Adsorption study of cationic dyes having a trimethylammonium anchor group on hectorite using electrooptic and...

Clay minerals are natural or synthetic material of colloidal dimensions. Due to the sheetlike structure clay minerals offer a huge specific surface area and hence optimal properties for modification through adsorption. The current work studies the adsorption of five cationic dyes on the synthetic clay mineral hectorite. All dyes have a trimethylammonium anchoring group in common. The adsorbed dye molecules are characterized by means of pulsed electric linear dichroism and UV-VIS spectroscopy. With increasing dye loading a continuous shift in the absorption spectra is observed. But there is no occurrence of a new absorption band. Therefore we conclude that the dyes preferentially adsorb as amorphous aggregates on the clay surface. At low dye loadings the dye molecules lie flat on the clay mineral surface. Increasing dye concentration leads to a continuous increase in average tilt angle. However the orientation of the dye molecules is very sensitive to functional groups. The introduction of a nitro group to a particular dye increases significantly the tendency to lie flat on the surface whereas the introduction of a methoxy group at the same position has the opposite effect.