Electrical Structure of Interphases

When an electrode is in contact with an electrolyte, the interphase as a whole is electroneutral. However, electric double layers (EDLs) with a characteristic potential distribution are formed in the interphase because of a nonuniform distribution of the charged particles. 

Two types of EDL are distinguished superficial and interfacial. Superficial EDLs are located wholly within the surface layer of a single phase (e.g., an EDL caused by a nonuniform distribution of electrons in the metal, an EDL caused by orientation of the bipolar solvent molecules in the electrolyte solution, an EDL caused by specific adsorption of ions). Tfie potential drops developing in tfiese cases (the potential inside the phase relative to a point just outside) is called the surface potential of the given phase k. Interfacial EDLs have their two parts in dilferent phases the inner layer with the charge density in the metal (because of an excess or deficit of electrons in the surface layer), and the outer layer of counterions with the charge density = -Qs m in the solution (an excess of cations or anions) the potential drop caused by this double layer is called the interfacial potential 

Hermann von Helmholtz put the concept of EDL formation at electrode surfaces forward in 1853. For a long time only the interfacial EDLs were taken into account. The considerable importance of various kinds of superficial EDLs was pointed out by Alexander N. Frumkin in 1919. 

The formation of any kind of EDL implies the development of strong electrostatic fields in the interphase. The distance between the two sides of an EDL as a rule is 

Each type of EDL and the potential drop produced by it contribute to the total Galvani potential, (pg, at the interface considered  

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