The Adsorption of Molecules at Electrodes

Experimental data relevant to molecular adsorption have been obtained for a wide variety of organic compounds, including alcohols, aldehydes, ketones, amines, and amides [G5j. Included are studies of liquids which are polar solvents themselves, for example, acetonitrile, acetone, ethylene carbonate, formamide, and methanol. Studies have also included organic acids which are weakly dissociated in water. 

An ideal thermodynamic cell which could be used to study the adsorption of adsorbate A at mercury is the following  

X is the concentration of the adsorbate A which is varied in order to study the effect of bulk concentration on the interfacial surface excess y is the concentration of the electrolyte whose activity is kept constant. The corresponding electrolyte concentration is kept reasonably high to provide electrical conductivity to the solution. For low values of x, the electrolyte concentration is constant. However, at higher concentrations of organic solute, the activity coefficient of the electrolyte varies with organic solute concentration. Thus, in general, the concentration of the electrolyte must also be varied in order to keep its activity constant. It is also important that the ions of the electrolyte not adsorb on the electrode to a significant extent. 

In the following discussion the relative nature of the surface excess is neglected and the role of water in the thermodynamic analysis ignored. 

In dilute solutions, the effects of the organic solute on the electrolyte activity can be ignored so that experiments can be carried at constant electrolyte concentration. The chemical potential of the electrolyte is given by 

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