Metachromasy in clay minerals. Sorption of acridine orange by montmorillonite

The adsorption of acridine orange by H-, Na-, Mg-, Al- and Cu-montmorillonite has been studied by visible and infrared spectroscopy and by X-ray diffraction methods. Adsorption of the dye takes place by the mechanism of cation exchange. The maximum amount of adsorbed dye depends on the exchangeable metallic cation, decreasing in the order Na > H > Cu > Mg > Al. The adsorbed cationic dye is located in the interlayer space. Adsorption leads to metachromasy of the dye molecule and a shift of the absorption bands to lower wavelengths in the visible range. Two types of associations are formed between montmorillonite and acridine orange: (i) a monolayer of the dye situated in the interlayer space, with the aromatic rings parallel to the aluminosilicate layer and (ii) a bilayer in the interlayer space or tilting of the cationic dye relative to the aluminosilicate layer. In the association of type (i) metachromasy cannot be attributed to polymerization of the organic dye but must be caused by interactions between the oxygen plane of the aluminosilicate and the aromatic dye.