Molecular dynamics simulations of sorption of organic compounds at the clay mineral/aqueous solution interface

Applications of molecular modeling procedures to adsorption processes at clay mineral surfaces are described. Using a recently developed empirical force field for molecular dynamics (MD) simulations of dioctahedral clays, the interactions of various classes of organic compounds with clay surfaces are being explored, including man-made pollutants, agrochemicals such as pesticides, naturally occurring organics such as proteins, and organic dyes, like methylene blue, which are used to determine clay properties. Full MD simulations of the sorption of trichloroethene, one of the most dangerous pollutants, have shown that the molecule can sorb with the molecular plane flat on the mineral surface, or by making contact with only a single chlorine atom. In the adsorption of proteins, clay mineral surfaces have a denaturing effect, either by replacing protein-internal interactions with surface-to-protein interactions, or by dehydration. In the case of pesticides, differences are being studied in their adsorption to pristine clays and protein coated clays. In the case of methylene blue, various arrangements of the compound on clay mineral surfaces were found, including both single and double layers, illustrating the role of the compound in procedures determining the surface areas and cationic exchange capacities of clay minerals. The atomicscale modelling of the clay mineral/aqueous solution interfacial region is expected to aid the interpretation of equilibrium, kinetic, and spectroscopic data. The enhanced understanding of basic mechanisms will help us make better management decisions needed for maintaining or improving the soil and environmental quality of our agri-ecosystem. Lothar Schäfer, Ching-Hsing Yu, Susan Q. Newton, Mya A. Norman, David M. Miller and Brian J. Teppen