Image charge repulsion

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). 

The description of both water-metal and ion-metal interactions by a short-range repulsive potential without significant adsorption energy, augmented by the image charge model of electrostatics, leads to contact adsorption of Br and L and to no adsorption in the case of Li+ and F . CL is a borderline case. 

When = e2, image charges are present in the system. The image repulsion between electroneutral planes is exactly canceled by the static van der Waals interaction, and the leading term for the total pressure is exactly the same as in the previous case. 

The individual elements of this potential energy diagram are readily explained When an atom or molecule approaches a metal surface with free electrons, image charges will be introduced through polarization. This leads to an attractive interaction, the so-called Van der Waals interaction, which is proportional to dr, where d is the distance between the atom/molecule and the surface. However, if the electrons in the surface and the molecule do not adjust there will be a strong repulsion proportional to e since this is the manner in which the electronic wave-functions decay far from the surface and the atom/molecule. The dotted line going very steeply upwards indicates this repulsive behavior. 

Ionic charges interact with a surface because of the field reflected by the surface on being polarized. This reflected field is the same as if there were an image charge on the other side of the surface at the same distance. If a is the permittivity of the aqueous medium and z is the permittivity of the surfaces, then an additional repulsive force due to this image charge interaction will occur if e < e. 

Ions in solution, apart from interacting with each other, also feel repulsion from the interface due to image-charge effects, as discussed by Onsager and Samaras [25]. It can be shown, however, that these effects become negligible as soon as the surface coverage exceeds about 2% [12]. 

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