Theoretical Study of Adsorption Sites on the (001) Surfaces of 1:1 Clay Minerals

Abstract

Quantum chemical calculations within the framework of a two-layered ONIOM approach using the B3LYP density-functional method were carried out for the study of adsorption sites on regular (001) surfaces of the kaolinite group of clay minerals. A molecular cluster of an appropriate size (about 80 atoms including H atoms saturating dangling bonds) was cut from a structure of a single kaolinite layer. This cluster contains two types of interaction sitesone at the tetrahedral side and one at the octahedral side of the layer. Interactions of water and acetic acid molecules with the tetrahedral side and of water molecule and acetate anion with the octahedral side were investigated. Geometry optimizations were carried out, and interaction energies between the clay surfaces and the molecular species were computed. Hydrogen bridges between the hydroxyl groups of the octahedral surface and the water molecule were observed with total adsorption energy about −8 kcal/mol. The carboxylate group of the acetate anion binds to the octahedral surface hydroxyls via hydrogen bridges also. The respective interaction energy amounts to about −70 kcal/mol. On the other hand, the water molecule and the acetic acid molecule interact with the tetrahedral (001) surface through weaker hydrogen bonds in comparison with those with the octahedral side. Adsorption energies are about −3 to −4 kcal/mol in these cases.