Monte Carlo simulations of Wyoming sodium montmorillonite hydrates

Abstract

Monte Carlo simulations have been used to predict the interlayer basal separations of sodium-saturated Wyoming clays at constant stress (NP zz T ensemble͒ and at constant chemical potential (VT ensemble͒. These simulations use the Ewald summation technique to incorporate long-range Coulombic interactions in the calculation of the total potential energy and the pressure tensor. A comparison is made between the use of one, two, and three sheets of clay. It is shown that, for small separations, at least two separate clay sheets must be used to avoid system-size effects. The stable interlamellar separations are determined by combining results from isostress-isothermal and grand canonical simulations. It is shown that, consistent with experiments, at the temperature and pressure studied here, the cations in the interlayer are hydrated, except at the smallest basal separations.

Simulation insights on the structure of nanoscopically confined poly(ethylene oxide)

“…This "double-slit" geometry was chosen because it allows for more accurate electrostatic calculations. 22 For the bulk, constant NPT simulations at six temperatures (273, 298, 323, 373, 398, and 423 K) were performed, in a manner similar to those described in refs. 19 and 20, with a cutoff radius of 0.9 nm and a distance-dependent dielectric constant.…”

Why Clays Swell

“…Because the properties of a liquid near a surface are totally different from those of the bulk liquid, macroscopic theories such as DLVO theory have failed to describe typical stepwise microscopic swelling. In this regime, swelling strongly depends on the molecular packing of intercalated water 5 and statistical-mechanical simulations have unveiled many details of the confined fluid's structure, [6][7][8][9][10][11][12][13][14][15] but not of the swelling mechanism due to their limitations to predict clay water content. Here we employ modern Monte Carlo methods, which enable us to compute water adsorption isotherms for sodium montmorillonite clay.…”

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

“…The location of the isomorphous substitution of silicon by aluminium atom in the tetrahedral sheets and aluminium replaced by magnesium in the octahedral sheet determines the distribution of excess negative charge on the clay layer. [2][3][4] Apart from the charge distribution, other structural factors influencing the swelling behavior of smectites include the amount of negative charges on the clay layers, 32,33 the charge and size of the interlayer cations, 34,35 and vacancies in cis-or trans-octahedral positions.…”

“…[1][2][3][4] Smectites are aluminosilicates consisting of negatively charged layers compensated by solvated cations such as sodium ions. Clays play an important role in extraction of gas and oil from unconventional geological reservoirs such as shale rocks, [5][6][7][8][9][10][11][12][13] carbon dioxide storage and sequestration, 1,14-23 and selective sorption.…”

“…[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.…”