Chemical adsorbates

Fig. XI-2. Variation of physically adsorbed (Pp) and chemically adsorbed (Pc) segments as a function of time for cyclic polymethylsiloxane adsorbing from CCI4 onto alumina (from Ref. 43). Note that the initial physisoiption is overcome by chemical adsorption as the final state is reached. [T. Cosgrove, C. A. Prestidge, and B. Vincent, J. Chem. Soc. Faraday Trans., 86(9), 1377-1382 (1990). Reproduced by permission of The Royal Society of Chemistry.]...

XI-1C) as well as alongside it. The infrared spectrum of CO2 adsorbed on 7-alumina suggests the presence of both physically and chemically adsorbed molecules [3]. 

Precipitate particles grow in size because of the electrostatic attraction between charged ions on the surface of the precipitate and oppositely charged ions in solution. Ions common to the precipitate are chemically adsorbed, extending the crystal lattice. Other ions may be physically adsorbed and, unless displaced, are incorporated into the crystal lattice as a coprecipitated impurity. Physically adsorbed ions are less strongly attracted to the surface and can be displaced by chemically adsorbed ions. 

Example of copredpitation (a) schematic of a chemically adsorbed inclusion or a physically adsorbed occlusion in a crystal lattice, where C and A represent the cation-anion pair comprising the analyte and the precipitant, and 0 is the impurity (b) schematic of an occlusion by entrapment of supernatant solution (c) surface adsorption of excess C. 

The flotation process is based on the exploitation of wettabiUty differences of particles to be separated. Differences of wettabiUty among soHd (mineral) particles can be natural, or can be induced by the use of chemical adsorbates. Because the largest segment of industrial appHcations is conducted in water, with air, the following discussion is confined mainly to these fluids. 

Chemical Bond Formation (Chemisorption). This is the mechanism that leads to the formation of the strongest bonds between coUectors and mineral surfaces. Chemically adsorbed reagents usuaUy form surface compounds at the active waU sites. The flotation of calcite (CaCO ) and... 

It has been suggested that gold does not have a stable surface oxide (227), and therefore, its surface can be cleaned simply by removing the physically and chemically adsorbed contaminants. However, more recently it has been shown that oxidation of gold by uv and ozone at 25°C gives a 1.7 0.4 — nm thick AU2O2 layer (228), stable to extended exposure to ultra high vacuum (UHV) and water and ethanol rinses. 

Most soHd catalysts used on a large scale are porous inorganic materials. A number of these and the reactions they catalyze are summarized ia Table 1 (10). Catalysis takes place as one or more of the reactants is chemisorbed, chemically adsorbed, on the surface and reacts there. The activity and selectivity of the catalyst depend strongly on the surface composition and stmcture. 

Estimating Minimum Sample Quantity for Moisture Measurement Estimates of material quantity for testing moisture content depend on mechanisms of moisture distribution in the material. Moisture is physically retained on particle surfaces, chemically adsorbed on surfaces and within pores of particulate solids, and contained as an internal constituent of solids. Significant internal moisture is most often encountered in organic and agricultural source materials. 

Having chosen a suitable refrigerant, the best adsorbent must be found. Zeolites, silica gels and chemical adsorbents have been used as well as carbons, but this chapter will concentrate on the carbon adsorbents. An indication as to the range of cop s that can be expected and the influence of the type of carbon used can be obtained by modelling the perfonnance of carbons with a range of adsorption parameters. For this purpose it is preferable to use the Dubinin-Raduschkevich... 

In the catalyzed decomposition, N20 is chemically adsorbed on the surface of the solid. A chemical bond is formed between the oxygen atom of an N20 molecule and a gold atom on the surface. This weakens the bond joining nitrogen to oxygen, making it easier for the N20 molecule to break apart. Symbolically, this process can be shown as... 

Our understanding of the nature of a solid surface and of the interaction between a molecule physically or chemically adsorbed on the surface is of... 

This review will endeavor to outline some of the advantages of Raman Spectroscopy and so stimulate interest among workers in the field of surface chemistry to utilize Raman Spectroscopy in the study of surface phenomena. Up to the present time, most of the work has been directed to adsorption on oxide surfaces such as silicas and aluminas. An examination of the spectrum of a molecule adsorbed on such a surface may reveal information as to whether the molecule is physically or chemically adsorbed and whether the adsorption site is a Lewis acid site (an electron deficient site which can accept electrons from the adsorbate molecule) or a Bronsted acid site (a site which can donate a proton to an adsorbate molecule). A specific example of a surface having both Lewis and Bronsted acid sites is provided by silica-aluminas which are used as cracking catalysts. 

Adsorption Coefficient (K c)—The ratio of the amount of a chemical adsorbed per unit weight of organic carbon in the soil or sediment to the concentration of the chemical in solution at equilibrium. 

Adsorption Ratio (Kd)—The amount of a chemical adsorbed by a sediment or soil (i.e., the solid phase) divided by the amount of chemical in the solution phase, which is in equilibrium with the solid phase, at a fixed solid/solution ratio. It is generally expressed in micrograms of chemical sorbed per gram of soil or sediment. 

Amongst the earliest measurements involving chemical functionality of the probe were those of Nakagawa et al. [69]. They investigated octadecyltrichlorosilane (OTS) chemically modified tips against chemically adsorbed monolayers of different alkyl-trichlorosilanes in ethanol, as shown schematically in Figure 14. When both tip and surface were modified by OTS, a large adhesive force was observed that was not present for the case of an unmodified silicon nitride tip on an OTS-modified surface. Additionally there... 

A water sample is usually analyzed as it is, but is filtered through a glass-fiber filter to remove SS or to analyze both water and SS separately when chemicals adsorbed on SS used to be determined for a special purpose of a study. All filtration apparatus should be washed with the sample water in order to avoid contamination. 

Air bubbles being chemically adsorbed to the insoluble solids in their original form or their flocculated structure... 

Desorption of (chemically) adsorbed species from the surface of the catalyst. 

Figure 15.4(A) shows the effect of the R = Zn2+/Al3+ ratio, which determines the charge density of the LDH layer, on the Freundlich adsorption isotherms. K values are far higher than those measured for smectite or other inorganic matrices. The increase in Kf with the charge density (Kf= 215, 228, 325mg/g, respectively, for R = 4, 3 and 2) is supported by a mechanism of adsorption based on an anion exchange reaction. The desorption isotherms confirm that urease is chemically adsorbed by the LDH surface. The aggregation of the LDH platelets can affect noticeably their adsorption capacity for enzymes and the preparation of LDH adsorbant appears to be a determinant step for the immobilization efficiency. [ZnRAl]-urease hybrid LDH was also prepared by coprecipitation with R = 2, 3 and 4 and Q= urease/ZnRAl from 1 /3 up to 2.5. For Q < 1.0,100 % of the urease is retained by the LDH matrix whatever the R value while for higher Q values an increase in the enzyme/LDH weight ratio leads to a decrease in the percentage of the immobilized amount.

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