Metal-electrolyte interface images

Models for the compact layer of the metal-electrolyte interface have become more and more elaborate, providing better and better representations of observed electrocapillary data for different metals, solvents, and temperatures, but almost always leaving the metal itself out of consideration, except for consideration of image interactions of the solvent dipoles. For reviews of these models, see Parsons,13 Reeves,14 Fawcett et a/.,15 and Guidelli,16 who gives detailed discussion of the mathematical as well as the physical assumptions used. 

Another interface that needs to be mentioned in the context of polarized interfaces is the interface between the insulator and the electrolyte. It has been proposed as a means for realization of adsorption-based potentiometric sensors using Teflon, polyethylene, and other hydrophobic polymers of low dielectric constant Z>2, which can serve as the substrates for immobilized charged biomolecules. This type of interface happens also to be the largest area interface on this planet the interface between air (insulator) and sea water (electrolyte). This interface behaves differently from the one found in a typical metal-electrolyte electrode. When an ion approaches such an interface from an aqueous solution (dielectric constant Di) an image charge is formed in the insulator. In other words, the interface acts as an electrostatic mirror. The two charges repel each other, due to the low dielectric constant (Williams, 1975). This repulsion is called the Born repulsion H, and it is given by (5.10). 

CV is usually one of the first methods to be applied to new polymer films. As in the usual solution-based CV, a triangular-shaped potential is applied to the cell, but in this case the working electrode is coated with the polymer to be studied. The cell is filled with electrolyte solution that does not contain any electroactive solute. When current is flowing there is electron transfer across the metal-polymer interface and simultaneously ion transfer across the polymer-solution interface. The only diffusion-controlled process occurs inside the polymer film, where ions have limited mobility. If the polymer film is very thin, the diffusion time of ions is very short and we expect that the reverse electron transfer occurs exactly at the same potential on the return sweep of CV i.e., we should have a voltammogram with symmetrical and mirror-image cathodic and anodic waves. The current in the reversible case is... 

This quantum mechanical treatment can be used also for describing the semiconductor/vacuum interface. As for the metal/vacuum system, image forces play an important role. In fact, if image forces are considered in Gurevich s theory, a similar expression to Kane s result is obtained, namely, the unity power law. The theory predicts that the -power law may be obeyed for emission into an electrolyte in all cases where the unity power law holds for emission into vacuum. Kane dervied a law for the vacuum case which pertains to emission from surface states in which the transition is direct and the threshold point lies at the Fermi level, Ep (Table IV). 

For in situ investigations of electrode surfaces, that is, for the study of electrodes in an electrochemical environment and under potential control, the metal tip inevitably also becomes immersed into the electrolyte, commonly an aqueous solution. As a consequence, electrochemical processes will occur at the tip/solution interface as well, giving rise to an electric current at the tip that is superimposed on the tunnel current and hence will cause the feedback circuit and therefore the imaging process to malfunction. The STM tip nolens volens becomes a fourth electrode in our system that needs to be potential controlled like our sample by a bipotentiostat. A schematic diagram of such an electric circuit, employed to combine electrochemical studies with electron tunneling between tip and sample, is provided in Figure 5.4. To reduce the electrochemical current at the tip/solution... 

The interface is, from a general point of view, an inhomogeneous dielectric medium. The effects of a dielectric permittivity, which need not be local and which varies in space, on the distribution of charged particles (ions of the electrolyte), were analyzed and discussed briefly by Vorotyntsev.78 Simple models for the system include, in addition to the image-force interaction, a potential representing interaction of ions with the metal electrons. 

Note the peculiarities of the work functions in a nonconducting medium (vacuum, pure solvent) and a conducting medium (electrolyte solution) when two metals contact each other, an electron equilibrium is always established between them, i.e., the condition te(l) = is met. The work function W is defined as the work of electron transfer from a metal to a point in the nonmetallic phase which is in the proximity to the interface at such a distance that the potential variation with distance can be ignored, i.e., beyond the superficial electric double layer, including the region in which the image forces are active ... 

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