Donnan equilibrium in Na-montmorillonite from a molecular dynamics perspective

We present molecular dynamics simulations of the Donnan equilibrium principle in compacted montmorillonite with three hydration layers of water in the interlayer. This draws attention to the misconception, frequently seen in the literature, stating that anions cannot enter interlayer space due to electrostatic repulsion forces, sometimes referred to as anion exclusion. However, the calculations presented here show that excess salt, i.e., both anions and cations enter interlayer space to the extent predicted by Donnan equilibrium. Thus the excess salt concentration is reduced in the interlayer in comparison to the external electrolyte but not totally excluded.