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.