Competitive adsorption of alkali ions on aqueous mica surface: A force field comparison molecular dynamics study

Adsorption and exchange of ions on clay mineral surfaces are the fundamental processes which control the elemental partition in soil or marine sediments and the geological storage of toxic and radioactive waste. The distribution of ions at the surface has been modeled as an electric double layer. Due to the specificity of ions, simple theoretical models are hard to well predict ionic behaviors. Molecular dynamic simulations with empirical force fields to describe ionic interactions seem to be a delicate approach. However, different force fields for alkali ions were used when simulating clay mineral/water solution systems. It raises the question whether the empirical force field is reliable. We perform simulations with four generally used force fields for alkali ions (Na+, K+, and Cs+) to study cationic exchanges on the muscovite mica surface. Through comparing results to experimental thermodynamic data, we determine that the Horinek+SPC/E force field delivers the most consistent results. The adsorption and hydration structure of ions do not vary much as different force fields are used. However, it is this small variation in structure which explains the cationic affinity difference. This study sheds light on how to verify an ionic force field with which to reveal the clay surface phenomena and how to optimize force field parameters.