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.

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The absorption curves of methyl orange (MO) and analogous p-aminophenylazobenzenes in organic and aqueous organic solvents are shown to consist of two severely overlapping bands. The curves have been resolved into two skewed component bands using a regression method. The blue shift of the absorption maximum obtained when organic solvents are added to aqueous solutions of MO, or when MO is bound to bovine serum albumin or a surfactant micelle, is the result of a change in relative intensities of the component bands. The low-frequency component is assigned to a n, -. nlx transition of a solvate in which there is specific hydrogen-bonding interaction between solvent and the azo nitrogens, and the high-frequency component to a n1 + nl* transition of a solvate in which the interaction is absent. The low-frequency component is favored by aqueous solvent compositions and by low temperatures. The free energies of interconversion of the species in various hydrogen-bonding solvents are correlated by the solvent surface tension but not by the dielectric constant. The results show that the shift in absorption maximum accompanying binding to a protein or micelle should be interpreted as a shift in an equilibrium rather than as a shift in transition energy.Les courbes d'absorption du methylorange (MO) et des p-aminophCnylazobenzenes analogues, en solvants organiques et organiques aqueux, comprennent deux bandes fortement enchevktrees. La resolution de ces courbes en deux bandes gauches elementaires a ete faite par la methode de regression. Le d6placement vers le bleu du nlaxilnuin d'absorption, obtenu par addition de solvants organiques aux solutions aqueuses de MO ou lorsque le MO est lie a l'albumine du serum bovin ou a une micelle surfactive, est le resultat du changement des intensites relatives des bandes composantes. La bande a basse frequence est attribue a la transition n, + nl*, d'un produit de solvatation dans lequel il y a interaction specifique par liaison hydrogene entre le solvant et les azotes de l'azo; la bande a haute frequence correspond a la transition nl + n," du produit de solvatation sans interaction. La bande a basse frequence est favorisee par la composition en eau des solvants et aux basses temperatures. Les energies libres de l'interconversion des diverses especes dans differents solvants a liaison hydrogene, ont ete reliies a la tension superficielle du solvant et non a la constante dielectrique. Les rtsultats montrent que le deplacement du maximum d'absorption consecutif a une liaison a une protkine ou ~nicelle devraient &tre interpretes pluttit comme un deplacement d'equilibre qu'un diplacement de 1'Cnergie de transition.[Traduit par le journal]Can. J. Chem.. 51. 628 (1973) Methyl orange (MO, 1) has been used as a probe for studying the interaction of organic anions with proteins (1-9), synthetic polymers (10, 1 I), polyelectrolytes (12), and surfactants (12-14) in aqueous solution. The interaction is usually accompanied by spectral changes that are not associated with the acid-base indicator p...

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