Molecular scale understanding of the structure and properties of aqueous interfaces with clays, metal (oxy-) hydroxides, layered double hydroxides, and other inorganic phases is strongly affected by significant degrees of structural and compositional disorder of the interfaces. ClayFF was originally developed as a robust and flexible force field for classical molecular simulations of such systems (Cygan, R. T.; Liang, J.-J.; Kalinichev, A. G. J. Phys. Chem. B 2004, 108, 1255–1266). However, despite its success, multiple limitations have also become evident with its use. One of the most important limitations is the difficulty to accurately model the edges of finite size nanoparticles or pores rather than infinitely layered periodic structures. Here we propose a systematic approach to solve this problem by developing specific metal–O–H (M–O–H) bending terms for ClayFF, E bend = k (θ – θ0)2 to better describe the structure and dynamics of singly protonated hydroxyl groups at mineral surfaces, particularly edge surfaces. On the basis of a series of DFT calculations, the optimal values of the Al–O–H and Mg–O–H parameters for Al and Mg in octahedral coordination are determined to be θ0,AlOH = θ0,MgOH = 110°, k AlOH = 15 kcal mol–1 rad–2 and k MgOH = 6 kcal mol–1 rad–2. Molecular dynamics simulations were performed for fully hydrated models of the basal and edge surfaces of gibbsite, Al­(OH)3, and brucite, Mg­(OH)2, at the DFT level of theory and at the classical level, using ClayFF with and without the M–O–H term. The addition of the new bending term leads to a much more accurate representation of the orientation of O–H groups at the basal and edge surfaces. The previously observed unrealistic desorption of OH2 groups from the particle edges within the original ClayFF model is also strongly constrained by the new modification.

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, comprised of 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 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 (DFT-MD) calculations indicate the relative stability of both AC1 and AC2 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.