Abstract
The association of new methylene blue (NMB+) in aqueous solution has been studied using absorption and fluorescence spectroscopy. The absorption spectra of NMB+ were found to depend on the concentration of NMB+. The deviation from the Beer absorption law was ascribed to the association of the dye molecule. The equilibrium constants of dimerization were estimated at various temperatures and pressures. The values of ΔH°, ΔS°and ΔV° for dimerization were obtained. It was found that the association of NMB+ was suppressed by addition of some organic solvents, but is enhanced by addition of KCl. These findings can be explained by the change of microscopic surface tension of solvent, because the surface tension of water is decreased by addition of organic solvents and increased by addition of inorganic salts. The addition of β-cyclodextrin suppressed and that of γ- cyclodextrin enhanced the dimerization of the dyes. β-cyclodextrin includes the monomer, and γ- cyclodextrin includes the dimmer exclusively. The interaction of NMB+ with some anionic surfactants was also studied over wide concentration range of surfactants. While in the submicellar concentration region of surfactants the higher aggregates of the dyes were found, in the micellar region the monomer of dyes predominated.
