Double Layer

DOUBLE LAYER (electrical), a term introduced by Helmholtz for such a distribution of ions, where at boundary surfaces positively and negatively charged ions partially separate from each other and arrange themselves unevenly: on one side ions of one sign predominate, in the immediately following layer—ions of the other sign. In terms of the arrangement of electrical charges, the double layer has the structure of a capacitor, whose plates correspond to both ionic layers. The distance between them is extremely small, while the electrical potentials on both sides differ sharply from each other. This sharp and sudden change in potential in the D.L. (as opposed to its constant value in the bulk of each phase) is called a potential jump. A D.L. can arise as a result of the most diverse processes in which ions participate. For example, if any solid body adsorbs an electrolyte present in a solution, then usually both of its constituent ions are not adsorbed equally strongly. More strongly adsorbed ions accumulate in the layer nearest to the surface, giving it their charge. By the action of this electric charge, oppositely charged ions are attracted from the solution, arranging themselves near the first ones and forming the outer plate of the D.L. The same beginning separation of oppositely charged ions, immediately balanced by the formation of a D.L., also occurs when both ions of the electrolyte have different solubilities in two contacting media. If a solution of an electrolyte borders on a pure solvent, then the same result is caused by the unequal mobility of both ions. It is further known that metals dissolve only in the form of positively charged ions. A metallic surface in contact with a solution therefore acquires a negative charge, with the outer plate of the D.L. being the cations that have passed into the solution: the resulting electrode potential difference forms the basis of the construction of a galvanic cell. The existence of D.L.s on boundary surfaces accounts for the electrokinetic phenomena observed when an electric current passes.