In engineered settings, clay mineral swelling promoted by Na + adsorption plays a significant role in petroleum extraction ( 6) and in the construction of environmental liners ( 7). Clay minerals precipitated from seawater in nearshore depositional environments can similarly influence the geochemical cycles of metal cations such as K + ( 4), as well as the oceanic buffering of atmospheric CO 2 on a global scale ( 5). Metal nutrients such as K + or Ca 2+ are retained in temperate-zone soils on negatively charged clay mineral surfaces but eventually can be released for consumption in the biosphere or for buffering these soils against excess acidity brought in by applied fertilizers or contaminated rainwater ( 3). This small particle size, in turn, endows these minerals with an important surface reactivity that plays a major role in the terrestrial biogeochemical cycling of metals, in the chemical homeostasis of the oceans, and in a broad variety of managed processes, including oil and gas production, industrial catalysis, pharmaceutical delivery, and radioactive waste disposal. Their name derives from the micrometer-sized particles into which they crystallize. The clay minerals are layer-type aluminosilicates, ubiquitous on our planet in geologic deposits, terrestrial weathering environments, and marine sediments ( 1, 2). 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. The larger the interlayer cation, the greater the influence of clay mineral structure and hydrophobicity on the configurations of adsorbed water molecules. 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. 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. Unraveling the surface geochemistry of hydrated clay minerals is an abiding, if difficult, topic in earth sciences research. These applications derive fundamentally from the colloidal size and permanent structural charge of clay mineral particles, which endow them with significant surface reactivity. They are also used as lubricants in petroleum extraction and as industrial catalysts for the synthesis of many organic compounds. 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.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. 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.
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