Adsorption in the Presence of an Applied Potential

We conclude this chapter with a discussion of adsorption at the interface between mercury and a solution (usually aqueous) under the influence of an applied potential. The y value for this interface is easily measured, and potential and electrolyte concentration can be studied as variables. The Nernst equation provides a familiar reminder that potentials can be dealt with by thermodynamics. 

Moving a charge of q coulombs to a surface at which the potential is J/ volts involves a quantity of work (in joules) 

21 An optical switching device based on Langmuir-Blodgett films. (Redrawn with permission of P. Ball 1994.) 

The general shape of the curves is roughly parabolic. The coordinates of the maximum in the electrocapillary curve depend on the electrolyte content of the system. Since 7 decreases on both sides of the electrocapillary maximum and since reductions in 7 are associated with adsorption, we conclude that adsorption increases as we move in either direction from the maximum that is, the electrocapillary maximum seems to be a point of minimum adsorption. 

22 Schematic illustration of an apparatus to measure the electrocapillary effect. 

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