A critical spectrophotometric analysis of the dimerization of some ionic azo dyes in aqueous solution

Abstract

The monomer-dimer equilibrium of several azo dyes has been treated by applying factor analysis to concentration-dependent absorption curves. The method provides tests of model fit and the simultaneous estimation of dimer formation constants, KO, and dimer absorption curves. Aggregates larger than dimers were found to be present at surprisingly low concentrations. Light scattering revealed that in some cases the larger aggregates were of colloidal size. The presence of the larger aggregates gave small spectral perturbations but were revealed by the factor analysis as a failure of the dimerization model to fit absorption curves at higher concentrations. Detailed study of 4,-nitrophenylazo-l-naphthol-2-sulfonate (I), orange II (II), and Solochrome violet (III) showed that aggregates larger than dimers are present in fresh solutions above concentrations of 0.5,1.0, and 0.5 mM, respectively. Aging of the dye solutions can lower the concentration range over which the dimerization model holds because fresh solutions are often supersaturated. Nucleation to colloidal dye occurs slowly. Dimer formation constants of I, II, and III in fresh aqueous solutions at 25 °C are 688, 709, and 2.55 X 103 L mol"1, respectively. Error analysis shows that the regression method provides values of KO which fall at minima in the error curves, but a ±30% variation in KO can still give acceptable residual errors. The variation in KO is accommodated by small changes in the estimated dimer spectrum. Because of this, temperature coefficients of Kq could not be measured with sufficient precision to give enthalpies of dimerization. The dimer spectra were analyzed in terms of molecular exciton theory to provide estimates of intermolecular separation and twist angles between component dyes in a plane-parallel dimer model.

Evaluation of hematin-catalyzed Orange II degradation as a potential alternative to horseradish peroxidase

“…This shift in optimal pH values may be related to the molecular structure of OII. Formation of molecular aggregates of OII at a dimerization stage has been claimed to occur in aqueous neutral solution (Reeves et al, 1979;Asakura and Ishida, 1989;Simon ci c et al, 1994). OII has a pKa of 10.7 corresponding to the eOH group (Abbott et al, 2009).…”

“…OII has a pKa of 10.7 corresponding to the eOH group (Abbott et al, 2009). Thus, negatively charged species at higher pH may favor disaggregation of OII (Reeves et al, 1979). This, and the stability lost due to the tautomerization to the hydrazone form may result in a gain of reactivity with pH.…”

The effects of aggregation, counterion binding, and added NaCl on diffusion of aqueous methylene blue

“…Methylene blue (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13), like many other dyes (14,15) and biochemicals (l6), self-associates in aqueous solutions. The flat methylene blue cations stack like plates to form dimers, trimers, and larger aggregates.…”

The effect of substituent groups on the reductive degradation of azo dyes by zerovalent iron

“…8B, there was a strong absorbance band at 483 nm due to the n-* transition of the azo bond of Orange II, and other bands at 308 and 228 nm, and a doublet at 261 and 254 nm were attributed to the -* transition related to aromatic rings of Orange II [1,3,22]. After 10 min reaction, the bands at 483, 308 and 228 nm decreased, and the doublet at 261 and 254 nm disappeared, but the band at 250 nm appeared, which also suggested the degradation of Orange II and the formation of very unstable intermediates.…”