SREL Reprint #2244

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

Brian J. Teppen1, Ching-Hsing Yu2, David M. Miller2, and Lothar Schäfer2

1Advanced Analytical Center for Environmental Sciences, Savannah River Ecology Laboratory,
Aiken, South Carolina 29802
2Departments of Agronomy and Chemistry, University of Arkansas, Fayetteville, Arkansas 72701

Abstract: The adsorption of trichloroethene, C2HCl3 on clay mineral surfaces in the presence of water has been modeled as an example describing a general program that uses molecular dynamics simulations to study the sorption of organic materials at the clay mineral/aqueous solution interface. Surfaces of the clay minerals kaolinite and pyrophyllite were hydrated at different water levels corresponding to partial and complete monolayers of water. In agreement with experimental trends, water was found to outcompete C2HCl3 for clay surface sites. The simulations suggest that at least three distinct mechanisms coexist for C2HCl3 on clay minerals in the environment. The most stable interaction of C2HCl3 with clay surfaces is by full molecular contact, coplanar with the basal surface. This kind of interaction is suppressed by increasing water loads. A second less stable and more reversible interaction involves adsorption through single-atom contact between one Cl atom and the surface. In a third mechanism, adsorbed C2HCl3 never contacts the clay directly but sorbs onto the first water layer. To test the efficacy of existing force field parameters of organic compounds in solid state simulations, molecular dynamics simulations of several representative organic crystals were also performed and compared with the experimental crystal structures. These investigations show that, in general, in condensed-phase studies, parameter evaluations are realistic only when thermal motion effects are included in the simulations. For chlorohydrocarbons in particular, further explorations are needed of atomic point charge assignments.

Keywords: molecular dynamics simulations; clay mineral adsorption; clay mineral parameter development; trichloroethene adsorption on clay minerals; organic chlorine electrostatic charges

SREL Reprint#2244

Teppen, B.J., C.-H. Yu, D.M. Miller, and L. Schäfer. 1998. Molecular dynamics simulations of sorption of organic compounds at the clay mineral/aqueous solution interface. Journal of Computational Chemistry 19:144-153.

This information was provided by the University of Georgia's Savannah River Ecology Laboratory (srel.uga.edu).