Variation of adsorption-energy with

In the preceding chapters we have seen how one can in a general way understand colloid formation and stability in terms of the variation of free energy with the separation between the surfaces of two particles. When this free energy is measured with respect to the state in which the two surfaces are in contact, it may be identified with the surface or interfacial tension (see Figure 2.4). A major contribution to this surface tension arises from the van der Waals attractive forces between the particles. It turns out, however, that the surface tension, and hence the force between the two surfaces, is also strongly influenced by the adsorption of molecules at the surfaces. 

Both surface atoms and adsorbates must participate to form the surface chemical bond. In order to determine the nature of the bond, the heat of adsorption is measured as a function of the pertinent variables. These include trends across the periodic table, variations of bond energies with adsorbate size, molecular structure and coverage, and substrate structure. Changes in the electronic and atomic structure of the bonding partners are determined and compared with their electronic and atomic (or molecular) structure before they formed the surface bond. 

The variation of activation energy as a function of alloy composition is shown in Fig. 23. There is a fairly sharp rise in activation energy from about 0.4 kcal/mole at 50% Ag to 5.9 kcal/mole on pure Ag, but the increase observed with Pd-Au wires 127) was more abrupt. Pd-Ag alloy wires 148) showed a gradual increase from about 2 kcal/mole for pure Pd to 4 kcal/mole at 60% Ag, followed by a more rapid, smooth increase from 4.9 kcal/mole at 80% Ag to 11.5 kcal/mole on pure Ag. The results on films were also used to derive heats of adsorption at equilibrium coverage. Values increased from 1.29 kcal/mole on the 68% Ag alloy to 2.89 kcal/mole on pure Ag. 

Figure 4.37. Variation of volcano plots with the molecular precursor adsorption energy.

FIG. 15 Linear variation of adsorption free energy, —AGa, with experimental temperatures (°Q, measured at infinite dilution. 

Figure 6.2 Variation of adsorption amount F with adsorption energy per segment...

Thus, on a nonuniform surface on which the activation energies for the rate of adsorption varies with coverage, it is expected that there will also be some corresponding variation in the heat of adsorption. However, one energy is associated with an activation barrier E, and the other with an equilibrium heat of adsorption —A.H there is therefore no a priori reason for a specific functional relationship to exist between... 

When several species are adsorbed on the surface, the variation of free energy of adsorption with coverage can be expressed in the general form (49)... 

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