Restructuring of Surfaces and Interfaces

We are rather familiar with the effects that small amounts of additives can have on surfaces. For example, one part per million of carbon monoxide in a gas stream can poison a platinum catalyst surface and inhibit oxidation reactions. Similarly, five parts per million of potassium ferricyanide can stop the crystallization of sodium chloride from brine solution, The example given in the previous section is another illustration of such surface effects a few parts per million of sodium polyacrylate can produce a repulsion between oxide surfaces, thus inhibiting adhesion. 

In adhesion studies, the first demonstration of this effect was by Lord Rayleigh, who showed in 1899 that a single layer of fatty acid at a surface could reduce surface tension, adhesion, and friction. Since that time there has been enormous effort to study the structure of surfaces and interfaces to understand these substantial phenomena. It has become clear over the past 50 years that clean solid surfaces have a different structure from the bulk, and that this difference in structure must depend on any impurities or adhering bodies brought down onto the surface. Several examples are described in Somorjai s book.  

Consider the surface of a solid shown schematically in Fig. 6.28. There are two simple reconstructions that can take place the first is a simple shrinking of the clean surface atoms into the bulk, as they are pulled in by the unbalanced atomic forces, rather like the surface tension effect in a liquid the second is a reordering that can occur if the surface atoms can move from side to side, as in the (100) crystal face of silicon. Here the surface atoms are loosely packed and move both down and sideways in the two top layers, as shown in Fig. 6.28(b). 

clean surfaces tend to restructure to satisfy the unbalanced atomic forces. When foreign atoms adsorb onto such surfaces, further reconstruction is possible. For example, the chemisorption of contaminant atoms can destroy the clean surface reconstructions described above. Alternatively, new structures may form, as when carbon is chemisorbed on nickel (100) surfaces. If such carbon-coated surfaces were brought together in an adhesion experiment, the carbon would have to diffuse out before full Ni-Ni adhesion could be attained. Such diffusion and restracturing effects could explain the observed changes of adhesion with time. Also, hysteresis in adhesion values could then be accounted for. 

The importance of these interface structuring phenomena stems from the large number of possible adhesion states that could exist as different molecules are arranged at the surfaces. Little is known about the possible triggering or 

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