Complex formation in cationic dye-organic anion systems in aqueous solution.

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The interaction between a cationic dye, Methylene Blue (MB), and organic anions (1-naphthalenesulfonate, 2-naphthalenesulfonate, 1-naphthaleneacetate, 2-anthracenesulfonate, 2,7-naphthalenedisulfonate, and 1,3,6-naphthalenetrisulfonate) in aqueous solution has been investigated by means of absorption and fluorescence spectra. The red shifts of the absorption and fluorescence spectra of MB caused by the addition of the organic monoanion are attributed to the 1:1 complex formation between MB and the organic anion. The thermodynamic quantities as well as the equilibrium constants for the complex formation have been determined. From the finding that the absorption spectrum of MB is not affected by the addition of ethanesulfonate, it is concluded that the complex formation is due to the interaction between the MB cation and the aromatic ring in the organic anion. The addition of polyanion (<I>e.g.</I>, 2,7-naphthalenedisulfonate) in the low-concentration range accelerates the dimerization of MB; whereas, in the high-concentration range, the formation of the 1:1 complex rather than the MB dimer is dominant. Studies on the complex formation with monoanions have also been made for Rhodamine 6G, Pyronine G, Safranin T, and Crystal Violet, and similar results to that for MB have been obtained.
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