The Electrical Double Layer Is an Example of Electrostatic Equilibrium

1 The Electrical Double Layer Is an Example of Electrostatic Equilibrium 

In examining the properties of the metal solution interfaee, two limiting types of behavior are found, namely, the ideal polarizable interfaee and the ideally non-polaiizable interface. In the former case, the interface behaves as a capacitor so that charge can be placed on the metal using an external voltage source. This leads to the establishment of an equal and opposite charge on the solution side. The 

LoLal system in which charge is separated in space is called the electrical double layer and its properties are characterized by electrostatic equilibrium. An electrical double layer exists in general at any interface at which there is a change in dielectric properties. It has an important influence on the structure of the interface and on the kinetics of processes occurring there. 

In many electrolytes, one or more of the constituent ions are specifically adsorbed at the interface. Specific adsorption implies that the local ionic concentration is determined not just by electrostatic forces but also by specific chemical forces. For example, the larger halide ions are chemisorbed on mercury due to the covalent nature of the interaction between a mercury atom and the anion. Specific adsorption can also result from the hydrophobic nature of an ion. Thus, tetra-alkylammonium ions, which are soluble in water, are specifically adsorbed at the mercury water interface because of the hydrophobic nature of the alkyl groups. Specific adsorption of molecular solutes, such as the alcohols, occurs for the same reason. 

Double layers are established at most other interfaces. An important example is the semiconductor solution interface. In this case, a space charge region or double layer exists both in the semiconductor phase and in the liquid solution. Double layers are also formed at the liquid liquid interface when both phases contain an electrolyte. 

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