Hydrophobic carbon surfaces

The most important and at the same time the most abundant heteroatom that affects the use of carbon is oxygen. It is usually either chemisorbed on the surface or arranged in the form of functional groups analogous to those existing in organic chemistry [51-54]. Since those groups increase the reactivity of the otherwise hydrophobic carbon surface, oxidation often opens the door to further... 

Water molecules are very weakly adsorbed on the surface of hydrophobic activated carbons [356], Nevertheless, when humidity is high water was found to interfere with adsorption of organic compounds. Although, some of them are able to replace preadsorbed water [75], these quasistable conditions affect the kinetics of the process and feasibility of adsorptive separation/removal [357]. The process is even more complex when the surface of carbon is decorated with functional groups. They provide the hydrogen bonding sites, which are the primary adsorption centers [358]. Then water- water interactions lead to the formation of clusters and condensation of water in micropores at much lower humidity than that at which it happens on a fully hydrophobic carbon surface. Due to the importance of this problem in industrial and military applications numerous reviews were published describing the effects of surface chemisby on the adsorption of water [356, 359]. 

The texture parameters of the above catalysts are given in Table 1. Sbet exhibits a typical decrease which is not in agreement with the change of th S ies and Vvibs there is a similar situation. In addition, the effect of the media (methanolic and aqueous) on the change of the above parameters depends on whether the carbon surface is oxidized or not. In this sense the active phase distribution of ACM is, due to the wettability of the hydrophobic carbon surface, much more uniform than is the case of OACM. The coinciding maxima of the distribution curves of the micropores (Fig.3) reflect the effect of the same media (methanol). 

In addition, surface oxygen complexes affect the surface hydrophobicity, which determines the hydrophobic interaction. Hydrophobic interaction or, more specifically, hydrophobic bonding, describes the unusually strong attraction between hydrophobic molecules and hydrophobic carbon surfaces. The surface hydrophobicity of carbon materials can be determined, for instance, by measuring their contact angle with test liquids of known surface tension, by inverse gas chromatography or by water adsorption. 

Wetting properties. A clean carbon surface is hydrophobic. Surface oxides provide sites of adsorption for water and other polar compounds. The more surface oxides there are, the more distinct is the hydrophilic behavior of the carbon. This was confirmed by Healey et al. (39) for graphitized carbon black, and by Kraus (40) and A. V. Kiselev and his group (41) for carbon black. Beebe and Dell (42) measured the sulfur dioxide adsorption on channel black and found an increase after oxidation at 600°. Further evidence for selective adsorption of polar compounds was provided by Gasser and Kipling (43). 

A wide range of carbonaceous materials can be modified with a stable DNA adsorbed layer. The multi-site attachment of DNA on carbon surfaces seems to be strongly dependent on hydrophobic interactions between DNA bases and carbon substrates such as GC and GC(ox)> HOPG, CNTs and GECs. Al-... 

The collector oil molecules are amphiphilic, with their polar groups exhibiting some affinity for particular metal ores, and so create a hydrophobic particle surface on adsorption. They can be anionic, cationic or non-ionic. Organic xanthates and thiophosphates are often used for sulphide ores and long-chain fatty acids for oxide and carbonate ores. 

Another modification easily assessed by immersion microcalorimetry is the change in hydrophobicity of a surface, e.g. by oxidizing a graphitized carbon surface. The energy of immersion in water was shown to increase almost linearly with decrease in hydrophobicity (Young et al., 1954) and the energies of immersion of hydrophobic and hydrophilic patches were estimated to be 31 and 730 J m-2, respectively (Healy et ah, 1955). 

In addition to pore size distribution, the surface chemistry of the activated carbon can have an important influence on the adsorption of certain compounds. As the adsorptive surface of most activated carbons is hydrophobic, they are best suited for the removal of neutral organic molecules, while polar and ionic compounds show much less affinity for adsorption. For the adsorption of polar compounds such as phenol, research has shown that the carbon surface chemistry is more relevant than the total available adsorption capacity or surface area [72-74]. It has been found that the presence of acidic surface oxides, whose concentration can be increased by oxygen adsorption or chemical treatment, leads to a decrease in adsorptive capacity for compounds such as phenols and increases the base adsorption capacity [75 [. 

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