Effect of n-Alkanols on the Solubilization of Sudan III and Sudan IV in Aqueous Ionic and Nonionic Micelles

Solubilization of aminoazobenzene-beta-naphthol (Sudan III) and orthotoluene-azo-orthotoluene-azo-beta naphthol (Sudan IV) dyes in anionic, cationic, and nonionic micelles in aqueous medium, have been studied spectrophotometrically at 30°C. The absorbance of either of the solubilized dye in dilute surfactant solutions does not change appreciably. But it was found to vary linearly with surfactant concentration above the CMC. Comparable solubilities of dyes in concentrated solutions of surfactants and in organic solvents provide some indication about the location of the solubilizate in the micelle interior. For a particular dye-surfactant system, moles of surfactant solubilizing a mole of dye, N_D, have been calculated from the straight live portions of the plots. N_D has been found to depend on the nature of the surfactant and the structure of dye. Decreased values of N_D with increased chain length of added alkanols have been explained in the light of partitioning of alcohol at the micelle water interface. Lower values of standard free energy of solubilization indicate smaller hydrophobic contribution in the solubilization process. The mechanism of solubilization has been found to depend on the nature of solubilizer, solubilizate, and the additives.