Surface gibbsite type

Schematic representation of the kaolinite structure. It reveals the 1 1 structure due to the alternation of silica-type (black) and gibbsite-type layers (white). Furthermore, the edge surface exposes aluminol and siianol groups.

Figure 2. Charging behaviour of silica type (panel a) and gibbsite type (panel b) surfaces.

The combination of site binding model and double layer model will be worked out in some more detail for silica and gibbsite type surfaces in the next two sections. 

For gibbsite type surfaces the characteristic protonation reaction is given by Eq. (43) and the surface charge by Eq. (62). In the presence of a simple 1-1 electrolyte of s.a. ions that weakly adsorb at the Stern plane Eqs. (62) and (55) to (59) can be combined to give... 

Figure 7. Surface electrostatics for gibbsite type surfaces. Panel (a) shows two sets of

The function depends on the type of surface oxygens. For gibbsite type (amphoteric) groups a combination of Eq. (60) and (62) with = 0 can describe the local... 

Kaolinite is a 1 1 (T-O) phyllosilicate. The fundamental unit of its structure is an extended sheet of two constituents a silica-type layer of composition (Si4O10)4- and a gibbsite-type layer of composition (0H)6A14(0H)204 (see schematic representation in Fig. 10). Ideally, kaolinite crystals are not permanently charged. However, due to isomorphic substitution of Si by Al at the siloxane surface, kaolinite platelets carry a small, permanently negative charge (Van Olphen, 1977). Lim et al. (1980) and Talibudeen (1984) postulate that the permanent charge of kaolins is caused by contamination with small amounts of 2 1 phyllosilicates rather than a consequence of isomorphic substitution. 

Surface protonation at the kaolinite surfaces. The excess proton density, Th.v. at the surface hydroxyl group is displayed as a function of pH. Surface protonation is interpreted as a successive protonation of two distinct types of OH groups localized at the gibbsite and edge surfaces. The pHpzc of the edge surface is about 7.5. 

Figure 4.12. Depiction of a likely structure of the glycine-Cu -AI(OH)3 ternary complex. This is a type A ternary complex, as the metal ion bonds simultaneously with the organic ligand and surface oxygens located at steps and edges of gibbsite crystals. Large open and shaded circles symbolize structural OH groups, while smaller broken circles represent structural AP ions.

Alumina. Alumina has a general formula AhOj, although it usually exists as the trihydrate. Activated aluminas are prepared by dehydrating aluminum hydroxide (Gibbsite) at 370-400 °C in a current of air. The surface areas are 180 mVg (type E) and 90 xrr/g (type T), and the pore sizes range up to 10 gm. 

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