Box 17-3 The Electric Double Layer

When a power supply pumps electrons into or out of an electrode, the charged surface of the electrode attracts ions of opposite charge. The charged electrode and the oppositely charged ions next to it constitute the electric double layer. 

A given solution has a potential of zero charge at which there is no excess charge on the electrode. This potential is —0.58 V (versus a calomel electrode containing 1 M KC1) for a mercury electrode immersed in 0.1 M KBr. It shifts to —0.72 V for the same electrode in 0.1 M KI. 

The first layer of molecules at the surface of the electrode is specifically adsorbed by van der Waals and electrostatic forces. The adsorbed solute could be neutral molecules, anions, or cations. Iodide is more strongly adsorbed than bromide, so the potential of zero charge for KI is more negative than for KBr A 

The next layer beyond the specifically adsorbed layer is rich in cations attracted by the negative electrode. The excess of cations decreases with increasing distance from the electrode. This region, whose composition is different from that of bulk solution, is called the diffuse part of the double layer and is typically 0.3-10 nm thick. The thickness is controlled by the balance between attraction toward the electrode and randomization by thermal motion. 

Electrode-solution interface. The tightly adsorbed inner layer (also called the compact, Helmholtz, or Stem layer) may include solvent and any solute molecules. Cations in the inner layer do not completely balance the charge of the electrode. Therefore, excess cations are required in the diffuse part of the double layer for charge balance. 

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