The Adsorption of Ions on Dielectric Surfaces

When an ion is adsorbed on the surface of a dielectric, itself consisting of ions, we may expect Coulomb forces to act between the ions of the adsorbent and the adsorbed ion. A positive ion, adsorbed on top of a negative ion of an adsorbent, is attracted by this ion, but it is repelled by the ions surrounding the one with which it is in direct contact, attracted again by the then following ions, etc. The result is a rather weak attraction. Hiickel (S3) derived the following equation for the electrostatic field emanating from a cubic face of the surface of an alkali halide crystal  

This energy is only 6.6% of the energy contribution by Coulomb forces which we would have found if the adsorbing ion of the crystal surface had not been surrounded by all other ions of the crystal. The collaboration of all the ions of the crystal results in relatively small energy contributions and, moreover, in attraction forces of a very short range. At a distance r = 2rc the force is negligibly small. 

When an ion approaches the adsorbing crystal, following a line perpendicular to the surface and ending in a surface ion of the same charge as its own, it will be repelled. The electrostatic part of the repulsion 

In all these electrostatic considerations the surface of the ionic crystal was idealized, as described in Sec. IV,2, as if it were cut by means of an ideally sharp razor blade. Our lack of knowledge of the structural deviations of the surface arrangements with respect to the structure inside the crystal renders it impossible for us to make any quantitative or semiquantitative statements regarding the actual adsorption energies caused by electrostatic forces. We can only say that in most ionic crystals negative ions i.e., halide ions or oxide ions, tend to form the outside (adsorbing) surface. We shall have an opportunity (see, for example, Secs. V,5 and VI,5) to revert to this phenomenon. 

The foregoing electrostatic calculations hold, moreover, only for positions in the middle of a cubic face of a crystal of the NaCl type. Any deviation from this situation may result in a stronger electrostatic bond. Corners and edges of crystals, other crystallographic faces, lattice disturbances, etc., may all form active spots where the electrostatic adsorption of ions is relatively strong. We shall return to the problem of active spots in Sec. V,12. 

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