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1  University of Medicine and Pharmacy “V. Babeş” Timişoara, Faculty of Pharmacy, Eftimie Murgu 2, 300041 Timişoara, Romania
2  Institute of Chemistry Timisoara of Romanian Academy, B-dul Mihai Viteazul 24, 300223 Timisoara, Romania

Abstract: Dyes ions have a tendency to self-associate in aqueous solutions. Since almost all textile dyes are applied from aqueous systems, it is important to understand the association of dyes in water, and to determine the number of chemical species presented in the system. The aggregation of the dyes in aqueous solution is influenced by: the dye concentration, electrolyte concentration, and temperature. In the present work, the qualitative and quantitative analysis of the aggregation of two disazo direct dyes (DSA, and DAS) is presented. The molecular aggregation of studied dye has been studied spectrophotometrically in aqueous solutions as a function of dye concentration (10-6 ÷10-3 M). In case of DSA dye, in concentration range 1.10-6 ÷ 1.10-5 M and 5.10-5 ÷ 1.10-3 M, as the concentration increases, the extinction coefficient at 416.5 nm decreased. Between concentrations 1.10-5 ÷ 5.10-5 M an increase of extinction coefficient was observed, and the shape of the spectra changed; all this shifts indicate that the molecules are beginning to aggregate. In case of DAS dye, a change in the absorption spectra shape was observed at concentrations bigger than 5.10-5 M. From the mathematical obtained data the number of species presented in the system, in domain of concentrations investigated, was determined. The mathematical calculations confirm the experimental data regarding the aggregation of the DSA, and DAN dyes. From the matriceal analysis, the number of chemical species presented in the system (in domain of concentrations studied) was determined to be three in case of DSA dye, respectively four in case of DAN dye.
Keywords: direct dye, aggregation, matriceal analysis