Animated molecular dynamics simulations of hydrated caesium-smectite interlayers

Abstract

Computer animation of center of mass coordinates obtained from 800 ps molecular dynamics simulations of Cs-smectite hydrates (1/3 and 2/3 water monolayers) provided information concerning the structure and dynamics of the interlayer region that could not be obtained through traditional simulation analysis methods. Cs 1 formed inner sphere complexes with the mineral surface, and could be seen to jump from one attracting location near a layer charge site to the next, while water molecules were observed to migrate from the hydration shell of one ion to that of another. Neighboring ions maintained a partial hydration shell by sharing water molecules, such that a single water molecule hydrated two ions simultaneously for hundreds of picoseconds. Cs-montmorillonite hydrates featured the largest extent of this sharing interaction, because interlayer ions were able to inhabit positions near surface cavities as well as at their edges, close to oxygen triads. The greater positional freedom of Cs 1 within the montmorillonite interlayer, a result of structural hydroxyl orientation and low tetrahedral charge, promoted the optimization of distances between cations and water molecules required for water sharing. Preference of Cs 1 for locations near oxygen triads was observed within interlayer beidellite and hectorite. Water molecules also could be seen to interact directly with the mineral surface, entering its surface cavities to approach attracting charge sites and structural hydroxyls. With increasing water content, water molecules exhibited increased frequency and duration of both cavity habitation and water sharing interactions. Competition between Cs 1 and water molecules for surface sites was evident. These important cooperative and competitive features of interlayer molecular behavior were uniquely revealed by animation of an otherwise highly complex simulation output.

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

“…On average, our layer spacing values for these multilayer hydrates are somewhat higher than the ones observed in other simulations, but they are very close to the simulation data that also used the CLAYFF force field. 36 The differences with other simulation data can be attributed to the different force fields used, [27][28][29][30][31]35,36,38 to the slightly different clay composition with no tetrahedral charge, 28,31,35 or the presence of both Na + and Cs + counterions in the clay interlayers. 31,35 In the range from x = 0 to x = 4.5 (0 to 100 mg water /g clay ) the simulated layer spacings are always very close to the experimental ones, being just slightly lower 17−19 or slightly higher, 23 depending on the sample.…”

Effect of the exchangeable cations on the spectral properties of methylene blue in clay dispersions

“…Estimates of the heat of hydration of Li + cations give values near −520 kJ mol −1 , compared with −405 kJ mol −1 for Na + and only −265 kJ mol −1 for Cs + cations [31]. Therefore, large alkali metal cations likely do not hydrate completely on clay surfaces, but associate rather with basal or edge oxygen atoms of the silicate [32]. Due to the sterical aspect described above they bind only partially to water molecules, frequently sharing one molecule with another cation.…”

Silicon control of strontium and cesium partitioning in hydroxide-weathered sediments

“…sorbents at low Cs + sorbate concentrations (Jeong et al, 1996;Wauters et al, 1996;Sutton and Sposito, 2002) is applicable even under the extremely high ionic strength and pH conditions imposed. The non-labile (residual) pool of sorbed Cs + also increases in the order HC > HF > RS (see top three graphs in Fig.…”