Azo dyes interactions with surfactants. Determination of the critical micelle concentration from acid-base equilibrium

The critical micelle concentrations (CMC) for three surfactants, CTAB, TX-100 and SDS, were determined from the shift in the ionization equilibrium of two optical probes, methyl orange (MO) and acid alizarin violet N (AVN). At constant pH, the ratio of the acid and base forms of these dyes was considerably altered in the presence of micelles, as evidenced by pronounced changes in their absorbance spectra. CMC were also obtained from surface tension measurements, in the presence and absence of the optical probes, under comparable experimental conditions. The results indicate that the optical probes have no effect on the CMC values. Good agreement between the conventional, non-invasive, surface tension method and our method was observed with TX-100. For CTAB, the difference is explained in terms of interactions between optical probes and surfactant preaggregates; for SDS it has been attributed to electrostatic repulsion. Thermochromic measurements show that both the enthalpy and entropy of ionization of MO undergo abrupt changes near the CMC of SDS and hence such measurements can form the basis for a new method for CMC determination.