No abstract

Aqueous dispersions of different smectite-type clay minerals were probed by ion-exchange of methylene blue (Mb). The evolution of the absorption spectra with time demonstrates the importance of the strength of the Mb−clay surface interaction, which is affected by the colloidal nature of the suspension and typical clay characteristics. A strong Mb−surface interaction is observed for clay minerals with tetrahedral substitution and a large basal surface area. Mb species adsorb in a more polar environment and show well-resolved bands. Their mobility is small. When the Mb−surface interaction is weaker, Mb−Mb interactions and interactions among the clay particles dominate. This is the case for octahedrally substituted clays with a large edge surface. Mb species experience the influence of the surrounding water phase and of neighboring Mb molecules. They remain mobile, even for longer aging periods. The absorption spectra evidence a less polar, “undefined” adsorption environment.

Nonlinear-optical investigations of new materials are important for 2 complementary reasons. first, the requirements for materials to be used in device applications that utilize nonlinear interactions are very demanding. New approaches to materials research can yield materials with favorable properties compared to traditional nonlinear materials, as shown recently in several supramolecular systems (Cox et a1. 1990; Stucky and MacDougall 1990; Clays et a1. 1993; Kelderman et a1. 1993; Kauranen et a1. 1995; Lehn 1995). This is particularly true for 2nd-order nonlinear optics that require noncentrosymmetric materials (Prasad and Williams 1991). On the other hand. nonlinear-optical processes can provide new techniques to study the properties of new materials. The nonlinear techniques have the potential of being more sensitive than the existing techniques or providing information that is not accessible at all using the existing techniques (Shen 1984). Here again, 2nd-order processes are particularly important because they provide an extremely sensitive tool to study symmetry properties of materials.

No abstract