Computer Calculation of Water-Clay Interactions Using Atomic Pair Potentials

Abstract

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...

Monte Carlo Simulation of Interlayer Molecular Structure in Swelling Clay Minerals. 1. Methodology

“…It also has the advantage of permitting a facile use of three-dimensional periodic boundary conditions and the Ewald sum technique in the MC simulations. Skipper et al (1989Skipper et al ( , 1991bSkipper et al ( , 1993 found good agreement between experimental data and predictions based on the present approach for the talcwater system and for both tri-and dioctahedral Nasmectite-water systems.…”

“…This step may seem drastic, but previous ab initio studies suggest that the valence electrons in OH, SiO4, and AIO4 groups are centered on the O atoms (Matsouka et al 1976, Allan and Teter 1987, Sauer 1989, Lasaga 1992) and, therefore, that the latter will dominate any short-range interaction with a clay mineral surface. We may then be justified in representing a clay layer solely by O atoms saturated by two H atoms (i.e., water molecules) to make optimal use of the model water-water and Na +-water potential functions described above (Skipper et al 1989(Skipper et al , 1991b. To represent the siloxane surface, water molecules are placed at the sites of the O atoms on the basal planes of talc (Brindley and Brown 1980) with the H atoms generally directed toward the cation positions in the tetrahedral sheet.…”

“…This charge, in turn, is balanced by an equal and opposite charge on the bonding O atoms. Semi-empirical estimates of the gross atomic charge on surface O atoms in uncharged silicate clusters suggest the range -0.7e to -0.9e (Hass et al 1981, Delville 1992; we have selected the value -0.8e, which has been adopted successfully for MC simulations of the talc-water system (Skipper et al 1989(Skipper et al , 1991. Electroneutrality then dictates a gross atomic charge of + 1.2e for Si in the tetrahedral sheet.…”

“…Foremost is the choice of potential function to describe interactions between water molecules in a condensed, liquid-like phase. This choice, which must be made from a broad group of candidates (Spohr and Heinzinger 1988), affects not only the quality of the MC simulation of interlayer water structure, but also the selection of potential functions with which to describe interactions between water molecules and counter-ions or the clay mineral surface (Skipper et al 1989(Skipper et al , 1991b, Copyright 9 1995, The Clay Minerals Society Delville 1991, Delville and Sokolowski 1993. Comparisons among leading models for water-water interactions should be made to ascertain their accuracy and any sensitivity to their differences that arise in MC simulations of clay mineral interlayers (Bleam 1993).…”

Hydrophobicity of Siloxane Surfaces in Smectites as Revealed by Aromatic Hydrocarbon Adsorption from Water

“…The preferential adsorption of hydrophobic organic compounds to smectites in the presence of bulk water implies that the siloxane surface is hydrophobic. This view is supported by Skipper et aL (1989) who made computer calculations of the waterclay interactions in talc using atomic pair potentials. They asserted that the surface oxygens can be regarded as hydrophobic because the bonding between the surface oxygens and water are not as strong as the inter- (Chen, 1976).…”