Contaminant monolayer

If the oxide-covered, contaminated surface is heated for 30 min. at 5(X)°, the surface is found to be identical with the original contaminated surface. Furthermore, the nickel oxide covered surface appears to be the same for both the initially clean and initially contaminated surfaces. These observations indicate that the oxide layer is formed on the outer surface of the contamination monolayer. 

From oil contaminated monolayer to thin oil film Pseudo-partial wetting... 

In choosing a SAM system for surface engineering, there are several options. Silane monolayers on hydroxylated surfaces are an option where transparent or nonconductive systems are needed. However, trichlorosilane compounds are moisture-sensitive and polymeri2e in solution. The resulting polymers contaminate the monolayer surface, which occasionally has to be cleaned mechanically. CarboxyUc acids adsorb on metal oxide, eg, AI2O2, AgO through acid—base interactions. These are not specific therefore, it would be impossible to adsorb a carboxyUc acid selectively in the presence of, for example, a terminal phosphonic acid group. In many studies SAMs of thiolates on Au(lll) are the system of choice. 

CO oxidation catalysis is understood in depth because potential surface contaminants such as carbon or sulfur are burned off under reaction conditions and because the rate of CO oxidation is almost independent of pressure over a wide range. Thus ultrahigh vacuum surface science experiments could be done in conjunction with measurements of reaction kinetics (71). The results show that at very low surface coverages, both reactants are adsorbed randomly on the surface CO is adsorbed intact and O2 is dissociated and adsorbed atomically. When the coverage by CO is more than 1/3 of a monolayer, chemisorption of oxygen is blocked. When CO is adsorbed at somewhat less than a monolayer, oxygen is adsorbed, and the two are present in separate domains. The reaction that forms CO2 on the surface then takes place at the domain boundaries. 

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). 

Before 1950, it was impossible to examine the true structure of a solid surface, because, even if a surface is cleaned by flash-heating, the atmospheric molecules which constantly bombard a solid surface very quickly re-form an adsorbed monolayer, which is likely to alter the underlying structure. Assuming that all incident molecules of oxygen or nitrogen stick to the surface, a monolayer will be formed in 3 x 10 second at 1 Torr (=1 mm of mercury), that is, at 10 atmosphere a monolayer forms in 3 s at 10 Torr, or 10 atmosphere but a complete monolayer takes about an hour to form at 10 Torr. The problem was that in 1950, a vacuum of 10" Torr was not achievable lO Torr was the limit, and that only provided a few minutes grace before an experimental surface became wholly contaminated. 

Davies et al. (D9) have recently measured the rates of absorption of various gases into turbulently stirred water both with carefully cleaned surfaces and with surfaces covered with varying amounts of surfactants. That hydrodynamic resistances, rather than monolayer resistances, are predominant in their work is consistent with the high sensitivity of kL to very small amounts of surface contamination and also with the observation that a limit to the reduction in kL is found (D7, D9). This is in agreement with the results of Lindland and Terjesen (L9), who found that after a small concentration of surfactant had been used further additions caused but little change in terminal velocity (L9). 

It has been proposed recently [28] that static friction may result from the molecules of a third medium, such as adsorbed monolayers or liquid lubricant confined between the surfaces. The confined molecules can easily adjust or rearrange themselves to form localized structures that are conformal to both adjacent surfaces, so that they stay at the energy minimum. A finite lateral force is required to initiate motion because the energy barrier created by the substrate-medium system has to be overcome, which gives rise to a static friction depending on the interfacial substances. The model is consistent with the results of computer simulations [29], meanwhile it successfully explains the sensitivity of friction to surface film or contamination. 

AES spectra on these surfaces after reaction also show only N at a density of approximately one monolayer with contaminants (typically,... 

Again, AES spectra arise from core levels, hence are characteristic for a given element. Surface elemental analysis (except H and He) with very high sensitivity, able to detect < 1% of a monolayer. Usually employed first to check that the surface is free from contamination... 

The first structural study of a UPD layer involved the formation of a series of Agl monolayers on Pt(lll) single crystals [132, 133], Pt is the quintessential catalytic metal, and thus reacts with almost any organic compound, other than simple alkanes [134], in other words, it is easily contaminated. In the early days of UHV-EC studies, contamination during transfer of Pt single crystals from the analysis chamber to the electrochemical cell and back was the chief stumbling block to well-defined studies of... 

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