ABSTRACT
Clay minerals are silicates that, due to their unique characteristics (layer or fibrous structures, ion exchange capacity, variability in chemical composition and in electrical charge of particles, etc.), present special physicochemical behaviors that determine their surface properties and, therefore, their interactions with organic substances. One of the most salient features of these interactions is that sorption of molecules of different functionality affects not only the external surface but also, in most cases, the intracrystalline region of the silicate structure. Moreover, such interactions involve a wide variety of bonding mechanisms and energies (Table 1) resulting in organicinorganic materials of variable stability. Due to the peculiarities just mentioned and the existence of well-crystallized species amenable to detailed structural analyses, clay minerals constitute appropriate model systems for the study of physical and chemical phenomena pertinent to adsorption and reactivity in constrained spaces. These processes have special characteristics different from those on open surfaces or in solution. The knowl-
TABLE 1 Mechanisms of Clay-Organic Interactions Nature of the interactions
Characteristics
Electrostatic Ion exchange of interlayer metal cations with organic cations
van der Waals forces Hydrogen bonding and water bridges Ion dipole and
Adsorption of neutral molecules by interactions with external or internal (intracrystalline region of silicates) surfaces
coordination Proton transfer Electron transfer Covalent bonding Grafting reactions of organic groups
edge already acquired on clay-organic interactions has been extended to other constrained systems, such as inorganic or organic layered compounds and zeolites. In fact, many of the studies published recently concerning adsorption of organic species on layered compounds have their antecedents in studies made some years ago on clay minerals.
