specific adsorption or disturbance of the adsorbed water layer, as for Ga).847 Important variations can be seen, on the other hand, for polycrystalline Ag, Zn, Ni, Fe, and Cu. For all these metals a drop of the pzc to much more negative values has been recorded this is evidently related to an improvement in the preparation of the surface with more effective elimination of surface oxides. All these metals, with the exception of Ag, are naturally sensitive to atmospheric oxygen. Values of pzc for single-crystal faces first appeared in a 1974 compilation,23 in particular for the three main faces of Ag and for Au (110). Values for a number of other metals were reported in 1986.25 However, for sd-metals, an exhaustive, specific compilation of available experimental data was given by Hamelin etal. in 1983.24... 

The enhanced adsorption of anions and other substances that occurs at increasingly positive potentials causes a gradual displacement of water (or other solvent) molecules from the electrolyte layer next to the electrode. This leads to a markedly slower increase in the rate of oxygen evolution from water molecules and facilitates a further change of potential in the positive direction. As a result, conditions arise that are favorable for reactions involving the adsorbed species themselves (Fig. 15.9). In particular, adsorbed anions are discharged forming adsorbed radicals ... 

In the early work of Bewick and Robinson (1975), a simple monochromator system was used. This is called a dispersive spectrometer. In the experiment the electrode potential was modulated between two potentials, one where the adsorbed species was present and the other where it was absent. Because of the thin electrolyte layer, the modulation frequency is limited to a few hertz. This technique is referred to as electrochemically modulated infrared reflectance spectroscopy (EMIRS). The main problem with this technique is that data acquisition time is long. So it is possible for changes to occur on the electrode surface. 

The charge density on the electrode a(m) is mostly found from Eq. (4.2.24) or (4.2.26) or measured directly (see Section 4.4). The differential capacity of the compact layer Cc can be calculated from Eq. (4.3.1) for known values of C and Cd. It follows from experiments that the quantity Cc for surface inactive electrolytes is a function of the potential applied to the electrode, but is not a function of the concentration of the electrolyte. Thus, if the value of Cc is known for a single concentration, it can be used to calculate the total differential capacity C at an arbitrary concentration of the surface-inactive electrolyte and the calculated values can be compared with experiment. This comparison is a test of the validity of the diffuse layer theory. Figure 4.5 provides examples of theoretical and experimental capacity curves for the non-adsorbing electrolyte NaF. Even at a concentration of 0.916 mol dm-3, the Cd value is not sufficient to permit us to set C Cc. 

A working definition of a surface state as any energy level within the bandgap that is bound to the surface sufficiently to allow inelastic electron transfer to or from the semiconductor bonds was introduced. This allows adsorbed electrolyte species, reaction intermediates and attached layers to be considered as surface states. The experimental observations discussed illustrate such states. 

Steric stabilization differs from electrostatic stabilization in not being a function of a net force, but of the thickness of an adsorbed layer. When < >, equals 5-10%, stabilizing and destabilizing forces extend beyond the length of the electrostatic, interparticle barrier (Cabane et al., 1989). At this distance, attraction and repulsion are inconsequential, and electrolytes therefore have little effect. Bergenstahl (1988) proposed that the steric stabilization of emulsions by gums in the presence of a surfactant involves adsorption of the gum on the surfactant to form a combined structure constituted by a primary surfactant layer covered by an adsorbed polymer layer. 

It is clear that a perfect agreement with experiment cannot be provided by a theory which ignores the additional interactions between ions, and ions and surfaces, not included in the mean field potential (such as image forces,14 excluded volume effects,15 and ion-dispersion16 or ion-hydration forces17). However, it will be shown that the experimental results reported by Lopez-Leon et al.1 can be more than qualitatively reproduced for uniunivalent electrolytes by the present polarization model for hydration/double layer forces, if one accounts for the association equilibria with the surface sites for all the ions present in the electrolyte (H+, OH , anions, and cations).11 Some additional reasons for the quantitative disagreements, involving the structural modifications of the adsorbed protein layer and the nonuniformity of the colloidal particles, will be also noted. 

An important issue that has to be emphasized is that the experimentally determined dependence ofthe stability ratio on electrolyte concentration, at low ionic strengths, exhibits (at least for NaSCN) a strongly non-DLVO behavior, in a range in which the DLVO theory is considered fairly accurate. Therefore, we are inclined to believe that the electrolyte (even at low ionic strength) induces indeed structural modifications of the adsorbed protein layer at least near the interface. 

The adsorption and structure of anions such as bromide, cyanide, sulfate/bisulfate, and iodide on metal electrodes have been extensively studied by in-situ STM in electrolyte solutions. Figure 41a displays a cyclic voltammogram for an Au(lll) electrode in 1-mM KI solution. The anodic/cathodic peaks below 0 V versus Ag/AgI are associated with adsorption/desorption of iodine at the surface. The smaller peaks at 0.5V are due to a phase transition in the adsorbed iodine layer, as can be observed by STM images taken at various electrode potentials. STM images shown in Figure 41b taken at a potential of —0.2 V show a periodic structure with perfect... 

If gaseous, electrochemicaUy active components of the measuring environment are not dissolved in the electrode, then the electrode process will consist of the following stages (also shown in Figure 1.18). They are adsorption-desorption of electrochem-icaUy active gaseous components on gas-electrolyte (GE) and gas-metal (GM) interfaces, ionization reaction (with electron transfer) on the metal-electrolyte (ME) and gas-electrolyte interfaces, and mass-transfer processes on all boundaries of three phases (gas-metal, gas-electrolyte, and metal-electrolyte). Furthermore, mass transfer of electrons and holes on the surface electrolyte layer may also occur. It is evident that the quantity of the current in the stationary state is equal to the quantity of the nonmetal component adsorbing on the gas-metal and gas-electrolyte surfaces as a result of ionization of this component on the ME and GE surfaces. 

In combination with adsorption measurements, electro-optics is proved to be a powerful technique for studying the structure of adsorbed polyelectrolyte layers. Our data show flat conformation of the adsorbed macromolecules, which slightly depends on the polyelectrolyte charge density in accordance with the theory for weak poly electrolytes. Counterion condensation is also suggested on the surface of weakly charged poly electrolytes, which has not been predicted from the theory. 

The determination of the real surface area of the electrocatalysts is an important factor for the calculation of the important parameters in the electrochemical reactors. It has been noticed that the real surface area determined by the electrochemical methods depends on the method used and on the experimental conditions. The STM and similar techniques are quite expensive for this single purpose. It is possible to determine the real surface area by means of different electrochemical methods in the aqueous and non-aqueous solutions in the presence of a non-adsorbing electrolyte. The values of the roughness factor using the methods based on the Gouy-Chapman theory are dependent on the diffuse layer thickness via the electrolyte concentration or the solvent dielectric constant. In general, the methods for the determination of the real area are based on either the mass transfer processes under diffusion control, or the adsorption processes at the surface or the measurements of the differential capacitance in the double layer region [56],... 

In this case, it is useful to apply the so-called Parsons-Zobel plot [58] that shows the dependence of the reciprocal measured differential capacity on the reciprocal of the calculated diffuse layer capacity at a constant charge density covering wide spectra of electrolyte concentrations. The slope of this plot must be a straight line and equal to the R when we have a non-adsorbing electrolyte. However, it has been observed that at a low electrolyte concentration the linearity is lost and the slope becomes lower [57] (electrolyte concentrations below 0.01 M). 

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