Gibbsite surface hydroxyls

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. 

The protonation of surface hydroxyl groups at the gibbsite and edge surfaces is displayed in Figure 14. The surface proton concentration FH) v denotes the excess proton density with respect to pHZPC = 7.5 of the hydroxyl groups at the edge face. The total excess proton density (solid line) may be assigned to two successive protonation equilibria at the kaolinite surface (broken lines). 

Surface hydroxyl groups coordinated to pairs of AP" cations appear on the plane perpendicular to the crystallographic c axis in gibbsite (Fig. 1.2). This basal plane makes up most of the surface of the mineral, but it appears that the hydroxyl groups bound to Lewis add sites on the edge plane perpendicular to the basal plane are more reactive.These Lewis acid sites comprise an Al " cation coordinated to a single water molecule. 

In summary, the model proposed on the basis of acid-base characteristics of kaolinite platelets explains the pH-dependent charge primarily to the protonation of the hydroxyl groups at the basal gibbsite and the edge surface. We will later illustrate how this charge characteristics (surface protonation) influences the reactivity (dissolution characteristics) of kaolinite. 

Oxides/Oxyhydroxides. For natural solids that are oxides or oxyhydroxides (e.g., quartz, Si02 goethite, a-FeOOH gibbsite, Al(OH)3), their water-wet surface is covered by hydroxyl groups (recall Fig. 11.2). These hydroxyl moieties can undergo proton-exchange reactions with the aqueous solution much like dissolved acids ... 

Cho et al. reported that a-alumina is formed from aluminum hydroxide prepared by precipitation with potassium hydroxide. However, when alkaline hydroxide is used as the precipitation agent, alkali cations are incorporated into the product, and commercial gibbsite samples are always contaminated with a small amount of sodium ions. Therefore their starting material seems to be contaminated with potassium, and the presence of potassium ions in their precursor seems to play an important role in the nucleation of a-alumina. They also reported that hydroxyl ions, acetic acid, and pyridine added to the glycothermal reaction system affect the morphology of the a-alumina particles because of their preferential adsorption to a specific surface. ... 

Figure 3.16. Surface charge of goethite (FeOOH) in solutions of 1.0, 0.1, and 0.01 M NaCl, estimated from the difference in proton and hydroxyl uptake (H" — OH ) by the oxide during acid-base titration. PZC, point of zero charge, (Adapted from F. J. Hingston, 1970. Specific adsorption of anions on goethite and gibbsite. Ph.D, dissertation. University of Western Australia, Perth.)...

The Aluminate Ion, Al(OH)4. Calculation of AG ° or log K° of the aluminate ion from gibbsite, boehmite and diaspore in alkaline solutions using equations (1) or (2) requires the corresponding standard state partial molal properties of the hydroxyl ion, OH . These may be determined from the dissociation constant for water, if it is accurately known over the range of temperatures and pressures investigated. The experimental measurements of diaspore solubility, reported here, extend to 350°C on the aqueous phase saturation surface, but experimental determinations of by Sweeton et al. ( ) attain only... 

Although kaolin can be regarded as chemically inert it does have a complicated surface chemistry. Hydroxyl ions either from the gibbsite layer or from adsorbed gels readily react with commercial bifunctional silanes [17],titanates or other coupling agents [18]. 

---