SURFACE CHEMISTRY. Interaction of Water and Clay Minerals. A. Origins of Charge Deficiencies. Permanent pH-dependent. (due to isomorphous substitution). (variable, due to edges). A. Origins of Charge Deficiencies. Imperfections in the crystal lattice - Isomorphous substitution .
(due to isomorphous substitution)
(variable, due to edges)
Al3+ in place of Si4+ (Tetrahedral sheet)
Mg2+ instead of Al3+(Octahedral sheet)
unbalanced charges (charge deficiencies)
Octahedral sheet neutral
Net negative charge
The broken edge can be positively or negatively charged.
3. Proton equilibria (pH-dependent charges)
H+ bound tightly, so the lower the pH, the less exchange there is (i.e., lower nutrient availability)
Especially important in kaolinite, humus, where no internal charge imbalance
Ions of outer sphere complexes do not lose their hydration spheres. The inner complexes have direct electrostatic bonding between the central atoms.
radius of hydrated ion: 0.358 nm
The water molecules wedge into the interlayer after adding water
The cations are fully hydrated, which results in repulsive forces and expanding clay layers (hydration energy).
(Interlayer)E.Clay-Water Interaction (Cont.)
2. Ion hydration
3. Osmotic pressure
From Oxtoby et al., 1994
The concentration of cations is higher in the interlayers (A) compared with that in the solution (B) due to negatively charged surfaces. Because of this concentration difference, water molecules tend to diffuse toward the interlayer in an attempt to equalize concentration.
Relative sizes of adsorbed water layers on sodium montmorillonite and sodium kaolinite
Holtz and Kovacs, 1981