Stability of metal uncharged ligand complexes in ion exchangers. Part 4.—Hydration effects and stability changes of copper–ethylenediamine complexes in montmorillonite

Abstract

The thermodynamic stability of the Cu(en)₂ complex in clay minerals of varying charge density has been determined by three different methods: ion exchange, complex formation function and dependence of selectivity on free ligand concentration.

The extra stability of the complex with respect to its stability in bulk solution is linearly related to surface charge density and vanishes at zero charge. The formation constant for the first complex (K₁) is ≃ 10^11.6 irrespective of the charge density and corresponds to a one order of magnitude stabilization. The charge density dependence is exclusively carried by variations in K₂ from 10¹⁰(Laponite) to 10^11.8(Otay montmorillonite). Hydration changes, as inferred from d₀₀₁ spacings account for this behaviour. The stability gain upon adsorption of the complex is explained in terms of an enhanced delocalization of the charge on the complex in the interlayer phase.