No 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.

No abstract

No abstract