Segregation to surfaces

Figure 13.5 shows that there is a clear preference for Sn to segregate to surface layers of Ni for Sn surface concentrations lower than l/3 ML. Further addition of Sn to the surface is not favorable and Sn atoms will preferentially occupy sites in the bulk. Figure 13.5 also shows that the separation of Sn in a pure Sn phase is not favorable. In the calculations of the differential segregation energies we have neglected entropic contributions. This is reasonable as the difference in energy of various Sn/Ni structures is substantial, and the addition of the entropic effects would not affect our conclusions. 

As indicated in previous chapters, low molar mass materials can segregate to surfaces and hence the molar mass dependence of the surface tension is an important factor in considering the performance of a polymer material. Wu has proposed that... 

This case is particularly interesting since the surface segregation energy can be directly compared to surface core level binding energy shifts (SCLS) measurements. Indeed, if we assume that the excited atom (i. e., with a core hole) is fully screened and can be considered as a (Z + 1) impurity (equivalent core approximation), then the SCLS is equal to the surface segregation energy of a (Z + 1) atom in a Z matrixi. in this approximation the SCLS is the same for all the core states of an atom. 

These standards also outline requirements for surface finish, grain size, heat treatment, metallurgical cleanness, absence of delta ferrite and alloy segregation to ensure that besides having a well balanced chemistry the alloys shall be in the proper metallurgical condition to yield optimum mechanical and corrosion resistant properties. 

In addition, it has fairly recently been recognised that impurities and alloying elements will also tend to segregate to free surfaces. The implications of this for corrosion resistance and particularly for passive-film formation have received relatively little attention. 

In alloys, the component with the lower surface free energy segregates to the surface, making the surface composition different from that of the bulk (Scheme 5.1). 

Impurities in metals, such as C, O, or S, segregate to the surface because there they lower the total energy due to their lower surface free energy. 

ZnO is, apparently, a very suitable support for the copper particles. Evidence exists, however, that its role does not have to be limited to that of a support only. Nakamura et al. have studied the influence of Zn on methanol synthesis on copper crystals by depositing Zn on the surface [J. Nakamura, I. Nakamura, T. Uchijima, Y. Kanai, T. Watanabe, M. Saito, and T. Fujitani, J. Catal. 160 (1996) 65]. They found that the rate was enhanced by a factor of six (see Fig. 8.14), suggesting that Zn atoms also act as a chemical promoter. Whether some of the ZnO in the real catalyst is actually reduced to such a degree that it can alloy into the copper particles and segregate to the surface, as suggested by Nakamura, is still a controversial topic. 

The data of Zink et al. (1998) illustrate the measurement by NRA of near-surface composition profiles in isotopically labelled polymer blends. If a mixture of polymers is adjacent to a phase interface (e.g. a solid or an air surface), often one of the components is preferentially attracted to the surface and will segregate to it, and this phenomenon will influence the tribological behaviour the interface (lubrication, wear and adhesion). 

The wall-PRISM theory has also been implemented for binary polymer blends. For blends of stiff and flexible chains the theory predicts that the stiffer chains are found preferentially in the immediate vicinity of the surface [60]. This prediction is in agreement with computer simulations for the same system [59,60]. For blends of linear and star polymers [101] the theory predicts that the linear polymers are in excess in the immediate vicinity of the surface, but the star polymers are in excess at other distances. Therefore, if one looks at the integral of the difference between the density profiles of the two components, the star polymers segregate to the surface in an integrated sense, from purely entropic effects. 

An inhomogeneous system of interest is one in which a monolayer or less of a material segregates to the surface of a particle. The fluorescent molecule dil(5) (Figure 8.14) is an example of a material which is expected to be surface active on polar liquids because of its hydrophilic head group and hydrophobic side chains. In fact, dil(5) has been used to prepare Langmuir-Blodgett films on water(21) and would be expected to be surface active on glycerol. 

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