Self-aggregation of Methylene Blue in aqueous medium and aqueous solutionsof Bu4NBr and urea

The self-aggregation of Methylene Blue [MB] (1x10(-6)-4x10(-4) M) in water and in aqueous solutions of Bu4NBr (0.1-0.3 M) and urea (0.1-4 M) has been investigated by recording electronic spectra in the wavelength range 550 to 700 nm. Analysis of the spectral data yielded the dimer dissociation const ant, and individual characteristic monomer and dimer spectra. By applying t he exciton model for the first time, the interaction energy between the MB molecules in the dimer species has been evaluated. Further, information abo ut the dimer geometry and the twist angle between the dipoles of the MB mol ecules in the dimer species has been obtained. Similar data have also been obtained for solutions of MB in aqueous urea and Bu4NBr solutions. The sign ificant observation is the change in the dimer geometry from sandwich type to end-on-end type in the presence of urea and Bu4NBr. It is observed that the contribution to the interaction energy is mainly from van der Waal's ty pe and dispersion forces, in addition to short-range forces involving multi poles. However, the contribution from hydrogen-bonding interactions between MB and water molecules is found to be significantly low. It is proposed th at water structural effects and hydrophobic interactions are the major fact ors in the phenomenon of aggregation.