Dimerization of methylene blue in aqueous and mixed aqueous organic solvent: A spectroscopic study

Abstract

The dimerization of methylene blue (MB) dye in aqueous and mixed aqueous organic solvents has been investigated spectrophotometrically at 303.15K. The organic solvents chosen for the study include ethylene glycol, 2-methoxy ethanol, 2-ethoxy ethanol, tetrahydrofuran and dioxane. The dimerization constant KD of MB in aqueous as well as the mixed aqueous media has been computed from their UV–visible spectra. Addition of the organic solvent of low dielectric constant resulted into lowering of the KD perhaps due to the overall decrease in dielectric constant of mixed media. However, decrease in KD in the mixed media was not in order of decreasing dielectric constant of the mixed media. In fact, introduction of hydrophobic methyl or ethyl group in ethylene glycol led to an increase in KD in the order 2-ethoxy ethanol>2-methoxy ethanol>ethylene glycol even though the dielectric constant of ethylene glycol is higher than that of methyl or ethyl substituted ethylene glycol. The change in the fraction of monomeric dye (α) in the presence of the mixed aqueous organic solvent as a function of the dye concentration has also been computed. The spectra for the monomer and the dimer species of MB have been re-constructed using the KD values and the results have been analysed in the light of the molecular exciton theory. Both the twist (θ1) and the tilt (θ2) angles between the transition dipoles of the MB molecules in the dimer have been computed. It was observed that the presence of the organic solvent led to decreasing not only the twist but also the tilt angle. The results indicated that the dimer essentially has sandwich geometry with a slight deviation from the exact parallel stacking arrangement of the dye molecular planes highlighting the importance of dispersive forces including the hydrophobic effects in the formation of the dye aggregates.