Electric double layer influence

In active matrix addressing of the TN LCD, the applied voltage is maintained during the holding period. In such a situation, ionic impurity transfer can occur because of the electric field maintained between electrodes. The polarity of the 

Holding Period Partial inversion ofPs direction Holding Period 

In the case of the simple matrix driving of SSFLCs, two electrodes are shorted during the holding period. 

When ionic impurities exist in an FLC, like (a) and (b), the spontaneous polarization is affected by the electric double layer formed by the ionic impurity (c). As a result, partial inversion of the spontaneous polarization direction to the opposite direction occurs. In extreme cases, the molecules return to the initial position (f). In this case, the result is the same as if no switching had occurred. The spontaneous polarization induces a dc electric field. This electric field causes the transfer of ionic impurities (d), which form a new electric double layer (e). 

In Fig. 5.2.10(a) [29], the charge on the upper surface due to the spontaneous polarization is in Ps per unit area. This charge can be cancelled by the charge — Ps of the accumulated ionic impurities on the surface. When an external voltage 

5 2005 Kohki Takatoh, Masaki Hasegawa, Mitsuhiro Koden, Nobujoiki Itoh, Ray Hasegawa and Masanoii Sakamoto 

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E° depends rather strongly on the type of surfactant used. The ° values for thin films of three different surfactants (Table 1) are all more negative than 50 mV vs, NHE found for Mb dissolved in pH 7 buffer [28]. The cationic surfactant DDAB gave the most positive °, followed by anionic DHP, then the zwitterionic phosphatidylcholines. ° also depended on electrode material (Table 2), in the order Pt > PG > Au > ITO. These data seem consistent with an electric double-layer influence on the electrode potential felt by the protein, dependent on electrode material and surfactant type. 

On the electrode side of the double layer the excess charges are concentrated in the plane of the surface of the electronic conductor. On the electrolyte side of the double layer the charge distribution is quite complex. The potential drop occurs over several atomic dimensions and depends on the specific reactivity and atomic stmcture of the electrode surface and the electrolyte composition. The electrical double layer strongly influences the rate and pathway of electrode reactions. The reader is referred to several excellent discussions of the electrical double layer at the electrode—solution interface (26-28). 

Activation Overpotential that part of an overpotential (polarisation) that exists across the electrical double layer at an electrode/solution interface and thus directly influences the rate of the electrode process by altering its activation energy. 

In the above we have assumed that no other forces than the electrical are acting at the surface of separation. In general, there will be the capillary forces as well, and we have to take account of the influence of the electrical double layer in considering the adsorption of an electrolyte. If w is the area of the surface, o the interfacial tension, e the charge per unit area, and E the difference of potential, we shall have ... 

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. 

Parsons, R., Equilibrium properties of electrified interfaces, MAE, 1, 103 (1954). Parsons, R., The structure of electrical double layer and its influence on the rates of electrode reactions, AE, 1, 1 (1961). 

A further electrokinetic phenomenon is the inverse of the former according to the Le Chatelier-Brown principle if motion occurs under the influence of an electric field, then an electric field must be formed by motion (in the presence of an electrokinetic potential). During the motion of particles bearing an electrical double layer in an electrolyte solution (e.g. as a result of a gravitational or centrifugal field), a potential difference is formed between the top and the bottom of the solution, called the sedimentation potential. 

Having chosen a particular model for the electrical properties of the interface, e.g., the TIM, it is necessary to incorporate the same model into the kinetic analysis. Just as electrical double layer (EDL) properties influence equilibrium partitioning between solid and liquid phases, they can also be expected to affect the rates of elementary reaction steps. An illustration of the effect of the EDL on adsorption/desorption reaction steps is shown schematically in Figure 7. In the case of lead ion adsorption onto a positively charged surface, the rate of adsorption is diminished and the rate of desorption enhanced relative to the case where there are no EDL effects. 

The electrical double layer has been dealt with in countless papers and in a number of reviews, including those published in previous volumes of the Modem Aspects of Electrochemistry series/ The experimental double layer data have been reported and commented on in several important works in which various theories of the structure of the double layer have been postulated. Nevertheless, many double layer-related problems have not been solved yet, mainly because certain important parameters describing the interface cannot be measured. This applies to the electric permittivity, dipole moments, surface density, and other physical quantities that are influenced by the electric field at the interface. It is also often difficult to separate the electrostatic and specific interactions of the solvent and the adsorbate with the electrode. To acquire necessary knowledge about the metal/solution interface, different metals, solvents, and adsorbates have been studied. 

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