Solid surface contaminants, effect

Trace contaminants are also significant at charged solid surfaces, affecting both the charging process and the surface conductivity. In ambient air atmospheres their effect is often determined by interaction with adsorbed water vapor, whose dominant concentration may be sufficiently large to form a monolayer. Topical antistatic agents for solids typically rely on interaction with adsorbed water and can lose effectiveness at low relative humidity (4-2.1). 

The data in Table 2.2 are for natural samples. It is notoriously difficult to prepare and maintain a clean solid surface, since any freshly created surface quickly becomes contaminated with adsorbed species even with a rather good laboratory vacuum. Any naturally occurring solid material must be considered to have a surface extensively populated by adsorbed atoms and molecules rather than a pristine surface. Such surfaces are what are examined in most laboratory experiments (e.g., those reported in Table 2.2), and, of coruse, just such surfaces are geochemically relevant for noble gas adsorption. It is interesting to note, however, that in other situations, noble gas adsorption can be rather a stronger effect. Thus, for example, Bernatowicz et al. (1983) examined Xe adsorption on a vacuum-crushed lunar rock and concluded that a small part of the freshly created surface had an adsorption potential as high as 14 kcal/mole but that in a few days at 10 8torr this surface was rendered inaccessible to Xe by other chemical species that were better competitors for the sorbent surfaces. 

As indicated previously, there is absolutely no measurable indication of formation of any mixed oxide associating antimony and molybdenum (10). The mixtures of a-Sb204 and M0O3 having worked catalytically under normal conditions (those of fig. 2), where mixed catalysts are perfectly stable compared to fresh catalysts, indicating that no solid-state reaction and no mutual surface contamination of the simple oxide takes place. In addition to surface area measurements and X-ray diffraction, whose sensitivity to small effects is low, the following techniques were used (10) ... 

Chemistry plays a very signiticant role in the CMP process. Several variables listed in Chapter 3, the fluid boundary layer formation at the solid-liquid interface, chemical composition of the surface being polished, the formation of the passivating layer at the solid surface caused by an oxidizer, dissolution of the solid surface or of the mechanically abraded solid fragments or atoms/molecules of the original or passivated layer, the isoelectric point (see Chapter 5) related to abrasive and solid surface charge layers, effective removal or redeposition of the polished material, polished surface contamination and post-CMP passivation, and lifetime and properties of the pad all are determined by the chemical interactions induced by the chemicals in the slurry and the solid surfaces. Thus the choice of chemicals (thus of an appropriate chemistry) in making the slurry is very important. 

Figure 4 compares several of these models with respect to the nature of the constants that each uses. The simplest model (linear sorption or Ai ) is the most empirical model and is widely used in contaminant transport models. values are relatively easy to obtain using the batch methods described above. The Aid model requires a single distribution constant, but the Aid value is conditional with respect to a large number of variables. Thus, even if a batch Aid experiment is carefully carried out to avoid introduction of extraneous effects such as precipitation, the Aid value that is obtained is valid only for the particular conditions of the experiment. As Figure 4 shows, the radionuclide concentration, pH, major and minor element composition, rock mineralogy, particle size and solid-surface-area/solution volume ratio must be specified for each Aid value. 

When applied as a unit operation, adsorption can be used to split mixtures containing significant percentages of adsorbable components or purify streams containing trace amounts of contaminants. These separations, whether for gases or liquids, are accomplished by allowing the fluid and solid phases to interact under controlled conditions. Molecules that are selectively taken up are called adsorbates, and the solid surface that attracts the adsorbate is called the adsorbent. Jargon used in connection with adsorbents and adsorption, as well as some prominent definitions, are listed in Table 14.1. Fundamental causes and effects of adsorption are discussed in Section 14.3. 

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