Stability of Two-Dimensional Ionic Clusters at Solid-Liquid Interfaces.

The stability of two-dimensional clusters (2DCs) at the interface between ionic crystals and their solutions was investigated by molecular dynamics simulations. We found that 2DCs show a remarkable feature of odd-even alternation in stability. In NaCl and NaBr systems, the clusters containing an odd number of ions are more stable than those with an even number of ions, while in KCl systems, it is the other way round. Accordingly, the stability of water molecules in the first hydration shell of 2DCs also shows an odd-even alternation, which is consistent with the associated 2DCs. The odd-even alternation is discussed based on a competition mechanism between two factors: the Coulomb repulsion in charged 2DCs and the interaction between charges and water dipoles. Our discussion indicates that this odd-even alternation should be a universal feature in similar systems and would be important for understanding the nucleation and crystallization of solutions on ionic crystal surfaces.