Double layer dipolar

The behavior of simple and molecular ions at the electrolyte/electrode interface is at the core of many electrochemical processes. The complexity of the interactions demands the introduction of simplifying assumptions. In the classical double layer models due to Helmholtz [120], Gouy and Chapman [121,122], and Stern [123], and in most analytic studies, the molecular nature of the solvent has been neglected altogether, or it has been described in a very approximate way, e.g. as a simple dipolar fluid. Computer simulations... 

The presence of an electrical potential drop, i.e., interfacial potential, across the boundary between two dissimilar phases, as well as at their surfaces exposed to a neutral gas phase, is the most characteristic feature of every interface and surface electrified due to the ion separation and dipole orientation. This charge separation is usually described as the formation of the ionic and dipolar double layers. The main interfacial potential is the Galvani potential (termed also by Trasatti the operative potential), which is the difference of inner potentials (p and of both phases. It is a function only of the chemical... 

In the absence of specific adsorption and dipolar contributions, there is no excess charge in the whole double layer when positive and negative ions are equally distributed at the plane of closest approach qM and A02 will be both zero. The corresponding electrode potential is the potential of zero charge (pzc) which can be evaluated from the minimum in the differential capacity—potential curve for a metal electrode in contact with a dilute electrolyte [6]... 

Adsorption of dipolar molecules will not contribute to a net surface charge, but the presence of a layer of orientated dipolar molecules at the surface may make a significant contribution to the nature of the electric double layer. 

More recent models of the double layer have taken into account the physical nature of the interfacial region. In dipolar solvents, such as water, it is clear that an interaction between the electrode and the dipoles must exist. That this is important is reinforced by the fact that solvent concentration is always much higher than solute concentration. For example, pure water has a concentration of 55.5 mol dm-3. 

Studies in nonaqueous dipolar aprotic solvents allowed the elucidation of the complicated role of the solvent nature in determining the - double layer structure and kinetics of electrochemical reactions. Special attention was paid to the phenomenon of ion - solvation and its effect on -> standard electrode potentials. Experimental studies of the various electrochemical systems in nonaqueous media greatly contributed to the advancement of the theory of elemental electron-transfer reactions across charged interfaces via the so-called energy of solvent reorganization. 

Molecular dipolar polarization was difficult to define from dielectric measurements. A large first dispersion in time for isothermal cures of Resin 5208 is attributed to charge migration in a viscous medium. High values of dielectric constant and dielectric loss factor are attributed to the formation of an ion double-layer and sample conductivity, respectively. Limited frequency data on a smaller second dispersion prevent unequivocal assignment of its relationship to molecular changes. 

One can identify three physical phenomena which lead to the observed values of Ax- First, an ionic double layer can be established if the distance of closest approach for cations and anions to the interface is not the same. Second, if one of the components of the solution has a dipole moment, it may assume a preferred orientation at the interface, thereby giving rise to dipolar potential drop. Finally, the presence of the solute can change the orientation of water molecules at the interface from that present in the pure solvent. The fact that Ax is usually positive is evidence that the anion approaches the surface more closely than the cation. This is not difficult to understand given that anions are more weakly solvated than... 

The importance of the dipolar nature of the solvent and of the interactions between solvent and electrode were recognized in the double layer model by Bockris, Devanathan and Muller [9], Water hydrates the electrode, which is regarded as a giant ion, and so contributes to the electric fields near the interface. Starting with the work of Damaskin and Frumkin [10] the differences between sp and transition metals were described by a series of chemical models. More details on double layer models can be found, e.g., in Refs. 2, 11, 12. 

At frequencies below 63 Hz, the double-layer capacitance began to dominate the overall impedance of the membrane electrode. The electric potential profile of a bilayer membrane consists of a hydrocarbon core layer and an electrical double layer (49). The dipolar potential, which originates from the lipid bilayer head-group zone and the incorporated protein, partially controls transmembrane ion transport. The model equivalent circuit presented here accounts for the response as a function of frequency of both the hydrocarbon core layer and the double layer at the membrane-water interface. The value of Cdl from the best curve fit for the membrane-coated electrode is lower than that for the bare PtO interface. For the membrane-coated electrode, the model gives a polarization resistance, of 80 kfl compared with 5 kfl for the bare PtO electrode. Formation of the lipid membrane creates a dipolar potential at the interface that results in higher Rdl. The incorporated rhodopsin may also extend the double layer, which makes the layer more diffuse and, therefore, decreases C. ... 

The components Rt and C of the dipolar element in figure 6 represent the resistance of the solution between the working and reference electrodes, the transfer resistance [48] and the capacitance of the double layer [49], respectively. The quantities Rt and C,... 

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