Adsorption nonpolar surfaces

Adsorption on a nonpolar surface such as pure siUca or an unoxidized carbon is dominated by van der Waals forces. The affinity sequence on such a surface generally follows the sequence of molecular weights since the polarizabiUty, which is the main factor governing the magnitude of the van der Waals interaction energy, is itself roughly proportional to the molecular weight. 

Hydrophilic and Hydrophobic Surfaces. Water is a small, highly polar molecular and it is therefore strongly adsorbed on a polar surface as a result of the large contribution from the electrostatic forces. Polar adsorbents such as most zeoHtes, siUca gel, or activated alumina therefore adsorb water more strongly than they adsorb organic species, and, as a result, such adsorbents are commonly called hydrophilic. In contrast, on a nonpolar surface where there is no electrostatic interaction water is held only very weakly and is easily displaced by organics. Such adsorbents, which are the only practical choice for adsorption of organics from aqueous solutions, are termed hydrophobic. 

Adsorbents Table 16-3 classifies common adsorbents by structure type and water adsorption characteristics. Structured adsorbents take advantage of their crystalline structure (zeolites and sllicalite) and/or their molecular sieving properties. The hydrophobic (nonpolar surface) or hydrophihc (polar surface) character may vary depending on the competing adsorbate. A large number of zeolites have been identified, and these include both synthetic and naturally occurring (e.g., mordenite and chabazite) varieties. 

If a surface is polar, its resulting electric field will induce a dipole moment in a molecule with no permanent dipole and, through this polarization, increase the extent of adsorption. Similarly, a molecule with a permanent dipole moment will polarize an otherwise nonpolar surface, thereby increasing the attraction. 

Fig. 5(a) contains the oxygen and hydrogen density profiles it demonstrates clearly the major differences between the water structure next to a metal surface and near a free or nonpolar surface (compare to Fig. 3). Due to the significant adsorption energy of water on transition metal surfaces (typically of the order of 20-50kJmoP see, e.g., [136]), strong density oscillations are observed next to the metal. Between three and four water layers have also been identified in most simulations near uncharged metal surfaces, depending on the model and on statistical accuracy. Beyond about...

Healy et al. (134) studied experimentally the heats of adsorption of many polar and nonpolar gases on polar and nonpolar surfaces by means of their heats of immersion. It was found that the heat of immersion of rutile on a series of straight-chain compounds was a linear function of the dipole moment of the wetting liquid. In a later article (135)- this work was extended and it is shown that nearly the entire heat effect on immersion of the clean solid surface is due to adsorption of molecules in the first layer. From the slope of the line, giving the values found for the net heat of adsorption as a function of the dipole moments, the average field strength, F, of rutile can be found by means of Eq. (22). The experimental value found by these investigators is... 

For more complex molecules a reasonable estimate of the heat of sorption can sometimes be made by considering group contributions. Such an approach works best for nonpolar sorbates on nonpolar surfaces but is subject to considerable error for polar systems in which the electrostatic energies of adsorption are large. 

The adsorption of nonionic surfactants on polar and nonpolar surfaces also exhibits various features, depending on the nature of the surfactant and the substrate. Three types of isotherms may be distinguished, as illustrated in Fig. 7. These isotherms can be accounted for by the different surfactant orientations and their association at the solid/liquid interface as illustrated in Fig. 8. Again, bilayers, hemimicelles, and micelles can be identified on various substrates. 

This temperature corresponds well with the desorption temperature observed in this study. Therefore the propene desorption peak observed here must be from the recombination of an adsorbed ir-allyl and hydrogen. Dissociative adsorption of propene requires a Zn-0 pair site which is abundant on the nonpolar surface. That the 0-polar surface, but not the Zn-polar surface adsorbs propene in this manner suggests that surface defects which expose the nonpolar surfaces (or other Zn-0 pairs) exist more abundantly on the 0-polar than on the Zn-polar surface. 

In dishwashing, one must consider soil and surfactant adsorption to both polar and nonpolar surfaces. Metals (aluminum, stainless steel, carbon steel, cast iron, silver, and tin), siliceous surfaces (china, glass, and pottery), and organics (polyethylene, polypropylene, polyethylene terephthalate (PET), polytetrafluoroethylene (PTFE), and wood) present a wide variety of surface characteristics. They span the range of high interfacial free energy (metals and many ceramics) to low interfacial free energy (hydrocarbon polymers) surfaces [27,28],... 

FIGURE 2-9 Adsorption via disperison forces on nonpolar surface. 

Formation of the air nuclei during mixing depends on the adsorption of surface active compounds present at the gas-liquid interface. The natural surface active compounds in flour are the proteins and lipids. The molecules that adsorb strongly at an air-water interface have a dual nature. They possess a strongly polar (or hydrophilic) moiety, which interacts with the polar phase (water) and a nonpolar (or hydrophobic) moiety... 

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