Gibbs energy of adsorption

Figure 26. Plot of the Gibbs energy of adsorption of organic substances at a = 0 vs. the interfacial parameter, AX. (1) 1-Hexanol, (2) 1-pentanol, and (3) acetonitrile. From Ref. 32, updated. Additional points (1) Au(l 11),910 Bi(l 11),152 and (2) Ga916...

Figure 27. Gibbs energy of adsorption of water from the bulk of the solution on the given metals as calculated by Afanasyev and Akulova.909 The figures on top of the bars are the values of the interfacial parameter, AX.

Figure 28. Dependence of the Gibbs energy of adsorption of diols on the number of carbon atoms in the molecule. Data for Hg from Ref. 912 data for Au from Ref. 911.

The term free enthalpy of adsorption is synonymonsly used for free energy of adsorption and Gibbs energy of adsorption. 

These considerations can also be used to derive the Dupre equation, where AG(s) is the Gibbs energy of adsorption of the solvent per unit area of the metal surface ... 

The standard Gibbs energy of adsorption AGads is mostly a function of the electrode potential. In the simplest model, the adsorption of a neutral substance can be conceived as the replacement of a dielectric with a larger dielectric constant (the solvent) by a dielectric with a smaller dielectric... 

If the Gibbs energy of adsorption AG-M is considered as independent of the coverage the resulting formula is known as the Langmuir isotherm this assumption is reasonable when the interaction between the adsorbed particles is small. 

The difference between the potential of the current peak for the desorption and the bulk deposition potential is known as the underpotential shift simple systems the value of Gibbs energies of adsorption and deposition shift both according to the Nernst equation. Deviations from this behavior may indicate coadsorption of other ions. 

The Gibbs energy of adsorption on a particular metal is difficult to measure, but it should be related to the strength jEm-H of the corre-... 

A similar correlation was found between the Gibbs energy of adsorption AG%ds and the interfacial parameter Mat = AG°adsincludes... 

Assuming this standard state, the AC° value expresses a change in the Gibbs energy of adsorption of one molecule B upon being moved from the hypothetical ideal solution onto the electrode surface. This enables the particle-particle interactions on the surface to be separated from any other interactions and to be included in the term/(/i). 

The relation between a universal Gibbs energy of adsorption AG°(= -RT np) and the energies calculated from different isotherms can be expressed as follows a. Frumkin isotherm... 

Therefore the determination of the standard Gibbs energies of adsorption at various symmetrical or unsymmetrical standard states leads directly to derivation of the particle-particle interaction parameter. The same result may be obtained from the difference of AG"" values calculated at zero surface coverage (0 = 0) and at saturated surface coverage (0=1), using Eqs. (30a) and (30b). 

The interdependence of the Gibbs energy of adsorption and the molecular interaction parameter was recently discussed in detail by Karol-czak, who used a six-parameter model. Contrary to the rather general Damaskin model, no relation between the molecular interaction parameter A and AGads was assumed. It was suggested that this is an arbitrary relation dependent on the theoretical model used in fitting experimental data within acceptable experimental errors. 

The Gibbs energy of adsorption is a measure of adsorbate-metal interactions. Its values depend, however, on the choice of standard states for the chemical potentials of the components involved in the process. Therefore AG° values determined for different systems can only be compared if they refer to the same standard-state conditions. AG° values of adsorption of thiourea (TU) on several metallic electrodes, calculated for the most often used standard states, are presented in Table 1. 

The determination of the Gibbs energy of adsorption at zero surface coverage AGg=o nd of the interaction parameter A as a function of an electrical variable, may become a valuable source of information on the interactions at the interface. The value of AG°can be considered as the energy required to replace n monomolecularly adsorbed solvent molecules from a fully solvent-covered electrode surface by one monomeric molecule of the solute... 

As mentioned earlier, the Gibbs energy of adsorption can be analyzed using one of two independent electrical variables potential or charge density. The problem was discussed by Parsons and others, but it was not unequivocally solved because both variables are interconnected. Recent studies of the phase transition occurring at charged interfaces, performed at a controlled potential, show that if the potential is... 

The contribution of various interactions to the Gibbs energy of adsorption can be described by an equation proposed by Trasatti ... 

The AG° values of acetone adsorption decrease slightly in the sequence H2O,Me0H, NM. They are indicative of a weak physical adsorption at the Hg/solution interface. It is also evident that the Gibbs energy of adsorption is enhanced by the electric field, particularly at the point of adsorption maximum. Small values of AG , similar to those determined at the solution/air interface, attest to the absence of specific interactions cf acetone with the mercury surface (which is opposite to the TU adsorption case). Hence, the solute-solvent interaction in the solution is an important factor in the adsorption of acetone, as shown for the zero charge on the Hg electrode in Fig. 11. 

Gomez etal. [160] have studied specific adsorption of potassium penicillin G (salt K) on Hg electrode from electrocapillary and capacity measurements. The Frumkin isotherm has been found applicable to the quantitative description of the data repulsive interaction parameter equaled —3.5 and standard free Gibbs energy of adsorption at the zero charge was —38.6 kJ mol h depended linearly... 

Figure 6.8. Gibbs energy profiles of a proton discharge process resulting in a metal-hydrogen bond formation. The difference in the Gibbs energy of adsorption of hydrogen between metal 1 to metal 2 lowers the activation barrier for the discharge and makes metal 2 the electrocatalytically more favorable (active) electrode material.

Fig. 6.89. Plot of the Gibbs energy of adsorption vs. electrode charge density for the adsorption of chloride ions at an Au(111) electrode by the chronocou-lometry technique. Adapted from Z. Shi and J. Upkowski, J. Electroanal. Chem. 403 225, copyright 1996, Fig. 7b, with permission of Elsevier Science.)...

The maximum surface concentration of benzoic acid obtained by extrapolation of the experimental data is rmax = 5.1 X 1014 molecules cm-2. Determine the parameters P and A in the Frumkin equation of adsorption. Calculate the Gibbs energy of adsorption. Compare the results with the Langmuir isotherm. (Sobkowski)... 

The value a, being an adsorption equilibrium constant, is related to the standard Gibbs energy of adsorption, AG °, by the equation... 

The number of sites, dl, with the standard Gibbs energy of adsorption within the limits of AGa° and AGa° + d(AGa°) provided that AGfl° satisfies (92) is given by the formula... 

Let us consider now the case (47) when there are molecules of two different gases on the surface, the ability to be adsorbed not differing greatly for the two kinds of molecules we shall make use of Assumption 3 formulated in Section IX (i.e., assume that the change in the standard Gibbs energy of adsorption at passing from one surface site to another is the same for both kinds of molecules). 

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