Structure of Hydrated Kaolinite Edge Surfaces: DFT Results and Further Development of the ClayFF Classical Force Field with Metal-O-H Angle Bending Terms

Toward the development of classical force fields for the accurate modeling of clay mineral-water systems, we have extended the use of metal-O-H (M-O-H) angle bending terms to describe surface Si-O-H bending for hydrated kaolinite edge structures. Kaolinite, comprising linked octahedral Al and tetrahedral Si sheets, provides a rigorous test by combining aluminol and silanol groups with water molecules in hydrated edge structures. Periodic density functional theory and classical force fields were used with molecular dynamics to evaluate the structure, dynamics, hydrogen bonding, and power spectra for deriving optimum bending force constants and optimal equilibrium angles. Cleavage energies derived from density functional theory molecular dynamics calculations indicate the relative stabilities of both AC(1) and AC(2) edge terminations of kaolinite where Si-OH and Al-(OH2) or Si-OH, Al-OH, and Al-(OH2) groups exist, respectively. Although not examined in this study, the new Si-O-H angle bending parameter should allow for improved modeling of hydroxylated surfaces of silica minerals such as quartz and cristobalite, as well as amorphous silica-based surfaces and potentially those of other silicate and aluminosilicate phases.