A comparative experimental study of the aggregation of Acid Red 266 in aqueous solution by use of 19F-NMR, UV/Vis spectroscopy and static light scattering

Abstract

The aggregation behaviour of aqueous solutions of the azo dye Acid Red 266 has been investigated by a combination of three different analytical methods: ¹⁹F NMR, UV/Vis spectroscopy, and static light scattering (SLS). For a better comparison the same range of concentration was studied by all three methods. The NMR data were interpreted in terms of a monomer/dimer equilibrium and, additionally, of a monomer/n-mer-equilibrium, based on an isodesmic model. No distinction between both equilibria was possible. Modelling the UV/Vis data by a monomer/dimer equilibrium gives access to all of the important thermodynamic quantities associated with a dimerization. The major driving force for aggregation is enthalpic with ΔH_D=−22.10±0.50 kJ mol⁻¹, whereas a positive entropy of 7 J mol⁻¹ K⁻¹ points to a solvent contribution. The value of the free dimerization enthalpy is in good agreement with that from NMR results. Furthermore, the UV/Vis data revealed information on the local arrangement of the nearest neighbours in the dyestuff aggregates. The constituting molecules are separated by an interplanar distance of 0.69 nm and are twisted against each other by an angle of 50° (calculated from exciton theory). This twist angle proves that the CF₃ group of one Acid Red 266 molecule must be covered by an adjacent benzene ring within the aggregate and supports the conclusion obtained from NMR measurements. Both spectroscopic methods indicate a vertical stacking of the dye molecules, confirming a columnar structure of the aggregates. In contrast, SLS indicates the existence of highly aggregated species in dilute solutions of 10⁻⁶–10⁻⁴ mol L⁻¹ with molecular weights and contour lengths in the order of magnitude of 10⁵–10⁶, and up to 700 nm, respectively. In addition to aggregation numbers, the overall shape of the aggregates were supplemented by SLS, suggesting a worm-like structure. Both, the contour lengths and the cross sections of the aggregates can be modified by addition of electrolyte, resulting in an increase of chain stiffness. Finally, the apparent discrepancies between the results of the spectroscopic methods and of SLS are discussed.