Blocking interface double layer

When particles or large molecules make contact with water or an aqueous solution, the polarity of the solvent promotes the formation of an electrically charged interface. The accumulation of charge can result from at least three mechanisms (a) ionization of acid and/or base groups on the particle s surface (b) the adsorption of anions, cations, ampholytes, and/or protons and (c) dissolution of ion-pairs that are discrete subunits of the crystalline particle, such as calcium-oxalate and calcium-phosphate complexes that are building blocks of kidney stone and bone crystal, respectively. The electric charging of the surface also influences how other solutes, ions, and water molecules are attracted to that surface. These interactions and the random thermal motion of ionic and polar solvent molecules establishes a diffuse part of what is termed the electric double layer, with the surface being the other part of this double layer. 

The mechanisms of detection and the functions of the conductor layer and of the semiconductor are the same in a C-I-S diode sensor as they are in a C-S diode sensor. The only difference between these two structures is the presence of the purposefully inserted interfacial layer (I-layer) between the conductor and the semiconductor in the C-I-S devices. In general, this I-layer is employed in the C-I-S sensor configuration for one of two reasons (1) either it is used to block chemical reactions between the conductor and the semiconductor or (2) it is used to augment or reduce the role of the interface in establishing the double layer or controlling transport. 

The Rb based on the sample cannot be calculated correctly, since the electric charge transfer resistance and the electric double layer in an electrode interface are also detected as a resistance, even if bias voltage is impressed to the measurement cell in order to measure the ionic conductivity. For the ionic conductivity measurement, a dc four-probe method, or the complex-impedance method, is used to separate sample bulk and electrode interface [4]. In particular, the complex-impedance method has the advantage that it can be performed with both nonblocking electrodes (the same element for carrier ion and metal M) and blocking electrodes (usually platinum and stainless steel were used where charge cannot be transferred between the electrode and carrier ions). The two-probe cell, where the sample is sandwiched between two pohshed and washed parallel flat electrodes, is used in the ionic conductivity measurement by complex-impedance method as shown in Figure 6.1. 

Figure 6.3 (a) Schematic representation of equivalent circuit for an ion conductor put between a pair of blocking electrode, and (b) the corresponding Nyquist plot. Ideally the sample-electrode interface is composed only of the double-layer capacitance. However, the practical Nyquist plot that corresponds to this frequency region is not vertical to the real axis. The rate-limiting process of this plot is that the ion diffuses to form a double layer. 

Figure 10 shows a solid electrolyte with two non-blocking electrodes. For non-blocking electrodes, no accumulation of charge occurs at the electrode-electrolyte interface.The Nyquist plot is expected to show a semicircle. The equivalent circuit may take the form of a resistor connected in parallel with a capacitor. In the presence of R, the expected Nyquist plot together with its equivalent circuit is depicted in Figure 11. On the other hand, for blocking electrodes, charge accumulates at the electrolyte-electrode interfaces. This contributes to the double layer capacitances at the interfaces, A vertical spike is thus expected to arise in the Nyquist plot due to the double layer capacitance (Figure 12).

When the polarizing voltage is applied to a cell with ionically blocking electrodes, a current decay is often observed before a stable value is reached. The reason for this is, that immediately after application of the voltage, the charging of the double layer at the electrode/electrolyte interfaces can sustain an ionic current. An electrode reaction is therefore not needed until the double layer is fully charged. For this reason it is common practice to determine the transference number of a species for which the electrodes are reversible as the ratio between the stationary current and the initial current. ... 

Besides this bulk phase elfects surfactants will adsorb at the liquid-liquid interface. Their influence on mass transfer may then be on different mechanism. A blocking effect of adsorption layers in a diffusional transport regime is well known and results in a reduction of mass transfer [54-57] and even Marangoni instabilities [58,59] are found. However, in the kinetical mass-transfer regime, both an enhancement and retartion of mass transfer [59] is with Gibbs surfactant layers. With extracting ionic species, ionic surfactants will induce an electrostatic double layer, which can be related to the -potential. As a result, there exists, in addition to the chemical potentials, an... 

It has been suggested that fractal pore structures lead to CPE behavior during double-layer charging (31 )(32). In afractal structure, all length scales are present, and for a process, such as diffusion, that contains a well-defined size scale ( [D/w it seems to be true that a CPE with an exponent closely related to the fractal dimension occurs in the response (33). For the case of double-layer charging, however, the situation is not so clear because there is no similar characteristic distance involved (34)(35). It appears that the fractal blocking interface does lead to CPE behavior, but the exponent is not simply related to the fractal dimension. 

This circuit is representative of a conductive solution with conductance inversely proportional to the R-parameter. At low frequencies a "blocking" fully capacitive interface emerges where only the double-layer charging represented by the capacitor C is present. Total impedance of the circuit and its real and imaginary components can be expressed as ... 

Electrolytic interface polarization results from different ion mobility and diffusion processes causing a charge build-up in non-metallically mineralized rocks. A prominent type is the membrane polarization at clay particles in the pore space. Ward (1990) describes the membrane polarization mechanism as follows Polarization arises chiefly in porous rocks in which clay particles partially block ionic solution paths. The diffuse cloud of cations (double layer) in the vicinity of a clay surface is characteristic for clay-electrolyte systems. Under the influence of an electrical potential, positive charge carriers easily pass through the cationic cloud, but negative charge carriers accumulate an ion-selective membrane, therefore, exists. Upon elimination of the electrical potential, all charges return to equilibrium positions. Consequently, a surplus of both cations and anions occurs at one end of the membrane zone, while a deficiency occurs at the other end. 

The simplest test-cell that can be envisaged consists of two blocking electrodes in contact with the polymer whose conductance is to be measured. The three-component equivalent circuit is shown in Fig. 1.4(a). The polymer acts as a resistor, R, which is in series with the double layer capacitor, Cdi, at the interface, and in parallel with the geometric capacitance, Cg, as discussed in Section 1.4.2. This simple network is dealt with by evaluating the parallel term first to give an impedance Zp, which is then added to the double-layer impedance. 

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