Computer simulation of interlayer water in 2:1 clays

Abstract

Monte Carlo computer simulation has been used to study water confined between the layers of 2:1 clay minerals. The model systems are based on natural Mg and Na smectites. The simulation cells contain one clay layer, 64 water molecules and four magnesium or eight sodium interlayer cations. These atoms and molecules interact with each other through a new set of effective pair potentials, which we discuss. The calculations are conducted in constant (N,p,T) ensembles, at T=300 K and with a uniaxial pressure, p, of 1 M Pa applied normal to the clay sheets. All the molecules, including the clay sheets, are therefore allowed to move during the simulations. The calculated equilibrium layer spacing is 14.7±0.1 Å with interlayer Mg2+ and 14.2±0.1 Å with interlayer Na+. These spacings compare with experimental values of 15.1 Å and 14.5 Å, measured for Mg and Na saturated Chambers montmorillonite, at 79% relative humidity. The corresponding densities and average potential energies of the interlayer water molecules are 1.38±0.04 g cm−3 and −17.63±0.02 kcal mol−1, respectively, for Mg smectite and 1.14±0.04 g cm−3 and −11.77±0.02 kcal mol−1, respectively, for Na smectite. We analyze and compare the interlayer structures in the two systems.

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

“…22 The clay layers are kept rigid, while the water and counterions are allowed to move during the MD simulation. The cation-clay and cation-water interaction potentials for Li ϩ , Na ϩ , and K ϩ are taken from Skipper et al 7 and Park and Sposito. 23 The MCY model was used for the water-water interactions.…”

“…The interaction model for the water-clay interactions ͑SRM28͒ is the one developed by Skipper et al 7 and is based on the ab initio MCY model for water. 22 The clay layers are kept rigid, while the water and counterions are allowed to move during the MD simulation.…”

“…These latter simulations have been difficult to apply since the probability of acceptance of insertion/ removal attempts in the geometrically confined and dense clay-water system is generally low, thereby making such studies expensive. Skipper et al 7,9 and more recently Young and Smith 15 circumvented this problem by performing simulations in the constant NpT ensemble, i.e., by applying constant stress on the clay layers and allowing volume changes at a given water content. A drawback of this method is that it cannot predict the clay water content.…”

“…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.…”

Theoretical Studies of Solid−Liquid Interfaces: Molecular Interactions at the MgO(001)−Water Interface

“…[2][3][4][5][6] Recently, classical molecular mechanical models appeared which can address the structural properties of interfaces at an atomistic level. [7][8][9][10] For an accurate description of quantum effects in bond breaking and bond forming processes, i.e., chemical reactions at interfaces, ab initio methods of quantum chemistry are needed. A few quantum mechanical calculations concerning the properties of water and hydroxyl groups on oxide surfaces have been performed using either periodic boundary conditions [11][12][13][14][15] or the molecular cluster model (for some recent examples, see refs [16][17][18].…”