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Ion Exchange and Interparticle Forces Between Clay Surfaces

Dep. of Chem., The Faculties, Australian Nat. Univ., Canberra, ACT, 2601, Australia

J. P. Quirk

Waite Agric. Res. Inst., The Univ. Adelaide, Glen Osmond, SA, 5064, Australia

*Corresponding author.

ABSTRACT

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.

Received for publication March 1, 1989.