Investigation on the effect of metal cation radius on montmorillonite hydration: Combining experiments with molecular dynamics simulation

Colloidal suspension stability is vitally important in industries, including wastewater treatment and mineral purification, and is greatly affected by electrostatic interaction and surface hydration in inorganic salt solutions. Combing the Static Multiple Light Scattering (SMLS), Quartz Crystal Microbalance with Dissipation Monitoring (QCM-D), and Molecular Dynamics (MD) Simulation, the effect of cation radius on the montmorillonite hydration is elucidated. It is found that in 1 mol/L chloride solution, the montmorillonite suspension instability decreases in the order of CsCl > CaCl 2  > MgCl 2  > KCl > NaCl. QCM-D experiments in 1 mol/L chloride solution and MD simulation demonstrate that the mobility of strongly hydrated cations and water molecules on the montmorillonite surface, such as Na + , Mg 2+ , and Ca 2+ , is depressed. The distribution of cation and oxygen of water molecules in the normal direction indicates that cation hydration modulates the hydration layer on montmorillonite. It could be concluded that strong hydration of small radius cations could enhance the hydration layer on the hydrophilic surface. The surface hydration is effectively depressed by weekly hydrated Cs + . Therefore, surface hydration depends on both surface hydrophilicity and cation hydration, which strongly corresponds to cation radius and valence. It provides insight into the clay suspension modulation in the engineering application and enrichment of natural elements.