The aggregation of thionine and methylene blue dye in smectite dispersion

Interaction of cationic dyes, thionine (Th) and methylene blue (MB), with smectites in the aqueous dispersions was investigated using visible spectroscopy. Smectite samples of various layer charge were used: low-charge laponite (Lap), middle-charge Kunipia F montmorillonite (Kun) and high-charge fluorohectorite (FH). Dyes cations formed monomers, dimers and larger-size aggregates of H-type (sandwich type) and in a few cases of J-type (head-to-tail association). Each form of the dye was identified using visible (VIS) spectroscopy. The effects of layer charge, the type of the dye and smectite, and the influence of dye/clay ratio on the formation of specific dye species were compared. Negligible influence of the dye loading was observed for most cases, being always much lower than the effect of the layer charge density. Dye/Lap reaction systems were dominated by isolated dye cations. H-dimers and larger-size molecular aggregates were formed on the surface of this clay only at the highest loading tested (0.50 mmol/g). On the other hand, the reaction systems with a middle-charge Kun montmorillonite contained roughly equal amounts of dye monomers and H-aggregates at low loadings and slightly more H-aggregates were formed at high loadings. The spectra of dye/FH reaction systems were always dominated by the bands assigned to H-aggregates. The surface of high-charge densities induced destabilization and decomposition of Th cations especially at high dye concentrations on clay surface. The effects of silicate structure and composition on the optical properties of adsorbed dyes were not identified.