Investigation of aggregation behavior using computational methods and absorption spectra for trisazo direct dyes

Direct dyes are likely to self-associate in aqueous solutions. Here, we present the aggregation characteristics of three trisazo direct dyes investigated using a procedure, which combines computational and experimental approaches. The geometric features of the molecules and their aggregates were elucidated by molecular modeling and optimization. The relative energies specific for the aggregation process yielded the optimum number of molecules forming an aggregate: two for AHDS dye and three for SDH and AIDS dyes. The results were further confirmed by using spectrometric determination and mathematical analysis. Accordingly, molecular aggregation was studied in aqueous solutions as a function of dye concentration (10(-6)-10(-3) mol/l) and solution pH (4-10). As the dye concentration increased, shifts in absorption spectra were observed, suggesting the formation of aggregates. The pH variation produced a change in the spectral maximum, confirming the aggregation. The mathematical processing of the absorption spectrum data confirmed the number of chemical species of each aggregate as resulted from computational calculations.