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

Abstract

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.

Understanding methane/carbon dioxide partitioning in clay nano- and meso-pores with constant reservoir composition molecular dynamics modeling

“…64 The interatomic interactions for the montmorillonite were obtained from the CLAYFF 62 force field, and the parameters for the broken edge sites were obtained from newly developed metal-O-H bending potentials consistent with ClayFF. 65,66 The CO 2 and CH 4 molecules were represented by the EPM2 67 and TraPPE-UA 68 models, respectively. Three-dimensional periodic boundary conditions were employed with a cutoff of 14.0 Å for short-range non-electrostatic interactions, and the long-range electrostatic interactions were computed using the particleparticle particle-mesh (PPPM) summation algorithm 69 with an accuracy of 10 À6 .…”

Structure and Dynamics of Water Confined in Imogolite Nanotubes

“…Thus the distribution of internal hydrogen atoms, fundamental for adsorption studies and the description of hydrated imogolites, is best described by the extended CLAYFF model. This aspect has been recently studied by Pouvreau et al 28 that similarly showed the necessity of a M-O-H bending term to properly describe basal and edge surfaces of gibbsite and brucite. From here on, we will 9 use exclusively the extended CLAYFF force field.…”

Atomistic Structure of Mineral Nano-aggregates from Simulated Compaction and Dewatering

“…4B ) near (5-coordinated) Al atoms at the edge (i.e., this peak is aligned with the first peak on the density profile of Ow-Al(Edge)). The adsorption of a water molecule to form a complete octahedral (6-coordinated) shell about each edge Al atom is expected 24 . The second peak at 2.75 Å is associated with the interaction of O w with the surface hydroxyl O and H atoms denoted OH-2 in Fig.…”

“…Numerous quantum mechanical simulations tried to address this issue by studying the stability 17 , 18 and acid-base properties of clay edge surfaces 19 – 22 . At the molecular level, one of the most widely used force fields to describe water-clay mineral systems (i.e., ClayFF 23 ) was initially developed to simulate the interaction of water with unreactive basal surfaces and interlayers of layered minerals, but the applicability of ClayFF to edge surfaces has only recently been investigated 24 – 26 . Consequently, the majority of MD simulation studies of clay-water systems have been carried out using a simple slit-shape pore without considering edge effects 13 , 15 , 17 , 27 – 32 .…”