Double Layer

two closely spaced layers of electrical charges, of opposite signs but with the same surface densities, that occur at an interface between two phases. As a whole, a double layer is electrically neutral. Upon passing through the layer, the electric potential changes abruptly. A double layer forms on a metallic surface because the electrons of the metal emerge somewhat beyond the limits of the lattice formed by positive ions. The potential difference in such a double layer is a component of the electron work function for the metal.

The double layer at a metal-electrolyte interface is of great importance in electrochemistry. When a metal is immersed in a solution containing ions of the metal, an ionic double layer that is specific for the electrode-solution boundary is formed in addition to the layer existing on the metal’s surface prior to its immersion and the layer produced by the orientation of the polar molecules of the solvent (such as water) on the metal’s surface. Thus, when a silver plate is immersed in a solution of KNO₃ that contains very little AgNO₃, Ag⁺ ions pass from the metal into the solution and the excess electrons in the metal charge its surface negatively and attract K⁺ ions from the solution, thus forming a second (positive) layer that coats the metal. The resulting potential difference stops further transfer of the Ag⁺ ions, and an equilibrium occurs between the electrode and solution. If the AgNO₃ concentration in the solution is high, then Ag⁺ ions pass from the solution into the metal,’ its surface becomes positively charged, and NO₃^- ions are attracted from the solution. An intermediate concentration of metal ions exists, at which the metal’s surface is not charged; the corresponding potential of the electrode is known as the neutral charge potential, or the zero point. The important concept of the zero point as a quantity that is characteristic for a given electrode was introduced into electrochemistry by the Soviet scientist A. N. Frumkin.

The ions in a double layer are acted upon simultaneously by electrostatic forces and the forces of thermal motion. As a result of these opposing forces, only a portion of the ions remain in the immediate neighborhood of the electrode surface (the compact portion of the double layer, or the Helm-holtz layer), whereas the remainder are diffusely distributed in the solution at some distance from the electrode (the diffuse double layer, or Goiiy layer). The degree of diffuse-ness increases as the temperature rises, and also as the concentration of the electrolytic solution decreases and the electrode charge diminishes. The average thickness of the compact portion of the double layer is on the order of an ion radius (several angstroms), so that it has high capacitance (~10^-5 farads per sq cm), and a strong electrical field (~10⁶ volts per cm) acts within it.

The structure of a double layer has a considerable influence on the electrical properties of interphase boundaries and on the processes taking place in them, primarily on the mechanism and kinetics of electrochemical reactions, on electrokinetic phenomena, and on the stability of colloidal systems. The methods used in studying the double layer include measurements of the surface tension and capacitance and adsorption.