Metachromasy in clay dye systems: the adsorption of acridine orange by Na-beidellite

The adsorption of the metachromic dye acridine orange (AO) by Na-beidellite was investigated by Visible spectroscopy. Different types of clay-AO assoc iation were identified from the appearance and location of absorption bands alpha or beta. The colloidal properties were determined from curves of the absorbance vs, the degree of saturation. Three regions were identified in the absorbance curve. In the first region beidellite is peptized with small amounts of AO and the dye penetrates into the interlayer space where it un dergoes metachromasy due to pi interactions between the aromatic entity and the oxygen plane of the clay. With larger amounts of AO (second region), t he clay flocculates due to the aggregation of the dye cations in the interp article space of the flocs. In excess AO (third region), beidellite is grad ually peptized, forming small tactoids with monomeric AO in the interlayer space and at the same time adsorbing dimeric and polymeric AO cationic spec ies at the solid-liquid interface. Compared with the other smectites, AO sh ows the greatest tendency to undergo metachromasy in the presence of beidel lite.