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.

Neutron diffraction, in conjuction with substitution of deuterium (D) for hydrogen (H), has been used to determine the structure of interlayer water in sodium- and nickel-substituted vermiculites. We have measured the intensities of the (00l) Bragg reflections as a function of relative humidity and H/D content, up to l=30. Difference analysis has then been used to obtain separate density profiles, ρ(z), for both the hydrogen atoms and the oxygen atoms plus the clay sheets. Ni–vermiculite was studied at 84% relative humidity, while Na–vermiculite was studied at both 88% and 30% relative humidity. At these values the layer spacings are 14.40, 14.96, and 11.78 Å, respectively. We find that each interlayer nickel ion is coordinated octahedrally to 6.0 water molecules. All of these water molecules are oriented to form a strong hydrogen bond to the adjacent clay surface. We also find that extra water is located close to the clay layers. This additional water is situated within the hexagonal rings of SiO4 and AlO−4 tetrahedra, which comprise the clay surfaces. In the 14.96 Å phase of Na–vermiculite there are an average of 4.9 interlayer water molecules per cation. About half of these water molecules are oriented to form a hydrogen bond to one of the clay surfaces. Additional water is found close to the clay surface, occupying the same hexagonal ring sites as in 14.40 Å Ni–vermiculite. In the 11.78 Å phase of Na–vermiculite there are an average of 2.1 water molecules per interlayer cation. The oxygen atoms of these interlayer water molecules are found close to halfway between the layers, while the hydrogen atoms are directed towards one of the adjacent clay sheets.

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A B S T RA C T :Existing data on interatomic potentials have been used to study the interactions between an uncharged clay sheet and a water molecule. Calculations show that most of the clay surface is relatively hydrophobic, with binding energies for a water molecule in the range 1-0-4.5 kcal mo1-1 . There is, however, a low-energy site for an oriented water molecule above the layer OH group and within the ring of six SiO4 tetrahedra. Using two different models for the interactions, the binding energy in this position is found to be either 13.2 or 21.8 kcal mol-1. The existence of the low-energy site accounts for the formation of the hydrated '10 A' phase of talc, which is known from high-pressure experiments. Data on the PT stability of this phase can be used to estimate its energy of dehydration. This quantity is shown to be consistent with the value of 21.8 kcal mol-1 for the binding energy of a water molecule and the energy associated with the expansion of the layers from the 9-35 A phase.An understanding of clay-water--cation interactions is directly relevant to a variety of industrial problems. Two examples may be chosen to illustrate this. The first concerns the role of clays in drilling muds. In this case one is particularly interested in the physical properties of the clay-water complexes, and the precise nature of the atomic-scale interactions which control their viscosity and non-Newtonian behaviour. A second important problem concerns the properties of clay-rich rocks, such as shales, when they come into contact with drilling fluids. Since the chemistry of the drilling fluid is generally markedly different from that of the natural pore-fluid, conditions of strong chemical non-equilibrium normally exist. This promotes the transfer by diffusion of both ~ater and cations between the drilling fluid and rocks forming the well-bore. The concomitant change in hydration state of the clays is usiially associated with a change in volume, and the resulting mechanical failure of the rock can lead to severe problems during drilling.'-To solve problems relating to clay-water complexes it is necessary to have a proper understanding of their behaviour at an atomic level. It is evident that there is a great deal of local structure developed in these systems, since the water present has a wide range of binding energies depending on its proximity to either interlayer cations or special sites on the clay surface. However, conventional structural studies are made difficult by the fact that these complexes comprise particles of colloidal dimensions. The interpretation of probe data, such as nuclear magnetic resonance or inelastic neutron scattering, in terms of specific claycation-water configurations is therefore not possible.Computer experiments provide one means of overcoming this problem. They can be used to explore the local structure and to characterize the transport properties of both water and cations, which are the subject of existing probe data. This information will be invaluable in 9 1989 The Mineralogical So...