ABSTRACT This study focused on the aggregation behavior of toluidine blue (TB) in aqueous and aqueous salt (NaCl and KCl) solutions. The absorption spectra of TB in aqueous and aqueous 0.1, 0.2, and 0.3 M NaCl and KCl solutions were recorded at room temperature over the wavelength range of 520–720 nm. The main maximum ( λ max ) for TB in water observed at 631 nm which is attributed to the monomeric form of TB while the inflection point at 590 nm is indicative of dimeric TB molecules. The effect of added salts was thoroughly analyzed in the concentration range where, monomer–dimer equilibria predominate. The dimer dissociation constants for TB in aqueous and aqueous salt solutions were determined by applying appropriate methodology. The monomer and dimer absorption spectra of TB and the absorption spectra of TB dimers separated into contributions from H– and J–dimers were resolved. Oscillator strength, transition dipole moment, interaction energy and twist angle were calculated by applying molecular exciton theory. The results are explained in terms of interactions between TB dye molecules in aqueous and aqueous salt solutions, which influence the molecular arrangement and subsequently affect the extent of aggregation.
Chaudhari et al. (Sat,) studied this question.