Electrostatic double-layer effects

DFT Modeling of Stark-Tuning Effect CO on Polarized Pd(lOO) as a Probe for Double-Layer Electrostatic Effects in... 

DFT modeling of Stark-tuning effect CO on polarized Pd(lOO) as a probe for double-layer electrostatic effects in electrochemistry... 

The extent to which ions, etc. adsorb or experience an electrostatic ( coulombic ) attraction with the surface of an electrode is determined by the material from which the electrode is made (the substrate), the chemical nature of the materials adsorbed (the adsorbate) and the potential of the electrode to which they adhere. Adsorption is not a static process, but is dynamic, and so ions etc. stick to the electrode (adsorb) and leave its surface (desorb) all the time. At equilibrium, the rate of adsorption is the same as the rate of desorption, thus ensuring that the fraction of the electrode surface covered with adsorbed material is constant. The double-layer is important because faradaic charge - the useful component of the overall charge - represents the passage of electrons through the double-layer to effect redox changes to the material in solution. 

On the assumption of a somewhat mobile double layer, electrostatic attraction between particles may occur, due to this displacement effect, even if the total net charge be not zero, i.e. coagulation may take place before the isoelectric point is reached. The data of Zsigmondy on gold particles, and of Powis Zeit. Phys. Ohem. Lxxxix 186, 1915) on oil particles, have indeed shown that the optimum point for precipitation is not actually at the isoelectric point, although in the case of gold, practically complete discharge of the double layer had to take place before coalescence. 

If a pressure gradient is imposed on the fluid, a flow which contains charges appears and thus generates an electrostatic field due to the charge displacement. The electrostatic potential that is derived from this field is called the streaming potential. Without a pressure gradient, the fluid can be moved using an electrical field hence an electroosmotic flow appears. The fluid velocity profile which develops without the electric double-layer (EDL) effect is parabolic the... 

In view of the assumed lack of individual lateral adsorbate-adsorbate interactions the only electrostatic energy to be accounted for in expressing the electrochemical potential, p j, of the adsorbate is the electrostatic energy of interaction of the adsorbate dipole with the effective double layer field. This is accounted for by ... 

As shown in Chapter 4 (section 4.5.9.2), Equation (8.14) can also be derived via a rigorous electrostatic model which takes into account the presence of the effective double layer on the catalyst surface and gives in general ... 

The parameter a in Equation (11.6) is positive for electrophobic reactions (5r/5O>0, A>1) and negative for electrophilic ones (3r/0Oelectrochemical promotion behaviour is frequently encountered, leading to volcano-type or inverted volcano-type behaviour. However, even then equation (11.6) is satisfied over relatively wide (0.2-0.3 eV) AO regions, so we limit the present analysis to this type of promotional kinetics. It should be remembered thatEq. (11.6), originally found as an experimental observation, can be rationalized by rigorous mathematical models which account explicitly for the electrostatic dipole interactions between the adsorbates and the backspillover-formed effective double layer, as discussed in Chapter 6. 

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. 

In order to describe the effects of the double layer on the particle motion, the Poisson equation is used. The Poisson equation relates the electrostatic potential field to the charge density in the double layer, and this gives rise to the concepts of zeta-potential and surface of shear. Using extensions of the double-layer theory, Debye and Huckel, Smoluchowski,... 

In the crudest approximation, the effect of the efectrical double layer on electron transfer is taken into account by introduction of the electrostatic energy -e /i of the electron in the acceptor into the free energy of the transition AF [Frumkin correction see Eq. (34.25)], so that corrected Tafel plots are obtained in the coordinates In i vs. e(E - /i). Here /i is the average electric potential at the site of location of the acceptor ion. It depends on the concentration of supporting electrolyte and is small at large concentrations. Such approach implies in fact that the reacting ion represents a probe ion (i.e., it does not disturb the electric held distribution). 

Clearly, then, the chemical and physical properties of liquid interfaces represent a significant interdisciplinary research area for a broad range of investigators, such as those who have contributed to this book. The chapters are organized into three parts. The first deals with the chemical and physical structure of oil-water interfaces and membrane surfaces. Eighteen chapters present discussion of interfacial potentials, ion solvation, electrostatic instabilities in double layers, theory of adsorption, nonlinear optics, interfacial kinetics, microstructure effects, ultramicroelectrode techniques, catalysis, and extraction. 

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