Kaolinite surface hydroxyls

Figure 4.5. Schematic of the kaolinite surface hydroxyls. In addition to the basal OH groups, the schematic is showing the aluminol groups, the silanol groups, and the Lewis acid sites contributing to water adsorption (from Sposito, 1984a, with permission).

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 3. [100] Projection of the kaolinite structure showing the position of the inner and inner-surface-hydroxyl groups of kaolinite.

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). 

We have also studied the desorption of TCE (Teppen et al., 1998a,c) from kaolinite surfaces. In these studies, a monolayer of TCE was first equilibrated with a kaolinite external (hydroxylated aluminol) surface, and then water was added. After 350 ps of MD simulation (NVT, 5.0-nm interlayer separation, 0.5-fs timestep), 60% of the TCE had desorbed (Fig. 8-2). In this process TCE first changed its interaction with the mineral surface from a planar association to one in which only one or two atoms contacted the clay. Subsequently the TCE desorbed into the water layer. 

Fig. 8-2. (a) A monolayer of TCE was equilibrated with a hydroxylated aluminol kaolinite surface, then water was added. (b) After 350 ps of molecular dynamics simulation, 60% of the TCE had desorbed. Desorption occurred in two steps first the TCE molecule stood up" on the surface so that only one or two atoms contacted the clay, and then after some time detachment occurred and the TCE moved very quickly to the external water surface. When the same procedure was repealed at the siloxane kaolinite surface, after 350 ps of molecular dynamics simulation only, ()% of the T( l i had desorbed. 

Figure 1.10. Surface hydroxyl groups (shaded) on kaolinite. Besides the OH groups on the basal plane, there are aluminol groups, associated with Lewis acid sites, and silanol groups protruding from the edge surface. The right side of the figure shows an outer-sphere surface complex between an ionized H2O and Na" ", as well as complexes between the silanol groups and OH (i.e., proton dissociation).

The mechanism of intercalation has not been well established, but the process may be envisaged as resulting from the tendency of the dipolar kaolinite layers to become solvated with molecules. It is evident that hydrogen bonds between the surface hydroxyls of kaolinite and the intercalated molecules, as demonstrated by IR spectroscopy, contribute to the intercalation process, but calculation of the energies involved indicates that other factors contribute to the energy balance (65). Other postulates are that a decrease of the electrostatic attraction between the layers is caused by a higher dielectric constant in the interlayer volume after intercalation or that compensation of the internal dipole moment of the kaolinite layers by the dipole moment of the intercalated species occurs. 

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