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. 1999 Mar 30;96(7):3358-64.
doi: 10.1073/pnas.96.7.3358.

Surface geochemistry of the clay minerals

G Sposito  1 , N T Skipper, R Sutton, S Park, A K Soper, J A Greathouse
Affiliations

Surface geochemistry of the clay minerals

G Sposito et al. Proc Natl Acad Sci U S A. .

Abstract

Clay minerals are layer type aluminosilicates that figure in terrestrial biogeochemical cycles, in the buffering capacity of the oceans, and in the containment of toxic waste materials. They are also used as lubricants in petroleum extraction and as industrial catalysts for the synthesis of many organic compounds. These applications derive fundamentally from the colloidal size and permanent structural charge of clay mineral particles, which endow them with significant surface reactivity. Unraveling the surface geochemistry of hydrated clay minerals is an abiding, if difficult, topic in earth sciences research. Recent experimental and computational studies that take advantage of new methodologies and basic insights derived from the study of concentrated ionic solutions have begun to clarify the structure of electrical double layers formed on hydrated clay mineral surfaces, particularly those in the interlayer region of swelling 2:1 layer type clay minerals. One emerging trend is that the coordination of interlayer cations with water molecules and clay mineral surface oxygens is governed largely by cation size and charge, similarly to a concentrated ionic solution, but the location of structural charge within a clay layer and the existence of hydrophobic patches on its surface provide important modulations. The larger the interlayer cation, the greater the influence of clay mineral structure and hydrophobicity on the configurations of adsorbed water molecules. This picture extends readily to hydrophobic molecules adsorbed within an interlayer region, with important implications for clay-hydrocarbon interactions and the design of catalysts for organic synthesis.

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Figures

Figure 1
Figure 1
(A) Crystal structure of 1:1 and 2:1 layer type clay minerals, where X (shaded circles) is usually OH and M can be Al, Mg, Fe, etc. (B) Siloxane cavity in the basal plane of a tetrahedral sheet.
Figure 2
Figure 2
Cartoon of the three types of small cation adsorption by a 2:1 layer type clay mineral. The “Stern Layer” comprises only surface complexes, which can form in the interlayer region (left) as well as on single siloxane surfaces (right). Characteristic residence time scales of the three adsorbed species are compared at upper right to the time scales of in situ spectroscopic methods used to detect them.
Figure 3
Figure 3
Visualization of Ca2+ (large black sphere) in an octahedral solvation complex (water molecules, with smaller black spheres as O and red spheres as H) in the interlayer region of Ca-vermiculite. Portions of the opposing clay mineral layers are shown, with structural protons also indicated in red. Compare to the left side of the cartoon in Fig. 2.
Figure 4
Figure 4
Visualization of Na+ bound in an outer-sphere surface complex in the interlayer region of Wyoming montmorillonite, based on MC simulation. A portion of the siloxane surface structure also is shown.
Figure 5
Figure 5
Visualization of K+ bound in an inner-sphere surface complex in the interlayer region of Wyoming montmorillonite, based on MC simulation. Green lines extend from K+ (black sphere) to nearest-neighbor O in the surface complex. Dashed lines indicate hydrogen bonds between water molecules. Portions of the opposing two siloxane surfaces also are shown, with the beige sphere at the bottom of the figure (center) indicating a site of Al3+ substitution for Si4+.
Figure 6
Figure 6
Visualization of the interlayer configuration in Li(H2O)3-hectorite, based on MC simulation (33). The Li+ are bound in outer-sphere surface complexes with two water molecules. Other water molecules are keyed into the siloxane surface cavities.
Figure 7
Figure 7
Visualization of a methane molecule adsorbed in the interlayer region of the three-layer hydrate of Na-montmorillonite, based on MC simulation. The typical 20-fold coordination between CH4 and O occurs, but with nearly half of the O being in the siloxane surface.
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