The adsorption of the cationic dye acridine orange (AO) by Na-saponite and the colloidal properties of the aqueous suspension were investigated by visible spectroscopy and XRD. The organic cation is adsorbed by the mechanism of cation exchange. When small amounts of the dye are adsorbed, the system contains small tactoids and is peptized. At this stage the dye penetrates into the interlayer space and most of it undergoes metachromasy due to interactions between the aromatic entity and the oxygen plane of the clay. When greater amounts of AO are adsorbed, the clay platelets flocculate to form book-house flocs which, with excess AO, are transformed into card-house flocs. At this stage metachromasy results from the aggregation of the dye in the interparticle space of the flocs, in addition to the pi interactions with the oxygen plane. In excess AO, the clay is gradually peptized. At this stage the dispersed clay platelets form small tactoids with monomeric AO in the interlayer space and at the same time adsorb dimeric and polymeric AO cationic species at the solid-liquid interface.

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