MOLECULAR-DYNAMICS SIMULATIONS OF SORPTION OF ORGANIC-COMPOUNDS AT THE CLAY MINERAL AQUEOUS-SOLUTION INTERFACE

The adsorption of trichloroethene, C2HCl3, on clay mineral surfaces in the presence of water has been modeled as an example describing a gen eral program that uses molecular dynamics simulations to study the sor ption of organic materials at the clay mineral/aqueous solution interf ace. Surfaces of the clay minerals kaolinite and pyrophyllite were hyd rated 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 sug gest that at least three distinct mechanisms coexist for C2HCl3 on cla y minerals in the environment. The most stable interaction of C2HCl3 w ith clay surfaces is by full molecular contact, coplanar with the basa l surface. This kind of interaction is suppressed by increasing water loads. A second less stable and more reversible interaction involves a dsorption through single-atom contact between one Cl atom and the surf ace. In a third mechanism, adsorbed C2HCl3 never contacts the clay dir ectly but sorbs onto the first water layer. To test the efficacy of ex isting force field parameters of organic compounds in solid state simu lations, molecular dynamics simulations of several representative orga nic crystals were also performed and compared with the experimental cr ystal structures. These investigations show that, in general, in conde nsed-phase studies, parameter evaluations are realistic only when ther mal motion effects are included in the simulations. For chlorohydrocar bons in particular, further explorations are needed of atomic point ch arge assignments. (C) 1998 John Wiley & Sons, Inc.