Structure, Energetics, and Dynamics of Smectite Clay Interlayer Hydration: Molecular Dynamics and Metadynamics Investigation of Na-Hectorite

Abstract

This paper presents a classical molecular dynamics (MD) and metadynamics investigation of the relationships between the structure, energetics, and dynamics of Na-hydroxyhectorite and serves to provide additional, molecular-scale insight into the interlayer hydration of this mineral. The computational results support a model for interlayer H2O structure and dynamics based on 2H NMR spectroscopy and indicate that H2O molecules undergo simultaneous fast librational motions about the H2O C2 symmetry axis and site hopping with C3 symmetry with respect to the surface normal. Hydration energy minima occur at one-, one-and-one-half-, and two-water-layer hydrates, which for the composition modeled correspond to 3, 5.5, and 10 H2O/Na+, respectively. Na+ ions are coordinated by basal O atoms (OMIN) at the lowest hydration levels and by H2O molecules (OH2O) in the two-layer hydrate, and H2O molecules have an average of three H-bonds at the greatest hydration levels. The metadynamics calculations yield activation energies for site hopping of H2O molecules of 6.0 kJ/mol for the one-layer structure and 3.3 kJ/mol for hopping between layers in the two-layer structure. Computed diffusion coefficients for water and Na+ are substantially less than in bulk liquid water, as expected in a nanoconfined environment, and are in good agreement with previous results

Cation diffusion in the interlayer space of swelling clay minerals – A combined macroscopic and microscopic study

“…These latter simulations, which are performed to obtain ionic mobilities, are based on a specific model (ClayFF in our case) that can describe, at the atomic level, the various interactions between water molecules, neutralizing cations and clay particles. This model has also been successfully applied to predict the behavior of a large class of clay/water interfacial systems in the presence of various counterions (Cygan et al, 2004;Ferrage et al, 2011;Michot et al, 2012;Morrow et al, 2013;Porion et al, 2013;Shapley et al, 2013;Holmboe and Bourg, 2014;Ngouana and Kalinichev, 2014;Zeitler et al, 2014).…”

Geometrical and Electronic Properties of Hydrated Sodium Montmorillonite and Tetracycline Montmorillonite from DFT Calculations

“…1,23 The high adsorption at low pH is usually 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 attributed to the presence of the fully protonated TC molecule, which is positively charged and adsorbs by cation exchange. Only a few ab initio studies have considered the smectites hydrated phases. [24][25][26][27][28][29][30][31][32] Berghout et al 13 The aim of the present work is to study by means of theoretical calculations the adsorption process of the fully protonated TC using Na-MMT as an adsorbent. DFT calculations are performed and geometric, electronic and magnetic properties are analyzed.…”

NMR and computational molecular modeling studies of mineral surfaces and interlayer galleries: A review

“…H-bonding is very significant for hydrous systems, and H-bond strengths typically range from 10 to 25 kJ/mol. The thermal energy at room temperature is ~2.5 kJ/mol, sufficient to occasionally overcome activation barriers of this magnitude (e.g., Morrow et al 2013). The interactions between CO 2 molecules are even weaker, with the dimer binding energy being ~2 to 3 kJ/mol (Saharay and Balasubramanian 2007).…”