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

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