Adsorption capacitive nature

Tlie capacitive nature of the adsorption pseudocapacitance can be further illustrated by considering its ac response. Let us assume that a low amplitude sinusoidal voltage signal is applied to a system at equilibrium ... 

As we noted earher, at w = 1, Eq. (8.24) transforms into the relationship for electrical capacitance. The average value of n for Q j is 0.9, and this is indicative of the capacitive nature of Q j, which can be considered the electrical analog to the double layer. Then, it is possible to assume that the parameter Eq approximately represents the double-layer capacitance, which amounts to 70—90 pF cm . Somewhat higher values of Eq were obtained for the Cu Cu(II) system in the absence of adsorption of organic substances [71, 76]. In the IPS series, the total Cu(ll) concentration varied from 6.5 to 15 mM. According to the well-known equation for Warburg impedance [77], it is possible to estimate the value of Eq, which amounts to about 0.1 cm s - for a 10 mM Cu(II) solution and the transfer of one elec-... 

Metal/molten salt interfaces have been studied mainly by electrocapillary833-838 and differential capacitance839-841 methods. Sometimes the estance method has been used.842 Electrocapillary and impedance measurements in molten salts are complicated by nonideal polarizability of metals, as well as wetting of the glass capillary by liquid metals. The capacitance data for liquid and solid electrodes in contact with molten salt show a well-defined minimum in C,E curves and usually have a symmetrical parabolic form.8 10,839-841 Sometimes inflections or steps associated with adsorption processes arise, whose nature, however, is unclear.8,10 A minimum in the C,E curve lies at potentials close to the electrocapillary maximum, but some difference is observed, which is associated with errors in comparing reference electrode (usually Pb/2.5% PbCl2 + LiCl + KC1)840 potential values used in different studies.8,10 It should be noted that any comparison of experimental data in aqueous electrolytes and in molten salts is somewhat questionable. 

In contrast to the ionizing electrode method, the dynamic condenser method is based on a well-understood theory and fulfills the condition of thermodynamic equilibrium. Its practical precision is limited by noise, stray capacitances, and variation of surface potential of the air-electrode surface, i.e., the vibrating plate. At present, the precision of the dynamic condenser method may be limited severely by the nature of the surfaces of the electrode and investigated system. In common use are adsorption-... 

The main, currently used, surface complexation models (SCMs) are the constant capacitance, the diffuse double layer (DDL) or two layer, the triple layer, the four layer and the CD-MUSIC models. These models differ mainly in their descriptions of the electrical double layer at the oxide/solution interface and, in particular, in the locations of the various adsorbing species. As a result, the electrostatic equations which are used to relate surface potential to surface charge, i. e. the way the free energy of adsorption is divided into its chemical and electrostatic components, are different for each model. A further difference is the method by which the weakly bound (non specifically adsorbing see below) ions are treated. The CD-MUSIC model differs from all the others in that it attempts to take into account the nature and arrangement of the surface functional groups of the adsorbent. These models, which are fully described in a number of reviews (Westall and Hohl, 1980 Westall, 1986, 1987 James and Parks, 1982 Sparks, 1986 Schindler and Stumm, 1987 Davis and Kent, 1990 Hiemstra and Van Riemsdijk, 1996 Venema et al., 1996) are summarised here. 

Adsorption of cyanide anions can be affected by adsorption of cations. In the solutions containing nonspecifically adsorbed anions, the nature of alkali metal cations was found to influence the measured value of the electrode capacitance at potentials more negative than —0.6 V (versus standard hydrogen electrode (SHE)). At < —l.OV adsorption of CN ions was enhanced in the presence of Li+ and Na+ cations, and inhibited in the presence of Cs+ ions [81]. A combined SERS and density-functional theory has been applied to study cyanide adsorption at Au electrode [82]. The authors have arrived at the conclusion that the polarity of Au—CN bonds falls between that of Au—Cl and Au—Br surface bonds. The binding strength for three different gold surfaces decreased in the order ... 

The objective of most electrochemical experiments is to allow the experimenter to investigate one or more of three types of parameters (1) the concentration and identity of one or more solution components, (2) the kinetics of chemical, charge transfer, or adsorption processes, and (3) the nature of the double-layer capacitance associated with the electrode-solution interface. Historically, most small-amplitude techniques have been developed in an attempt to allow an easier separation of the contributions of these basic parameters. 

