Surface groups, chemical reactivity

Development of conditional (or variable) surface charges involves chemical reactions in the interfacial layer. Therefore, the individual material features, (i.e., the chemical properties of both the potentially charged sohd material and the dissolved species) have to be considered. When a chemically reactive surface group is exposed to an aqueous solution, the surface may become charged due to a surface reaction (e.g., dissociation, association, complexation) if the aqueous solution contains the other reactant as a dissolved species. The charging process on variable-charge sites is determined by not only the quality and quantity of active sites but also the composition of aqueous solution. An elechical double layer develops around particles due to the distribution of ionic species between the solid-liquid interfacial layer and the equilibrium Uquid phase. 

The pore structure and surface area of carbon-based materials determine their physical characteristics, while the surface chemical structure affects interactions with polar and nonpolar molecules due to the presence of chemically reactive fimctional groups. Active sites—edges, dislocations, and discontinuities—determine the reactivity of the carbon surface. As shown in Fig. 1, graphitic materials have at least two distinct types of surface sites, namely, the basal-plane and edge-plane sites [11]. It is generally considered... 

Supports used for obtaining Ziegler-Natta catalysts can differ essentially from one another. Some of the supports may contain reactive surface groups (such as hydroxyl groups present in specially prepared metal oxides) while others do not contain such reactive functional groups (such as pure anhydrous metal chlorides). Therefore, the term supported catalyst is used in a very wide sense. Supported catalysts comprise not only systems in which the transition metal compound is linked to the support by means of a chemical covalent bond but also systems in which the transition metal atom may occupy a position in a lattice structure, or where complexation, absorption or even occlusion may take place [28]. The transition metal may also be anchored to the support via a Lewis base in such a case the metal complexes the base, which is coordinatively fixed on the support surface [53,54]. 

The supports for the majority of bonded-phase packings for partition chromatography are prepared from rigid silica, or silica-based, compositions. These solids are formed as uniform, porous, mechanically sturdy particles commonly having diameters of 1.5-10 pm. with 3- and 5-pm particles being most common. The surface of fully hydrolyzed silica (hydrolyzed by heating with 0,1 M HCI for a day or two) is made up of chemically reactive silanol group.s, Thai is. 

Another method used to improve the strength of a bond is to introduce the chemical reactive functional groups directly on the surface of the material. For example, the bonding of Teflon or polyethylene can be improved by introducing OH groups onto the plastic surface by sparking moist air next to the surface to be joined. This is best done by means of a Tesla coil or a Corona discharge. 

Densification occurs as a result of condensation of reactive surface chemical groups on network particles that appear as a result of the shrinkage. In order to avoid this reaction, a treatment of the internal surface is then required to protect reactive surface groups such as Si-OH and thus prevent freezing-in of collapsed structures by condensation. In silica gels, the surface modification is commonly done through a chemical reaction with reactive alkylsilane compounds to form hydrolytically stable Si-R groups on the surface. 

Adsorption and Surface Chemical Grafting. As with siHca and many other siHcate minerals, the surface of asbestos fibers exhibit a significant chemical reactivity. In particular, the highly polar surface of chrysotile fibers promotes adsorption (physi- or chemisorption) of various types of organic or inorganic substances (22). Moreover, specific chemical reactions can be performed with the surface functional groups (OH groups from bmcite or exposed siHca). 

Conceivably, many compounds of inorganic elements have chemical reactivity that could contribute to improved adhesion to mineral surfaces. A fairly wide range of compounds, not restricted to any particular group in the periodic table, have been proposed as coupling agents as is shown in the following list ... 

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