Influence of charge density on the acid–base equilibrium of ethylenediamine in montmorillonites

Abstract

The ethylenediammonium (enH₂²⁺)–Ca ion-exchange reaction has been studied at three temperatures on four montmorillonites of different charge densities. The exchange reaction is exergonic and exothermic at all charge densities because of the higher thermodynamic functions of hydration for Ca. Both ΔG^⊖ and ΔH^⊖ of exchange decrease with decreasing charge density and tend to vanish at zero charge, an observation which is explained by the increasing interlamellar hydration as evidenced by the concomitant increase in the d₀₀₁ spacing of the homoionic end members Ca and EDAH₂²⁺. The protonation constants in the interface, as obtained from the enH₂²⁺–H⁺ exchange constants, exceed the value in bulk solution and decrease to the latter at zero charge density. The extra stability gained by the enH₂²⁺ ion upon adsorption in the interface is ascribed to its higher polarizability (compared with the proton) and to its smaller hydration state in the interlamellar phase, analogous to the case of transition metal ion complexes.