Additive effect on the dimer formation of thiazine dyes

Visible absorption spectra of thionine Cm), azure A (AA), and methylene blue (MB) in aqueous and aqueous additive solutions (0.1-3 M) were studied as a function of the dye concentration. The added additives were urea and aminoethyl ethanolamine (AEEA). Due to the strong structural similarity of the investigated dyes, their spectral parameters were compared at room temperature. The spectral data for each dye were analyzed using DECOM Program. The dimerization behavior of the investigated dyes in water with or without urea was analyzed in terms of monomer-dimer equilibria. The dimerization constant, K-d, values of Th, AA, and MB in aqueous solutions were calculated to be (5.6 +/- 0.4) x 10(2) M-1, (2.5 +/- 0.1) x 10(3) M-1, and (6.5 +/- 0.3) x 10(3) M-1, respectively. However, a reduction of the dimerization constant for the dyes in aqueous urea solutions was observed. The dimer structure and the nature of the interacting pairs in these dyes were discussed using the exciton theory. Finally, due to the unique properties of AEEA, different absorption characteristics were observed for these dyes in pure AEEA and its aqueous solutions. (C) 2012 Elsevier B.V. All rights reserved.