Ion exchange selectivity (Mg2+, Ca2+ and K+) in hydrated Na-montmorillonite: insights from molecular dynamic simulations

The cation exchange selectivity in the Na-montmorillonite interlayer has been extensively studied as a part of the mechanism of the cation exchange reaction, which also provides an important insight into the inhibition of montmorillonite expansion by inorganic salts. While the variation of interlayer structure associated with interlayer Na+ and water molecules during the ion exchange process has less been explored, to determine the possibility of replacing interlayer Na+ ions by the ion exchange reaction, the present study focuses on the effects of the added electrolyte cation (Mg2+, Ca2+ and K+) on the structure properties, diffusion features and the number of surface hydrogen bonds in hydrated Na-montmorillonite at different hydration stages by using molecular dynamic (MD) simulations. The simulation results showed that Ca2+ has a better tendency to exchange Na+ ions at a high hydration degree, and K+ is easier to exchange Na+ at a low hydration degree. The lower mobility of interlayer species and the relativity large change in the number of hydrogen bonds with the addition of Mg2+ reflected its weak possibility of exchanging Na+ .