Time dependence of the spectra of methylene blue-clay mineral suspensions

Aqueous dispersions of different smectite-type clay minerals were probed by ion-exchange of methylene blue (Mb). The evolution of the absorption spect ra with time demonstrates the importance of the strength of the Mb-clay sur face interaction, which is affected by the colloidal nature of the suspensi on and typical clay characteristics. A strong Mb-surface interaction is obs erved for clay minerals with tetrahedral substitution and a large basal sur face area. Mb species adsorb in a more polar environment and show well-reso lved bands. Their mobility is small. When the Mb-surface interaction is wea ker, Mb-Mb interactions and interactions among the clay particles dominate. This is the case for octahedrally substituted clays with a large edge surf ace. Mb species experience the influence of the surrounding water phase and of neighboring Mb molecules. They remain mobile, even for longer aging per iods. The absorption spectra evidence a less polar, "undefined" adsorption environment.