Chapter 10: The surface properties of clay minerals
Robert A. Schoonheydt and Cliff T. Johnston
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Clay minerals have interlayer surfaces and edge surfaces, the former being the most important, especially in the case of swelling clays or smectites. Water is by far the most important adsorbed molecule in the interlayer space, where it interacts with the exchangeable cations and with the siloxane surface. Transition metal ion complexes are selectively ion-exchanged in the interlayer space of smectites. Poly-amine complexes easily lose their axial ligands to adopt a square planar configuration. The more stable and bulky tris(bipyridyl) and tris(phenanthroline) complexes in the interlayer space give chiral clay mineral composites that can be used in columns for chiral chromatography, in asymmetric catalysis and in non-linear optics. The formation of clay mineral-dye complexes is a two-step process: instantaneous adsorption of the dye molecules, mainly as aggregates, followed by a slower redistribution process over the clay mineral surface. With careful choice of dye molecules, non-linear optical materials can be prepared which exhibit properties such as second harmonic generation and twophoton absorption. Ion exchange of cationic proteins is a three-step process: (1) instantaneous adsorption at the edges; (2) adsorption in the interlayer space, followed by; (3) weak adsorption in excess of the cation exchange capacity. The extent to which these three processes occur depends on (1) the kind of exchangeable cation in the interlayer; and (2) the molecular weight, shape and charge of the protein molecules.
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