Ion Exchange and Interparticle Forces Between Clay Surfaces
1989
Pashley, R. M. | Quirk, J. P.
The forces between muscovite mica surfaces have been measured in water and in NaCl and CaCl₂ solutions. Above a critical electrolyte concentration, strong, short-range repulsive forces arise which are not predicted by DLVO theory. These forces have been identified as hydration forces and have an oscillatory force superimposed with a periodicity corresponding to the diameter of the water molecule. Hydration forces are significant for the swelling of Ca-clays. Below the critical hydration concentration exchange favors H⁺. Protons are so strongly adsorbed that as the surfaces are brought to a distance of separation of 2 to 3.5 nm the hydrated cations are replaced by H⁺ and the surfaces jump into contact due to attractive van der Waals forces. The absence of the hydration forces in the presence of H⁺ indicates that the water structure is not perturbed by the siloxane layer alone. The strong affinity of the H⁺ is attributed to the H⁺ proton being able to approach closely to the surface by forming H₃O⁺ with water H-bonded to oxygen atoms on the mica basal plane surface. The sites for adsorption are the Al tetrahedra for which the charge is confined to three surface O atoms. Other cations can only approach to within one H₂O molecule of the surface because of their primary hydration shell. By comparison, the H⁺ associated with the surface of montmorillonite hydrate to the full extent and the crystalline swelling (d₀₀₁ > 4 nm) is similar to Li-montmorillonite. This difference must be related to the fact that the surface charge for montmorillonite is less discrete being spread over about 10 surface O atoms.
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