Double Layer

(redirected from Double layers)
Also found in: Acronyms.

double layer

[¦dəb·əl ′lā·ər]
(physical chemistry)

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.

REFERENCES

Frumkin, A. N., V. S. Bagotskii, Z. A. Iofa, and B. N. Kabanov. Kinetika elektrodnykh protsessov. Moscow, 1952.
Parsons, R. “Ravnovesnye svoistva zariazhennykh mezhfaznykh granits.” In Nekotorye problemy sovremennoi elektrokhimii. Moscow, 1958. (Translated from English.)
Delahay, P. Dvoinoi sloi i kinetika elektrodnykh protsessov. Moscow, 1967. (Translated from English.)

IU. V. PLESKOV

dual layer

Typically refers to optical media that contain two layers on a single side in order to double or nearly double the storage capacity. Also called "double layer." See DVD-9 and Blu-ray.
References in periodicals archive ?
Soestbergen, "Counterion volume effects in mixed electrical double layers," Journal of Colloid and Interface Science, vol.
Caption: Figure 1: (a) Counterion and (b) co-ion concentration profiles in the diffuse electric double layers near negative (left) and positive (right) interfaces, calculated using the CS and BMCSL full models and aqueous NaCl and KCl solutions with the indicated parameters.
When the concentration of NS is increased, complete double layers can be observed for both sizes of NS.
A set of acceleration and concentration has been tested for both diameters in order to identify the optimal parameters allowing the deposition of single or double layers. The surface coverage was measured by analyzing 20 adjacent SEM scans of 75 [micro]m x 50 [micro]m.
When the diffuse layers of the electrical double layers overlap in binary colloidal systems, the charges of the particles on the oppositely charged colloids can be balanced by each other.
So, double layers of the electrospun membranes were stacked together via hot-press in order to make the membranes remain their thickness in around 25-30 [micro]m.
Transfer the crisps to a double layer of kitchen paper and leave to cool.
Similarly, at the positive electrode, holes pair up with negative ions, forming a second electronic double layer that itself is a capacitor.
There is also a potential problem with internal wear of triple layers, and triple layer wires traditionally have higher void volume than double layers. Higher void volume means there is more water inside the fabric structure that must first be removed over the flatboxes and couch before water can be removed from the sheet.
This comprises two double layers of glass -- the outermost layer made of curved panels to produce a scalloped effect -- with dramatic maintenance walkways in the interstitial voids.
Contemporary use of translucent or semi-transparent glazing materials (such as frosted glass, translucent plastic sheeting, double layers of glass and perforated screens) expresses the current fascination for an architecture of material and spatial lightness.
The shell consists of two double layers of 40 x 20mm softwood laths at 400mm centres pin-jointed together with more than 1000 bolts.