Electrosorption of neutral organic molecules

I f electrosorption is restricted to a monolayer, the fractional surface coverage can be related to the surface excess by simply writing  

This is a satisfactory approximation in the present and similar cases, since the interaction of a neutral organic molecule with the surface does not extend beyond a monolayer. 

The concept of the potential of zero charge (PZC or Ez), has already been discussed in the context of electrocapillary thermodynamics, where we showed that, for an ideally 

Determination of Ez on base metals such as copper, nickel or iron can be complicated by the formation of oxide layers. The value of Ez measured may then correspond to an oxide-covered surface, rather than to the bare metal. 

The occurrence of Faradaic reactions of any kind, and particularly those leading to the formation of adsorbed intermediates, can severely interfere with the determination of Ez. when based on measurement of the capacitance minimum. The high values of the adsorption pseudo-capacitance, C, which extends over a significant range of potential, can distort the measurements of double-layer capacitance in dilute solutions, as discussed in Section 11.2.3. 

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Electrosorption is a replacement reaction. We have already discussed the role of the solvent in the interphase, in the context of its effect on the double-layer capacitance. It is most important for our present discussion to know that the electrode is always solvated and that the solvent molecules are held to the surface both by electrostatic and by chemical bonds. Adsorption of a molecule on such a surface requires the removal of the appropriate number of solvent molecules, to make place for the new occupant, so to speak. This is electrosorption. In this chapter we shall restrict our discussion to the electrosorption of neutral organic molecules from aqueous solutions, without charge transfer. Using the notation RH for an unspecified organic molecule, we can then represent electrosorption in general by the reaction... 

In any study of electrosorption of neutral molecules on metallic electrodes, the ions of supporting electrolytes should not be specifically adsorbed. Nevertheless, the interaction of the electrolyte ions with the electrode surface may depend on the interaction of the ions with the solvent. Usually, the stronger the ion-solvent interaction, the weaker the adsorption of the ion. Since the ions are more weakly solvated in nonaqueous solvents than in water, the ions that are not adsorbed from aqueous solutions may still be adsorbed from organic solvents. However, even in the absence of... 

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. 

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