Surface coordination functional groups

In recent years the understanding of colloid surfaces and soils constituents has increased tremendously. Surface coordinating functional groups on particulate inorganic and humic materials are viewed as complexant ligands (Stumm, 1992). 

Fig. 4.16 provides an illustration of the adsorption of a neutral polymer, polyvinyl alcohol, on a polar surface, and the resulting effects on the double layer properties. Adsorption of anionic polymers on negative surfaces - especially in the presence of Ca2+ or Mg2+ which may act as coordinating links between the surface and functional groups of the polymer - is not uncommon (Tipping and Cooke, 1982). 

Metal-organic frameworks (MOFs) are three-dimensional extended structures in which metal ions or clusters are linked through organic molecules that have two or more sites through which links can be formed. Unlike coordination polymers, MOFs are exclusively crystalline the trademarks of MOFs are their extremely high surface areas, tunable pore size, and adjustable internal surface properties. Functional groups that link metals or metal clusters within MOFs are molecules or ions that have two or more Lewis basic sites (for example, carboxylates, triazolates, tetrazolates, ° and pyrazolates (Figure 9.36)). 

Molecular simulation methods can be a complement to surface complexation modeling on metal-bacteria adsorption reactions, which provides a more detailed and atomistic information of how metal cations interact with specific functional groups within bacterial cell wall. Johnson et al., (2006) applied molecular dynamics (MD) simulations to analyze equilibrium structures, coordination bond distances of metal-ligand complexes. 

The elemental composition, oxidation state, and coordination environment of species on surfaces can be determined by X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) techniques. Both techniques have a penetration depth of 5-20 atomic layers. Especially XPS is commonly used in characterization of electrocatalysts. One common example is the identification and quantification of surface functional groups such as nitrogen species found on carbon-based catalysts.26-29 Secondary Ion Mass spectrometry (SIMS) and Ion Scattering Spectroscopy are alternatives which are more surface sensitive. They can provide information about the surface composition as well as the chemical bonding information from molecular clusters and have been used in characterization of cathode electrodes.30,31 They can also be used for depth profiling purposes. The quantification of the information, however, is rather difficult.32... 

At the water-air interface hydrophilic groups are oriented toward the water, hydro-phobic groups are oriented toward air. At solid-water interfaces, the orientation depends on the relative affinities for water and for the solid surface. The hydrophilic groups of amphipathic molecules may - if the hydrophobic tendency is relatively small - interact coordinatively with the functional groups of the solid surface (Ulrich et al., 1988) (see Fig. 4.10). 

How could one distinguish experimentally in the interaction of a hydrous oxide surface with a fatty acid, whether the interaction is due to hydrophobic bonding or to coordinative interaction (ligand exchange of the carboxyl group with the surface functional groups of the hydrous oxide) ... 

The acidity of organic ligands is enhanced by coordination with the oxide surface, i.e. the surface promotes deprotonation of the functional groups (COOH or OH). Such ligands, therefore, adsorb on the surface at a pH 2-3 units lower than that at which complexation with Fe in solution would occur (Kummert and Stumm, 1980). An example of this is the deprotonation of the alcoholic OH group of tartaric acid upon adsorption on the goethite surface (Cornell and Schindler, 1980). The appropriate reaction for the acid in water is. 

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