Electric double layer formation

It is commonly stated that the first readily observable event at the interface between a material and a biological Quid is protein or macromolecule adsorption. Clearly other interactions precede protein adsorption water adsorption and possibly absorption (hydration effects), ion bonding and electrical double layer formation, and the adsorption and absorption of low molecular weight solutes — such as amino acids. The protein adsorption event must result in major perturbation of the interfacial boundary layer which initially consists of water, ions, and other solutes. 

Liu, H.-Y., Wang, K.-P., and Teng, H. A simplified preparation of mesoporous carbon and the examination of the carbon accessibility for electric double layer formation. Carbon 43, 2005 559-566. 

Liquid-solid and liquid-liquid mass transfer is highly dependent upon surface area, or particle size. Mass transfer is involved in simple wetting, dissolution, hydration, swelling of product components, ion exchange, electric double layer formation,... 

Figure 3.2 Transport phenomena in NF, (a) concentration polarisation (b) sieving (c) charge effects (e.g. charge repulsion or electric double layer formation).

The electrical response of such a system is comparable to that of a capacitor. Being of faradic origin and non-electrostatic, this capacitance is distinguished from the double-layer one and is called pseudo-capacitance. In summary, the electrical double-layer formation is a universal property of a polarized material surface, and pseudo-capacitance is an additional property which depends both on the type of electrode material and electrolyte. Compared to the double-layer normalized capacitance ( 10 pF cm ), it has generally a high value (100-400 pF cm ), because it involves the bulk of the electrode and not only the surface. From a practical point of view, pseudo-capacitance contributes to enhancing the capacitance of materials and their energy density. 

Thus the theory of change in the water structure on the mineral particle surface is close to the theory of the electric double layer with the difference that in the first case the water acts as an electrolyte and mechanisms intensifying the process of the electric double layer formation are not the electric fields but the thermodynamic ones (temperature and pressure). 

Fig. 5 (a) Cross-sectional model of nanoporous carbide-derived carbon-based actuator, (b) Simplified notation of electrical double-layer formation inside porous CDC during charging, (c) Bending displacement of actuator under applied voltage, (d, e) SEM images of cross section of actuators with polymer-supported and gold-foil-modified CDC electrodes (Reproduced from Torop et al. 2012)... 

More recently, Possart has used potential contrast scanning electron microscopy to study electric double layer formation associated with the interface between a thin layer of solvent-cast low-density polyethylene and aluminum (Possart 1988). While arguing that the electrical discharge energy of the double layer makes a contribution to the fracture energy, this study... 

A current in an electrochemical cell due to the electrical double layer s formation. 

The situations would be totally different when the two surfaces are put in electrolyte solutions. This is because of formation of the electrical double layers due to the existence of ions in the gap between solid surfaces. The electrical double layers interact with each other, which gives rise to a repulsive pressure between the two planar surfaces as... 

For solid surfaces interacting in air, the adhesion forces mainly result from van der Waals interaction and capillary force, but the effects of electrostatic forces due to the formation of an electrical double-layer have to be included for analyzing adhesion in solutions. Besides, adhesion has to be studied as a dynamic process in which the approach and separation of two surfaces are always accompanied by unstable motions, jump in and out, attributing to the instability of sliding system. 

Surface forces measurement is a unique tool for surface characterization. It can directly monitor the distance (D) dependence of surface properties, which is difficult to obtain by other techniques. One of the simplest examples is the case of the electric double-layer force. The repulsion observed between charged surfaces describes the counterion distribution in the vicinity of surfaces and is known as the electric double-layer force (repulsion). In a similar manner, we should be able to study various, more complex surface phenomena and obtain new insight into them. Indeed, based on observation by surface forces measurement and Fourier transform infrared (FTIR) spectroscopy, we have found the formation of a novel molecular architecture, an alcohol macrocluster, at the solid-liquid interface. 

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 Nernst equation is of limited use at low absolute concentrations of the ions. At concentrations of 10 to 10 mol/L and the customary ratios between electrode surface area and electrolyte volume (SIV 10 cm ), the number of ions present in the electric double layer is comparable with that in the bulk electrolyte. Hence, EDL formation is associated with a change in bulk concentration, and the potential will no longer be the equilibrium potential with respect to the original concentration. Moreover, at these concentrations the exchange current densities are greatly reduced, and the potential is readily altered under the influence of extraneous effects. An absolute concentration of the potential-determining substances of 10 to 10 mol/L can be regarded as the limit of application of the Nernst equation. Such a limitation does not exist for low-equilibrium concentrations. 

The charges present on the insulator surface in contact with the solution give rise to an accumulation of ions of opposite sign in the solution layer next to the surface, and thus formation of an electric double layer. Since straightforward electrochemical measurements are not possible at insulator surfaces, the only way in which this EDL can be characterized quantitatively is by measuring the values of the zeta potential in electrokinetic experiments (see Section 31.2). 

Studies of the adsorption of surface active electrolytes at the oil-water interface provide a convenient method for testing electrical double layer theory and for determining the state of water and ions in the neighborhood of an interface. The change in the surface amount of the large ions modifies the surface charge density. For instance, the surface ionic area of 100 per ion corresponds to 16, /rC/cm. The measurement of the concentration dependence of the changes of surface potential were also applied to find the critical concentration of formation of the micellar solution [18]. 

A charging current (non-faradaic) due to the formation of an electric double layer on the surface of the growing drop most polarographs permit a... 

While the formation of an electrical double layer at interfaces is a general phenomenon, the electrode-electrolyte solution interface will be considered... 

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. 

If the electrolyte components can react chemically, it often occurs that, in the absence of current flow, they are in chemical equilibrium, while their formation or consumption during the electrode process results in a chemical reaction leading to renewal of equilibrium. Electroactive substances mostly enter the charge transfer reaction when they approach the electrode to a distance roughly equal to that of the outer Helmholtz plane (Section 5.3.1). It is, however, sometimes necessary that they first be adsorbed. Similarly, adsorption of the products of the electrode reaction affects the electrode reaction and often retards it. Sometimes, the electroinactive components of the solution are also adsorbed, leading to a change in the structure of the electrical double layer which makes the approach of the electroactive substances to the electrode easier or more difficult. Electroactive substances can also be formed through surface reactions of the adsorbed substances. Crystallization processes can also play a role in processes connected with the formation of the solid phase, e.g. in the cathodic deposition of metals. 

The formation of a membrane potential is connected with the presence of an electrical double layer at the surface of the membrane. For a thick, compact membrane, an electrical double layer is formed at both interfaces. The electrical double layer at a porous membrane is formed primarily in the membrane pores (see Section 6.2). The electrical double layer at thin membranes is formed on both membrane surfaces. It is formed by fixed ions on the surface of the membrane and the diffuse layer in the electrolyte. 

Mobility of The Anion-Free Water. It is well known that water in the electrical double layer is under a field strength of 10 -10 V/cm and that the water has low dielectric constants (36). Since anion-free water is thought to be the water in the electrical double layer between the clay and the bulk solution, at high electrolyte concentrations, the double layer is compressed therefore, the water inside is likely quite immobile. At low electrolyte concentrations, the electrical double layer is more diffuse, the anion-free water is expected to be less immobile. Since the evaluation of the shaly formation properties requires the knowledge of the immobile water, experiments were conducted to find out the conditions for the anion-free water to become mobile. 

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