Thermodynamic study of the aggregation of azo dyes containing two trifluoromethyl groups in aqueous solutions

Abstract

The aggregation behaviour of azo dyes containing two trifluoromethyl groups, sodium 5-(3,5-bistrifluoromethyl-phenylazo)-6-hydroxynaphthalene-2-sulfonate, 1, and disodium 4-(3,5-bistrifluoromethylphenylazo)-3-hydroxy-naphthalene-2,7-disulfonate, 3, in aqueous solutions has been investigated by ¹⁹F NMR and visible absorption spectroscopy. The visible absorption spectra of the corresponding azo dyes containing two methyl groups, sodium 5-(3,5-dimethylphenylazo)-6-hydroxynaphthalene-2-sulfonate, 2, and disodium 4-(3,5-dimethylphenylazo)-3-hydroxynaphthalene-2,7-disulfonate, 4, in aqueous solutions have also been measured. From the visible absorption spectra, the aggregation number has been estimated to be 2 for all the dyes in the concentration region under examination (1 × 10^–5 to 1 × 10^–3 mol dm^–3) and the aggregation constants have been calculated, the temperature dependence of which led to the determination of the thermodynamic parameters. Consequently, the substituents in the dyes affected entropically the aggregation processes. The aqueous solutions of 1 at concentrations greater than 5 × 10^–3 mol dm^–3 became gelatinous while those of 2, 3 and 4 did not. ¹⁹F NMR spectra have been recorded to elucidate the behaviour of 1 and 3 at the higher concentrations. The ¹⁹F NMR spectra of both dyes consisted of one singlet whose shapes did not change with dye concentration. The fluorine signals exhibited a progressive shift to higher and lower magnetic field with increasing dye concentration for 1 and 3, respectively. From this concentration dependence, the aggregation constants and the differences in the chemical shifts between the dye monomers and aggregates have been estimated. The aggregation constants determined by ¹⁹F NMR were in fair agreement with those calculated using visible absorption spectra. Furthermore, the differences in the chemical shifts between the dye monomers and aggregates have given important information concerning the relative spatial location between a fluorine atom and an adjacent aromatic ring.