Theoretical Study of the Roles of Na+ and Water on the Adsorption of Formamide on Kaolinite Surfaces

The adsorption of formamide (FA) on kaolinite surfaces and the roles of Na+ and water on the adsorption were investigated theoretically using a density functional theory. The calculations reveal that adsorbed FA is able to form stable complexes with the tetrahedral and octahedral surfaces of kaolinite. The octahedral surface possesses a larger binding affinity toward FA than the tetrahedral site of kaolinite partially due to the presence of the surface hydroxyl groups that are more active in the intermolecular interactions than the basal oxygen atoms of the siloxane or tetrahedral sites. The calculated (basis set superposition error corrected) binding energies of FA on the kaolinite octahedral and tetrahedral surfaces are -14.8 and -13.7 kcal/mol, respectively. FA also forms slightly less stable complexes with the negatively charged kaolinite fragments than with neutral ones. The addition of a sodium cation plays a key role in this adsorption, while addition of one water molecule affects the binding strength insignificantly. A chemical reaction occurs involving the Na octahedral surface complex, during which hydrogen is removed from FA and binds with the surface oxygen. Full solvation decreases the adsorption affinity of FA toward both kaolinite surfaces. Estimated Gibbs free energies indicate that the adsorption of FA on most of the studied kaolinite clusters is thermodynamically feasible from both gas phase and water solution.