Double layer, electric surface conduction

Clay and grain surface phenomena create a double layer or interface conductivity . The electrical double layer can be described with the Gouy-Chapman model or the Stem model. A detailed description of the physical properties and processes in the interface region is given by Revil et al. (1997) and Revil and Glover (1998). 

The existence of Galvani potentials between two different conducting phases is connected with the formation of an electric double layer (EDL) at the phase boundary (i.e., of two parallel layers of charges with opposite signs, each on the surface of one of the contacting phases). It is a special feature of such an EDL that the two layers forming the double layer are a very small (molecular) distance apart, between 0.1 and 0.4nm. For this reason EDL capacitances are very high (i.e., tenths of pF/cm ). 

The adsorption of ions at insulator surfaces or ionization of surface groups can lead to the formation of an electrical double layer with the diffuse layer present in solution. The ions contained in the diffuse layer are mobile while the layer of adsorbed ions is immobile. The presence of this mobile space charge is the source of the electrokinetic phenomena.t Electrokinetic phenomena are typical for insulator systems or for a poorly conductive electrolyte containing a suspension or an emulsion, but they can also occur at metal-electrolyte solution interfaces. 

A classic definition of electrochemical ultracapacitors or supercapacitors summarizes them as devices, which store electrical energy via charge in the electrical double layer, mainly by electrostatic forces, without phase transformation in the electrode materials. Most commercially available capacitors consist of two high surface area carbon electrodes with graphitic or soot-like material as electrical conductivity enhancement additives. Chapter 1 of this volume contains seven papers with overview presentations, and development reports, as related to new carbon materials for this emerging segment of the energy market. 

The central issue which has to be addressed in any comprehensive study of electrode-surface phenomena is the determination of an unambiguous correlation between interfacial composition, interfacial structure, and interfacial reactivity. This principal concern is of course identical to the goal of fundamental studies in heterogeneous catalysis at gas-solid interfaces. However, electrochemical systems are far more complicated since a full treatment of the electrode-solution interface must incorporate not only the compact (inner) layer but also the boundary (outer) layer of the electrical double-layer. The effect of the outer layer on electrode reactions has been neglected in most surface electrochemical studies but in certain situations, such as in conducting polymers and... 

The ssDNA was immobilized stronger and faster on the GC surface in the presence of the lipid membrane than on a bare GC surface and using milder conditions [61]. The lipid membrane enhanced the stabihty of ssDNA towards desorption from the GC surface [61,62]. Moreover, the adsorption of ssDNA on BLM induced a conductance enhancement due to (1) structural changes (i.e., defect sites) within the membrane and (2) the increase in negative surface charge density of the membrane. The charge of the phosphate groups of ssDNA induced an increase of cation concentration in the electrical double layer [63]. 

Jote the greater complexity of defining adsorption here in studies of electric double layers than, e.g., for metal-gas systems. With electric double layers, one is concerned with the whole interphasial region. The total adsorption is the sum of the increases of concentration over a distance, which in dilute solutions may extend for tens of nanometers. Within this total adsorption, there are, as will be seen, various types of adsorptive situations, including one, contact adsorption, which counts only Arose ions in contact with the electronically conducting phase (and is Aren, like the adsorption referred to in metal-gas systems, the particles on Are surface). Metal-gas systems deal with interfaces, one might say, whereas metal-electrolyte systems deal primarily with interphases and only secondarily with interfaces. 

By means of special experiments in vacuum, reversible photodesoption of oxygen under ultraviolet irradiation was shown [275]. Therefore the electric double layer on the polymer surface is modified. The increase of the emf obtained and the decrease in the dark conductivity and the photoconductivity with the action of water vapor confirms the proposed model [276],... 

Solid state materials that can conduct electricity, are electrochemically of interest with a view to (a) the conduction mechanism, (b) the properties of the electrical double layer inside a solid electrolyte or semiconductor, adjacent to an interface with a metallic conductor or a liquid electrolyte, (c) charge-transfer processes at such interfaces, (d) their possible application in systems of practical interest, e.g. batteries, fuel cells, electrolysis cells, and (e) improvement of their operation in these applications by modifications of the electrode surface, etc. 

For conducting and completely polarizable surfaces, the capacitance can be measured directly with great precision. In the most simple capacitance measurement, called chronoamperome-try, a potential step AU is applied to an electrode (Fig. 5.10). A current flows due to charging of the diffuse electrical double layer. The current flows until the capacitance is fully charged. By measuring the current as a function of time and integrating the curve with respect to time we get the charge Q. The total capacitance C is easily obtained from C = CA A = Q/AU. 

The impedance for the study of materials and electrochemical processes is of major importance. In principle, each property or external parameter that has an influence on the electrical conductivity of an electrochemical system can be studied by measurement of the impedance. The measured data can provide information for a pure phase, such as electrical conductivity, dielectrical constant or mobility of equilibrium concentration of charge carriers. In addition, parameters related to properties of the interface of a system can be studied in this way heterogeneous electron-transfer constants between ion and electron conductors, or capacity of the electrical double layer. In particular, measurement of the impedance is useful in those systems that cannot be studied with DC methods, e.g. because of the presence of a poor conductive surface coating. 

In its most simple form, this means without effects such as adsorption or formation of coatings at the electrode surface36. The resistance, Rc, represents electrical conductivity of the electrolyte and is not a property of the electrode itself. The differential double-layer capacity, Cmetal surface of the metal-electrolyte interface, which is in equilibrium with an equal excess of charge but opposite in sign at the side of the electrolyte. 

Consider the motion of liquid in the diffuse part of the double layer relative to that of a non-conducting flat surface when an electric field... 

The distribution of ions in the diffuse part of the double layer gives rise to a conductivity in this region which is in excess of that in the bulk electrolyte medium. Surface conductance will affect the distribution of electric field near to the surface of a charged particle and so influence its electrokinetic behaviour. The effect of surface conductance on electrophoretic behaviour can be neglected when ka is small, since the applied electric field is hardly affected by the particle in any case. When tea is not small, calculated zeta potentials may be significantly low, on account of surface conductance. 

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