Monte Carlo and Molecular Dynamics Simulations of Electrical Double-Layer Structure in Potassium−Montmorillonite Hydrates

Abstract

Monte Carlo and molecular dynamics simulations of interlayer molecular structure in the one-, two-, and (hypothetical) three-layer hydrates of K-montmorillonite were performed concurrently in order to elucidate counterion speciation and water structure in the electrical double layer of this clay mineral. Calculated layer spacings, interlayer water potential energies, and counterion mobilities were in agreement with available experimental data. In the one-layer hydrate, both outer-sphere and inner-sphere surface complexes of K+ were observed, the latter always near sites of tetrahedral charge substitution, with the counterion species exchanging readily on the simulation time scale (up to 200 ps). In the two- and three-layer hydrates, the surface complexes persisted, but an incipient diffuse layer of counterions also was observed, with all three types of surface species engaging in sluggish exchange. Water molecules in the one-layer hydrate resided at the interlayer midplane, whereas in the two-layer hydrate they lay in two planes between outer- and inner-sphere K+ surface complexes, as well as at the midplane. Hydrogen bonds in the one-layer hydrate were longer and more bent than in bulk liquid water. For all three hydrates, water and cation interlayer mobilities remained below those observed in bulk solution, principally because of the restricted geometry and the retarding effect of clay layer surface charge. Most of our results can be understood in terms of the weak solvation of the counterions by water molecules, which permits significant competition between K+ and water protons for negatively charged sites in the clay mineral layer.

Adsorption isotherms of water in Li–, Na–, and K–montmorillonite by molecular simulation

“…The reason for this most probably relates to the fact that the fixed d(001) spacing is the equilibrium spacing for Na-montmorillonite, 20 but not for the other two clays. 11,12 This allows a more efficient packing of the water molecules and water condensation at lower water fugacity for Namontmorillonite. Strikingly, the water content in K-montmorillonite remains lower than in its Na-and Licounterparts at high water fugacity.…”

“…Especially, the molecular structure of water and the distribution of counterion species in the interlayer have been topics widely addressed by molecular dynamics and Monte Carlo methodologies. [7][8][9][10][11][12][13][14][15] Several interaction potentials have been used such as the ab initio-based MCY model, [7][8][9][10][11][12]14 the TIP4P model, 11,13 and the SPC/E model. 15 Generally, the results from these studies are qualitatively in fair agreement.…”

Hydration Properties and Interlayer Organization of Water and Ions in Synthetic Na-Smectite with Tetrahedral Layer Charge. Part 1. Results from X-ray Diffraction Profile Modeling

“…20 Positional disorder of interlayer H 2 O molecules is, however, suggested by the 3D structure refinement of vermiculites, [45][46][47] and by computational studies. 36,37,40,42,44,[48][49][50][51] The present XRD modeling study thus integrates this positional disorder in the determination of the fine interlayer structure of smectite and of its evolution upon dehydration using low-and high-charge synthetic saponites. The water content determined from the modeling procedure is compared to results obtained from water vapor adsorption/desorption isotherms experiments.…”

Effects of Clay Fraction and Temperature on the H2O Self-Diffusivity in Hectorite Gel: A Pulsed-Field-Gradient Spin-Echo Nuclear Magnetic Resonance Study

“…Thus, measurements of the coefficients of diffusion of H20 in clay are important in applied clay science, particularly for nuclear waste disposal (e.g., Madsen, 1998). Many studies exist on H20 mobility or diffusivity in clays (e.g., Mokady and Low, 1968;Low, 1976Low, , 1979Cebula et al, 1981;Fripiat et al, 1984;Tuck et al, 1985;Miyahara et al, 1991;Sato et al, 1992;Chang et al, 1995Chang et al, , 1998Yu and Neretnieks, 1997;Ichikawa et al, 1999). One of the most important results of these studies is that H20 diffusivity in clay depends on the water fraction.…”

“…One of the most important results of these studies is that H20 diffusivity in clay depends on the water fraction. For example, Cebula et al (1981) and Chang et al (1995Chang et al ( , 1998 argued that diffusivity increases with increasing water fraction for relatively dry (<30 wt. % H20) clay.…”