In fig. 3.20b specific adsorption Is also accounted for. The notion of specific adsorption has been defined In sec. 3.3. In disperse systems, its occurrence is de facto Inferred from the dependence of certain double layer properties on the natures of counter- and co-lons Generally, ions interacting specifically (non-electrostatlcally) with the surface approach it to shorter distance p < d). The plane where these specifically adsorbed ions reside is called the inner Helmholtz plane (iHp) In colloid science, the model of fig. 3.20b Is also known as the triple layer model. In this model three charges and three capacitances can be distinguished. For the two inner layer differential capacitances... 

Nature of the Surface Complexes. The constant capacitance model assumes an inner-sphere molecular structure for surface complexes formed in reactions like equation 5a or 7. But this structure does not manifest itself explicitly in the composition dependence of Kc everything molecular is buried in which is an adjustable parameter. This encapsulating characteristic of the model was revealed dramatically by Westall and Hohl (13), who showed that five different surface speciation models, ranging from the Gouy-Chapman theory to the surface complex approach, could fit proton adsorption data on AL O., equally well, despite their mutually contradictory underlying molecular hypotheses [see also Hayes et al. (19)]. They concluded that "... no model will yield an unambiguous description of adsorption. .. . To this conclusion one may add that no model should provide such a description,... 

Some adsorption models attempt to adjust for the nonconstant nature of the equilibrium constants. The constant-capacitance model uses K values for dissociation and metal complexation that are adjusted for the efiect of the electrical potential at the surface, employing the empirical reaction... 

More information about the ubiquitous presence in seawater of natural organic surfactants has been obtained by the use of seawater/solid interfaces than from studies of the interface between seawater and air. For practical reasons, it is usually very much simpler to study the adsorption of organic matter from solution onto solid surfaces, where a variety of powerful techniques such as electrocapillarity, electrical double-layer capacitance measurements, electrophoresis and ellipsometry can be used to study the progress of adsorption and the nature of the adsorbed layer. Neihof and Loeb (1972, 1974) and Loeb and Neihof (1975, 1977) have demonstrated by electrophoresis and ellipsometry that a wide variety of solid surfaces become covered by a strongly adsorbed film of polymeric acids upon exposure to seawater. Hunter (1977) found the same type of effect and has shown by electrophoretic studies at different pH and metal ion concentrations that phenolic and carboxylic groups are probably responsible. This adsorbed organic material seems hkely to represent an important part of the natural surfactants in seawater and, as such, will adsorb at the air/sea interface as well. 

Comprehensive recent discussions of the constant capacitance model are given in P. W. Schindler, Surface complexes at oxide-water interfaces, in Adsorption of Inorganics at Solid-Liquid Interfaces (M. A. Anderson and A. J. Rubin, eds. Ann Arbor Science, Ann Arbor, Michigan, 1981) and in H. Hohl, L. Sigg, and W. Stumm, Characterization of surface chemical properties of oxides in natural waters, in M. C. Kavanaugh and J. O. Leckie, op. cit. A computer-based algorithm for the constant capacitance model is described by J. Westall, op. cit. ... 

Is the smeared stop-spike for ATh (Fig. 14) what we see for a hump on the measured capacitance curve, or is it screened by molecular reorientations. We don t know that. Moreover, the specific adsorption of ions, which was assumed absent, can give rise to a hump too. Somehow this feature is interesting because of its quantum-mechanical origin and direct dependence on the nature of the metal. The more densely packed the metal surface, the more abrupt the stop, the sharper the spike. Open surfaces should favor soft landing and the hump would... 

The popularity of the cychc voltammetry (CV) technique has led to its extensive study and numerous simple criteria are available for immediate anal-j sis of electrochemical systems from the shape, position and time-behaviour of the experimental voltammograms [1, 2], For example, a quick inspection of the cyclic voltammograms offers information about the diffusive or adsorptive nature of the electrode process, its kinetic and thermodynamic parameters, as well as the existence and characteristics of coupled homogeneous chemical reactions [2]. This electrochemical method is also very useful for the evaluation of the magnitude of imdesirable effects such as those derived from ohmic drop or double-layer capacitance. Accordingly, cyclic voltammetry is frequently used for the analysis of electroactive species and surfaces, and for the determination of reaction mechanisms and rate constants. 

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