Standard Gibbs energies of adsorption

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... 

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 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). 

It was later demonstrated that if the reaction mechanism corresponds to scheme (295) and the linear relation between standard Gibbs energy of adsorption and Gibbs activation energy of adsorption is obeyed [see (91)], then the kinetic (305) corresponds in general to the exponential nonuniformity of the surface with even nonuniformity included as a particular case (44). In the general case the exponent m is not equal to transfer coefficient a, but is connected with it according to (143).5... 

One word about the Gibbs energies of adsorption. In equilibrium the molar Gibbs energy of adsorption is zero AadGm = /P — pT 0. The reason is simple. In equilibrium and for constant P and T the chemical potential of the molecules in the gas phase n9 is equal to the chemical potential of adsorbed molecules /P. What is not zero is the standard Gibbs energy of adsorption... 

When two contacting liquid phases are in equilibrium, the adsorption of surfactant at the interface can take place from either of them. Accordingly, standard Gibbs energies of adsorption can be calculated for each phase using Eq. 2. 

Standard Gibbs energies of adsorption are often encountered. When A j G is accurately known as a function of temperature, standard enthalpies and entropies of adsorption can also be obtained, using the appropriate Gibbs Helmholtz relations (sec. 1.2.15). 

For ideal systems the standard Gibbs energy of adsorption or, for that matter, Kjj is Independent of composition and a function of T only. Then, g may be... 

The calculation of the log K j values by Hiemstra et al. [33] is based on Paulings idea of local charge neutralisation [19] in combination with the assumption that the standard Gibbs energy of adsorption of a proton according to Eq. (39a) or (39b) is depending on the local electrostatics. In principle this type of approach had been used before by Parks [35] and Yoon et al. [36] to study the pzc of oxides. Hiemstra derives the following simple expression for log Kn i ... 

Early studies were carried out at the liquid gas interface [22, 23]. Castro et al. [24] studied the adsorption of / -propyl-phenol from aqueous solutions at the air interface as a function of phenol concentration in the bulk. They showed that the square root of the second-harmonic intensity plotted against bulk phenol concentration followed a Langmuir isotherm with a standard Gibbs energy of adsorption equal to -24.3 kJmol Similar results were obtained for other alkylphenols and alkylanilines. In other work with phenols, the orientation of phenol at the water air interface was determined by studying the phase of the xfl component of the susceptibility. As expected, the OH was oriented toward the water phase [25] so that it could participate in the hydrogen-bonded structure of water. The same conclusion was reached for / -bromophenol and -nitrophenol. 

As in the case of fluid interfaces, the question of whether the adsorption of proteins onto solids is reversible or irreversible is very important for correct estimation of physicochemical characteristics of the process. In a reversible process, dilution of sorbate in the bulk phase should lead to spontaneous desorption of some portion of adsorbed molecules up to elimination of a transient difference in the chemical potential of the sorbate at the interface and in the solution the ascending and descending branches of the isotherm must overlap at all values of Cb. Only in this case the isotherm represents thermodynamic equilibrium, and the equilibrium constant Kads and the standard Gibbs energy of adsorption AG°ads = A/7°ads - rAS°ads can be determined. 

The model assumes, that each site is characterized by a definite adsorption energy of a particular substance and there is certain distribution of the total number of sites with this energy. The values of adsorption energy on different sites lie between a certain minimum and a certain maxium value (AG°a)min standard Gibbs energy of adsorption within (AG°a) and (AG°a)+d(AG°a) is given by an exponential dependence... 

The standard Gibbs energy of adsorption, AGajx, depends quadratically upon the electrical variable X (= , A(j) or a ). 

Figure 4. Interaction energy parameter (A) and standard Gibbs energy of adsorption (B) for hydrogen (squares) and OH (circles) on Pt(lll) from 0.1 M HCIO4, plotted as a function of the temperature. Lines are the linear fit of the experimental data. Adapted from Refs. 16,17 and 19.

To quantify the effect of additives in the mixture on the micellization process, the standard Gibbs free energy change of micellization, dmicG , and the standard Gibbs energy of adsorption, dadsC , were calculated by using equations (5) and (6),... 

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