Aggregation of o,o′-dihydroxyazo dyes -: 1.: Concentration, temperature, and solvent effect

The aggregation behavior and tautomerism of three o,o'-dihydroxy and one o-hydroxy-o'-methoxy azo dyes were studied by UV-visible spectroscopy. Concentration dependent spectroscopic changes with the formation of isosbestic points were observed indicating dimer-monomer equilibria. Combining all obtained data, the conclusion was reached that the spectroscopic changes were due to a shift of the monomer-dimer equilibrium of the hydrate form caused by the inversion of the intermolecular hydrogen bonding to intermolecular hydrogen bonding. The k(aggregation) for the dye selected as an example was evaluated statistically and was found [(1.82+/-0.30)x10(4) liter mol(-1)]. The effect of temperature (10-60 degrees C) on the monomer-dimer ratio and the effect of solvents upon stabilization of one form over the other, supported the conclusion that the driving forces for the dimerization were the hydrophobic effect and the high degree of entropy in solution. (C) 1998 Elsevier Science Ltd. All rights reserved.