Free energy, energy, and entropy of swelling in Cs–, Na–, and Sr–montmorillonite clays

Abstract

A Monte Carlo method for grand canonical and grand isoshear ensemble simulations has been used to characterize the free energy, energy, and entropy of clay mineral swelling. The Monte Carlo approach was found to be more efficient at simulating water content fluctuations in the highly constrained clay environment than a previously developed molecular dynamics method. Swelling thermodynamics calculated for Cs-, Na-, and Sr-montmorillonite clays indicate a strong dependence of swelling on the interlayer ion identity, in agreement with various experimental measurements. The Sr clay swells most readily, and both the Na and Sr clays prefer expanded states (two-layer hydrate or greater) when in contact with bulk water. In contrast, swelling is inhibited in the Cs clay. Differences in swelling behavior are traced directly to the tendency of the different ions to hydrate. The swelling free energies are decomposed into their energetic and entropic components, revealing an overall energetic driving force for the swelling phenomena. Entropic effects provide a smaller, mediating role in the swelling processes. The results provide a unique molecular perspective on experimentally well-characterized swelling phenomena.

Molecular Dynamics Simulations of Turbostratic Dry and Hydrated Montmorillonite with Intercalated Carbon Dioxide

“…This composition produces a bilayer configuration of water molecules (as confirmed by the density profile) in the interlayer with the equilibrated d 001 − spacing equal to 14.67 ± 0.03Å, the value that falls into the experimentally determined 2W hydration state range (14.5-15.5Å) 13 and close to the calculated stable hydration state. 15,35,55 In this case, both methods predict that rotation from θ = 0°is an endothermic process accompanied by the increase in the Comparison of the density maps for the nonzero θ value (Figure 11b) and for θ = 0°(Supporting Information Figure S8) has revealed that the CO 2 distribution is affected by the Moire patterns formed by the basal surfaces. However, the distribution is less structured, 28 to be recognized as the Moire pattern like that depicted in Figure 10b for water molecules.…”

“…That equilibrated spacing falls within the experimentally determined range of the 1W hydration state (11.5-12.5Å) 13 and corresponds to the calculated stable hydration state. 15,35,55,56 As a function of θ, Figure 7 (upper part) displays the various contributions to the relative potential energy. The figure shows that electrostatics plays the primary role in the overall increase of the relative potential energy upon rotation.…”

Acidities of confined water in interlayer space of clay minerals

“…The unit cell formula is X Previous experimental and simulation studies have proved that Na + and Mg 2+ smectites can swell to 1 and 2 layer hydrates and in water rich environments, the 2-layer states are thermodynamically favorable over the 1-layer states (Brindley and Brown, 1980;Bérend et al, 1995;Cases et al, 1997;Whitley and Smith, 2004;Liu and Lu, 2006). Therefore, the models containing two water layers are focused on in this study.…”

Hydration properties of synthetic high-charge micas saturated with different cations: An experimental approach

“…The swelling properties of clays have been studied extensively, both experimentally (Weiss et al, 1990;Cases et al, 1992;Bérend et al, 1995;Michot et al, 2002;Rinnert et al, 2005;Trausch et al, 2006;Salles et al, 2008) and theoretically (Boek et al, 1995a,b;de Siqueira et al, 1997;Young and Smith, 2000;Hensen et al, 2001;Hensen and Smit, 2002;Whitley and Smith, 2004;Tambach et al, 2004;Liu and Lu, 2006;Smith et al, 2006;Tambach et al, 2006) and are now relatively well understood. Mooney et al 3 (1952a,b) were among the first authors to show that smectites are able to sorb up to half their mass in water and that the water sorption behavior is strongly dependent on the nature of the exchangeable cation.…”