It was originally formulated to address outer sphere electron transfer reactions, in which two chemical species change only in their charge, with an electron jumping. Devanathan and Klaus Müller  proposed the BDM model of the double-layer that included the action of the solvent in the interface. This restricts the value of the Debye length and particle radius as following:. The figure shows the localised perturbation of potential produced by an idealised double layer consisting of two oppositely charged discs.
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You will also see this noted in checkout. Through the centers of these ions pass the OHP. The diffuse layer is the region beyond the OHP. Further research with double layers on ruthenium dioxide films in by Sergio Trasatti and Giovanni Buzzanca demonstrated that the electrochemical behavior of these electrodes at low voltages with specific adsorbed ions was like that of capacitors.
The specific adsorption of the ions in this region of potential could also involve a partial charge transfer between the ion and the electrode. It was the first step towards understanding pseudocapacitance.
Between and Brian Evans Conway conducted extensive fundamental and development work on ruthenium oxide electrochemical capacitors. In he described the difference between 'Supercapacitor' and 'Battery' behavior in electrochemical energy storage. In he coined the term supercapacitor to explain the increased capacitance by surface redox reactions with faradaic charge transfer between electrodes and ions. His "supercapacitor" stored electrical charge partially in the Helmholtz double-layer and partially as the result of faradaic reactions with "pseudocapacitance" charge transfer of electrons and protons between electrode and electrolyte.
The working mechanisms of pseudocapacitors are redox reactions, intercalation and electrosorption. The physical and mathematical basics of electron charge transfer absent chemical bonds leading to pseudocapacitance was developed by Rudolph A. Marcus Theory explains the rates of electron transfer reactions—the rate at which an electron can move from one chemical species to another. It was originally formulated to address outer sphere electron transfer reactions, in which two chemical species change only in their charge, with an electron jumping.
For redox reactions without making or breaking bonds, Marcus theory takes the place of Henry Eyring 's transition state theory which was derived for reactions with structural changes. Marcus received the Nobel Prize in Chemistry in for this theory. There are detailed descriptions of the interfacial DL in many books on colloid and interface science    and microscale fluid transport.
As stated by Lyklema, " This surface charge creates an electrostatic field that then affects the ions in the bulk of the liquid.
This electrostatic field, in combination with the thermal motion of the ions, creates a counter charge, and thus screens the electric surface charge. The net electric charge in this screening diffuse layer is equal in magnitude to the net surface charge, but has the opposite polarity.
As a result, the complete structure is electrically neutral. The diffuse layer, or at least part of it, can move under the influence of tangential stress. There is a conventionally introduced slipping plane that separates mobile fluid from fluid that remains attached to the surface.
The electric potential on the external boundary of the Stern layer versus the bulk electrolyte is referred to as Stern potential. Electric potential difference between the fluid bulk and the surface is called the electric surface potential.
Usually zeta potential is used for estimating the degree of DL charge. A characteristic value of this electric potential in the DL is 25 mV with a maximum value around mV up to several volts on electrodes  . It is usually determined by the solution pH value, since protons and hydroxyl ions are the charge-determining ions for most surfaces.
Zeta potential can be measured using electrophoresis , electroacoustic phenomena , streaming potential , and electroosmotic flow. It is reciprocally proportional to the square root of the ion concentration C. In aqueous solutions it is typically on the scale of a few nanometers and the thickness decreases with increasing concentration of the electrolyte.
These steep electric potential gradients are the reason for the importance of the DLs. The theory for a flat surface and a symmetrical electrolyte  is usually referred to as the Gouy-Chapman theory. There is no general analytical solution for mixed electrolytes, curved surfaces or even spherical particles. There is an asymptotic solution for spherical particles with low charged DLs.
In the case when electric potential over DL is less than 25 mV, the so-called Debye-Huckel approximation holds. There are several asymptotic models which play important roles in theoretical developments associated with the interfacial DL. The first one is "thin DL". Where we have a charged surface, however, there must be a balancing counter charge, and this counter charge will occur in the liquid.
The charges will not be uniformly distributed throughout the liquid phase, but will be concentrated near the charged surface.
Thus, we have a small but finite volume of the liquid phase which is different from the extended liquid. This concept is central to electrochemistry, and reactions within this interfacial boundary that govern external observations of electrochemical reactions. It is also of great importance to soil chemistry, where colloidal particles with different surface charges play a crucial role. There are several theoretical treatments of the solid-liquid interface.
We will look at a few common ones, not so much from the position of needing to use them, but more from the point of what they can tell us. This theory is a simplest approximation that the surface charge is neutralized by opposite sign counterions placed at an increment of d away from the surface. The surface charge potential is linearly dissipated from the surface to the contertions satisfying the charge. The distance, d , will be that to the center of the countertions, i. The Helmholtz theoretical treatment does not adequately explain all the features, since it hypothesizes rigid layers of opposite charges.
This does not occur in nature. Gouy suggested that interfacial potential at the charged surface could be attributed to the presence of a number of ions of given sign attached to its surface, and to an equal number of ions of opposite charge in the solution.
In other words, counter ions are not rigidly held, but tend to diffuse into the liquid phase until the counter potential set up by their departure restricts this tendency. The kinetic energy of the counter ions will, in part, affect the thickness of the resulting diffuse double layer.
Gouy and, independently, Chapman developed theories of this so called diffuse double layer in which the change in concentration of the counter ions near a charged surface follows the Boltzman distribution. Already, however, we are in error, since derivation of this form of the Boltzman distribution assumes that activity is equal to molar concentration. This may be an OK approximation for the bulk solution, but will not be true near a charged surface. Now, since we have a diffuse double layer, rather than a rigid double layer, we must concern ourselves with the volume charge density rather than surface charge density when studying the coulombic interactions between charges.
The volume charge density, r , of any volume, i, can be expressed as.
XtremPro DVD +R DL 8X GB Min Recordable & White Inkjet Printable Printable Double Layer DVD 25 Pack Blank Discs in Spindle - Add To Cart There is a problem adding to cart. Double layer technology Optical Quantum OQDPRDL08WIP-H 8 X GB DVD+R DL White Inkjet Printable Double Layer Recordable Blank Media, Disc Spindle by Optical Quantum. A double layer or Helmholtz double layer (HDL) is an electrical double layer of positive and negative charges with a thickness equal to one molecule. This occurs at a surface where two different materials are in contact or at the surface of a metal or other substance capable of existing in a solution as ions and immersed in a dissociating solvent.