The Adsorption Process of Organic Molecules

THE ADSORPTION PROCESS OF ORGANIC MOLECULES 6.9.1. The Relevance of Organic Adsorption... 

Iowever, as explained in Section 6.8.2.1 for the case of ionic adsorption, partial transfer of electrons may occur during the adsorption process of organic molecules. 

If the organic molecule under study follows the behavior described in these three tests, it can be said that the molecule is undergoing an adsorption reaction. Although it was explained why the current should be almost zero when adsorption occurs, we did not explain why the adsorption should follow a parabolic shape with a maximum close to the pzc. This behavior will be explained shortly, but before that, some fundamentals of the adsorption process of organic molecules should be pointed out. 

Thus, Eq. (6.270) is an isoconc (see Section 6.8.6), which describes the adsorption of organic molecules on electrodes as a substitution process of solvent molecules and takes into account the surface heterogeneity and lateral interactions among the adsorbed species. This isoconc is able to successfully describe and reproduce the parabolic shape—with its maximum in the vicinity of the pzc—of the adsorption process of organic molecules. 

Similarly, the crystal structure of the electrode influences the adsorption process of organic molecules. Adsorption on the most compact surfaces is generally stronger than in the most open surfaces. Thus, organic molecules tend to adsorb in the following order (111) > (100) > (110), as shown in Figure 6.115 for the adsorption of various organic substances on different crystal faces of various metals (Trasatti, 1995). 

It is also possible to use 3 0 Voltammetry for the elucidation of adsorption processes of organic molecules on... 

The specific methodology involved in these simulations is beyond the scope of this chapter. So, instead of extending our knowledge of adsorption of ions on electrodes, in the next section we will study the characteristics of the adsorption process of other types of species, the adsorption of organic molecules. 

Fig. 6.109. Coverage vs. potential curve for the adsorption of organic molecules on electrodes. The coverage due to the adsorption process of an organic molecule from solution follows a parabolic path, with a maximum close to the pzc.

The choice of chemical is usually based on trial-and-error procedures hence, demulsifier technology is more of an art than a science. In most cases a combination of chemicals is used in the demulsifier formulation to achieve both efficient flocculation and coalescence. The type of demulsifiers and their effect on interfacial area are among the important factors that influence the coalescence process. Time-dependent interfacial tensions have been shown to be sensitive to these factors, and the relation between time-dependent interfacial tensions and the adsorption of surfactants at the oil-aqueous interface was considered by a number of researchers (27, 31-36). From studies of the time-dependent tensions at the interface between organic solvents and aqueous solutions of different surfactants, Joos and coworkers (33—36) concluded that the adsorption process of the surfactants at the liquid-liquid interface was not only diffusion controlled but that adsorption barriers and surfactant molecule reorientation were important mecha-... 

Adsorption processes at natural phase boundaries occur from the aqueous phase containing complex mixtures of different organic and inorganic compounds and they are not in equilibrium. Adsorption behavior of organic molecules can be described by competitive adsorption of the mixture, in which very often less adsorbable materials are present at higher concentrations and strongly adsorbable substances are present at lower concentrations. Since the adsorption process depends on both the adsorption constant and the adsorption kinetics, which include the mass transfer of adsorbable molecules from the bulk phase toward the surface as well as the intrinsic rate of attachment to the surface, the adsorption layer formed is influenced by qualitative and quantitative composition of the complex mixture of adsorbable solutes. 

Many organic electrode processes require the adsorption of the electroactive species at the electrode surface before the electron transfer can occur. This adsorption may take the form of physical or reversible chemical adsorption, as has been commonly observed at a mercury/water interface, or it may take the form of irreversible, dissociative chemical adsorption where bond fracture occurs during the adsorption process and often leads to the complete destruction of the molecule. This latter t q)e of adsorption is particularly prevalent at metals in the platinum group and accounts for their activity as heterogeneous catalysts and as... 

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