Molecular dynamics modeling of clay minerals .1. Gibbsite, kaolinite, pyrophyllite, and beidellite

A molecular dynamics model for clays and the oxide minerals is desirable for studying the kinetics and thermodynamics of adsorption processes. To this end, a valence force field for aluminous, dioctahedral clay minerals was developed. Novel aspects of this development include the bending potential for octahedral 0-A1-0 angles, which uses a quartic polynomial to create a double-well potential with minima at both 90 degrees and 180 degrees. Also, atomic point charges were derived from comparisons of ab initio molecular electrostatic potentials with X-ray diffraction-based deformation electron densities. Isothermal-isobaric molecular dynamics simulations of quartz, gibbsite, kaolinite, and pyrophyllite were used to refine the potential energy parameters. The resultant force field reproduced all the major structural parameters of these minerals to within 1% of their experimentally determined values. Transferability of the force field to simulations of adsorption onto clay mineral surfaces was tested through simulations of Na+, Ca2+, and hexadecyltrimethylammonium (HDTMA(+)) in the interlayers of beidellite clays. The new force field worked rather well with independently derived nonbonded parameters for all three adsorbates, as indicated by comparisons between experimental and molecular-dynamics-predicted d((001)) layer spacings of the homoionic beidellites.