Quantum chemical modelling of ion exchange for alkali earth cations localised in interlayer structural gaps of layer silicate montmorillonite

Calculations of energetic and structural characteristics of clusters are performed to model certain states of the systems involved in the process of Mg2+→Ca2+ exchange in water solution — montmorillonite system. The energy of this ion, exchange of ΔF≈−215 kJ mol−1, was calculated. While the sign of ΔF corresponds to the substitution Ca2+→Mg2+, which actually takes place the absolute values of the energetic yield of the exchange reaction exceed those found in the experiments by two orders of magnitude. This discrepancy is shown to be mainly due to the energetic effects caused by the expansion of the montmorillonite interlayer gap Δd=0.14 nm during the substitution of [Mg(H2O)12]2+ aquacations by larger [Ca(H2O)16]2+ ones. It follows from the estimations based on the dilatometric data for the interlayer swelling of layer silicates and related variations in the thermodynamic characteristics that this increase in Δd corresponds to the increase in the internal energy of the adsorbent by ΔU∼200 kJ kg−1, or ∼200 kJ per one gram-equivalent of exchange cations, which agrees satisfactorily with the calculated values ΔF.