Molecular Simulation Study of Hydrated Na-Rectorite

Abstract

The swelling behavior of clay minerals is an important issue in industrial processes and environmental applications. Mixed-layer clay minerals containing a smectite fraction, such as rectorite, are neglected even though they could swell and exist in nature widely. The hydration of rectorite has not been well comprehended even though they are meaningful to mineralogy and industry. This study combines molecular dynamics (MD) and Monte Carlo (MC) simulations to disclose the swelling behavior of rectorite and compare with montmorillonite. From grand canonical Monte Carlo (GCMC) and MD simulations, we obtain swelling curves and swelling free-energy curves of rectorite with a relative humidity of 100%. With the comparisons of swelling free-energy minima, we find that the bilayer hydrate of Na-rectorite is more thermodynamically stable than the monolayer hydrate, which is similar to Na-montmorillonite. However, the interlayer sodium ions in rectorite show an asymmetrical distribution quite different from the symmetrical distribution in montmorillonite. Because of unequal layer charges between the smectite part and illite part of retorite, sodium ions prefer to distribute close to the illite part surface.

Overview of the Adsorption and Transport Properties of Water, Ions, Carbon Dioxide, and Methane in Swelling Clays

“…26−91,127−129 There have been few simulation studies for the swelling properties of other smectites (such as beidellite, hectorite, and saponite) 16,50,51,54,84 and mixed-layer clay minerals (such as rectorite and Illite-montmorillonite clay). 130,131 Importantly, the simulation results obtained for the montmorillonite clays are generally similar to those obtained for other swelling clays. For example, irrespective of the smectite mineral type, the stable basal d-spacings were in the ranges of about 9.5−10.5, 11.5−12.5, and 14.5−15.5 Å for 0W, 1W, and 2W H 2 O formations, respectively.…”

Swelling Pressure in Systems with Na-Montmorillonite and Neutral Surfaces: A Molecular Dynamics Study

“…However, for clay in contact with bulk water (same chemical potential as RH = 100%) the proposed barriers are so large that montmorillonite would not swell at all (see below), in contrast to experimental facts. For example, the barrier between 1WL and 2WL was estimated to 3.4 k B T /nm 2 , and still higher values can be deduced from other MC calculations. , Even for the clay-layer size used in the present study, a barrier of 3.4 k B T /nm 2 would amount to a total free-energy barrier of 53 k B T , and thus completely prevent swelling.…”

“…For example, the barrier between 1WL and 2WL was estimated 27 to 3.4 /nm 2 and still higher values can be deduced from other MC calculations. 24,59 Even for the clay-layer size used in the present study, a barrier of 3.4 /nm 2 would amount to a total freeenergy barrier of 53 , and thus completely prevent swelling.…”

Supercritical Carbon Dioxide at Smectite Mineral–Water Interfaces: Molecular Dynamics and Adaptive Biasing Force Investigation of CO2/H2O Mixtures Nanoconfined in Na-Montmorillonite

“…Computational molecular modeling approaches are playing an important role in addressing these issues. 6,7,8,10,38,43,51,57,60,64,65,67,68,73,[74][75][76][77][78] Most MD studies of these systems have investigated primarily the structure, dynamics, and energetic of H 2 O in smectite interlayers. For instance, the orientation of H 2 O molecules with respect to the basal surface and cations, the cation coordination shell structure and dynamics, the location of cations and H 2 O molecules with respect to the hexagonal rings of the basal surface, the dynamics and energetics of cation and H 2 O site-hopping can all be readily addressed through classical molecular dynamics (MD) simulations and enhanced sampling free energy methods [79][80][81][82] There have been few computational studies of CO 2 in 2-dimensional nano-confinement in hydrophilic systems.…”