Ab initio molecular dynamics study of adsorption sites on the (001) surfaces of 1:1 dioctahedral clay minerals

Ab initio molecular dynamics room-temperature simulations and full relaxation of atomic positions are performed in the study of adsorption sites on the octahedral and tetrahedral surfaces (parallel to the (001) plane) of the kaolinite group of clay minerals. Interactions of water and acetic acid molecules with both surfaces are investigated. Simulations showed that both polar molecules are strongly bound to the octahedral surface forming several simultaneous hydrogen bonds. Surface hydroxyl groups located at the octahedral surface act as proton donors as well as proton acceptors demonstrating their amphoteric character. Moreover, proton jumps between the carboxyl group of acetic acid and one surface hydroxyl group were observed in the molecular dynamics simulations. On the other hand, both molecules interact only very weakly with the tetrahedral side of the kaolinite layer showing the hydrophobicity of this surface. The octahedral surface is hydrophilic.