Surface and intergallery catalytic properties of Cu(II)-exchanged hectorite: A scanning force microscope study

Abstract

Many layered silicate clay minerals such as hectorite readily adsorb or intercalate organic species. Surface or intergallery structural effects, electric fields, or charged particles may then facilitate polymerization or other reactions, resulting in novel inorganic/organic host–guest composite systems. We have studied the interaction of Cu(II)-exchanged hectorite with the polar monomer aniline, nonpolar styrene, and the amino acid glycine. Hectorite is known to readily intercalate aniline into its intergallery regions, where spontaneous polymerization occurs, resulting in nearly two-dimensional sheets of the conducting polymer polyaniline. In addition, aniline vapor is spontaneously polymerized on the surface of Cu(II)-exchanged hectorite, due to the presence of micro-pore sites on the silicate surface where oxidizing Cu2+ cations are available. For the nonpolar styrene molecule, little or no intercalation occurs. Surface polymerization of styrene is observed for long, intense exposures to the monomer in solution. Cu(II)-exchanged hectorite was also exposed to the amino acid glycine and subjected to alternate cycles of heating and wetting. Concentration and condensation of the amino acid into short glycine oligomers was observed. These reactions were seen to occur only at surface step edges and micro-pore sites. Polymerization of biological molecules on the surface of clay minerals under simulated prebiotic conditions may help to explain the emergence of these more complex molecules on the early Earth.