Electrosorption features

Several important features of electrosorption follow from this simple equation. First it becomes clear that the thermodynamics of electrosorption depends not only on the properties of the organic molecule and its interactions with the surface, but also on the properties of water. In other words, the free energy of electrosorption is the difference between the free energy of adsorption of RH and that of n water molecules ... 

An additional unique feature of electrosorption is that the coverage is a function of potential, at constant concentration in solution. Thus, we can discuss two types of isotherms those yielding 0 as a function of C and those describing the dependence of 0 on E. This is not a result of faradaic charge transfer. Neither is it due to electrostatic interactions of the adsorbed species with the field inside Ihc compact part of the double layer, since a potential dependence is observed even for neutral organic species having no permanent dipole moment. As we shall see, it turns out that the potential dependence of 0 is due to the dependence of the free energy of adsorption of water molecules on potential. 

As a case study we discuss some aspects of the adsorption of non-ionic surfactants, non-ionics for short, from aqueous solution. Such surfactants have Invariably long molecules and strongly associate In solution to form micelles. The latter aspect is beyond the confines of the present chapter. Here we shall briefly Introduce some main features of the adsorption of non-ionics. Ionic surfactant adsorption belongs to the domain of electrosorption see sec. 3.12d. 

Electric double layers at phase boundaries pervade the entire realm of Interface and colloid science. Especially in aqueous systems, double layers tend to form spontaneously. Hence, special precautions have to be taken to ensure the absence of charges on the surfaces of particles. Insight into the properties of double layers is mandatory, in describing for Instance electrosorption, ion exchange, electrokinetics (chapter 4), charged monolayers (Volume III), colloid stability, polyelectrolytes and proteins, and micelle formation of ionic surfactants, topics that are intended to be treated in later Volumes. The present chapter is meant to Introduce the basic features. 

Perhaps the most important feature which distinguishes electrosorption from gas phase adsorption is the presence of the metal-solution potential difference as an additional degree of freedom of the system, which can be controlled and varied externally. The potential region over which adsorption occurs and the manner in which adsorption depends on potential will greatly influence the oxidation behavior of a given compound. 

The most significant feature of electrosorption as compared with gas phase adsorption is that the former is a replacement reaction while the latter is not. Bockris, Green, and Swinkels have shown that for an organic species R adsorbing on the surface and replacing n water molecules, the adsorption process may be represented as... 

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