Structural Arrangements of Isomorphic Substitutions in Smectites: Molecular Simulation of the Swelling Properties, Interlayer Structure, and Dynamics of Hydrated Cs–Montmorillonite Revisited with New Clay Models

Abstract

Three new structural models of montmorillonite with differently distributed Al/Si and Mg/Al substitutions in the tetrahedral and octahedral clay layers are systematically developed and studied by means of MD simulations to quantify the possible effects of such substitutional disorder on the swelling behavior, the interlayer structure, and mobility of aqueous species. A very wide range of water content, from 0 to 700 mg water /g clay is explored to derive the swelling properties of Cs−montmorillonite. The determined layer spacing does not differ much depending on the clay model. However, at low water contents up to 1-layer hydrate (∼100 mg water /g clay ) the variation of specific locations of the tetrahedral and octahedral substitutions in the two TOT clay layers slightly but noticeably affects the total hydration energy of the system. Using atom−atom radial distribution functions and the respective atomic coordination numbers we have identified for the three clay models not only the previously observed binding sites for Cs + on the clay surface but also new ones that are correlated with the position of tetrahedral substitution in the structure. The mobility of Cs + ions and H 2 O diffusion coefficients, as expected, gradually increase both with increasing water content and with increasing distance from the clay surface, but they still remain 2 to 4 times lower than the corresponding bulk values. Only small differences were observed between the three Cs− montmorillonite models, but these differences are predicted to increase in the case of higher charge density of the clay layers and/or interlayer cations.

Multi-Quanta Spin-Locking Nuclear Magnetic Resonance Relaxation Measurements: An Analysis of the Long-Time Dynamical Properties of Ions and Water Molecules Confined within Dense Clay Sediments

“…By taking into account the average size of the hectorite platelets determined by TEM (L ∼ 0.4 µm), we obtain a self-diffusion coefficient (D ≈ L 2 /2τ c ) smaller than 8 × 10 −11 m 2 /s. That upper limit is fully compatible with the cesium mobility within clay sediments obtained by numerical simulations of molecular dynamics [55,[69][70][71][72][73][74]. …”

Molecular Dynamics Simulations of Carbon Dioxide, Methane, and Their Mixture in Montmorillonite Clay Hydrates

“…[2][3][4]34,[37][38][39][40][41][42][43][44][45][46] The swelling process typically exhibits two regimes: crystalline swelling at basal d-spacings < ∼ 19Å and osmotic swelling corresponding to basal d-spacings > ∼ 30Å. 47,48 The stable basal dspacing is about 10Å for dry clays (no water molecules in the interlayer) and increases upon contact with water to the range 11.5-12.5Å yielding a fully saturated monolayer (1W) water arrangement.…”

Concentrated aqueous sodium chloride solution in clays at thermodynamic conditions of hydraulic fracturing: Insight from molecular dynamics simulations

“…Its unit cell formula is Si 8 Al 4 O 20 (OH) 4 with unit cell parameters: a = 5.28 Å, b = 9.14 Å, and c = 6.56 Å. 27 The Na-MMT was represented by the RanTO model 11 with sodium compensating cations. The Na-MMT TOT structure considers a random distribution of both octahedral Mg/Al and tetrahedral Al/Si substitutions and each TOT layer bears a negative charge of 0.75e per unit cell; see Ref.…”

“…The Na-MMT TOT structure considers a random distribution of both octahedral Mg/Al and tetrahedral Al/Si substitutions and each TOT layer bears a negative charge of 0.75e per unit cell; see Ref. 11 and Sec. IV for more details.…”

“…The affinity of interlayer cations for water and the relatively weak electrostatic forces that hold the TOT structure together result in an easy hydration of the chargecompensating cations in the presence of water and swelling of the MMT. 11 A challenge associated with molecular-level simulations of salt crystals and aqueous salt solutions, confined in the clay pores, involves the accurate description of the water, ions, and clay interactions. The recent review on progress in molecular simulation of bulk aqueous electrolyte solutions critically examined the performance of ten non-polarisable and two polarisable force fields for aqueous electrolytes to predict their thermodynamic and transport properties.…”