Solubilization of direct black 2 in mixed micellar media: insights from spectroscopic and conductometric measurements

The core-shell architecture of surfactant micelles serves as a super solvent to remove inevitable organic pollutants from industrial wastewater. In the present study, the synergistic mechanism improving the aqueous solubility of Direct Black 2 dye by a single cationic surfactant, cetyltrimethylammonium bromide (CTAB), and its composite solutions with nonionic surfactants, Triton X-100 (TX-100) and Tween 20 (Tw-20), has been studied in detail. The spectral behavior of dye was monitored in the presence of a surfactant from its pre-to-post micellar concentration. It was found that solubilization was greatly enhanced in the cationic-nonionic mixed micelle systems. The value of the binding constant (K-b) was 9.10 x 10(4) dm(3)/mol for CTAB, which was increased significantly increased to 58.45 x 10(4) dm(3)/mol for CTAB + Tw-20 and 86.82 x 10(4) dm(3)/mol for CTAB + TX-100. The value of the partition coefficient (K-x) also increased in a similar manner. The observed order of the extent of dye solubilization in micellar system was: CTAB < CTAB + Tw-20 < CTAB + TX-100. The enhanced solubility of dye in mixed surfactant solutions was attributed to higher aggregation number, lower critical micelle concentration (CMC) and more hydrophobic interior of micelle. Moreover, the degree of ionization degree (beta), the CMC and related thermodynamic parameters were calculated from conductometric measurements, the results of which showed that the solubilization process in all types of surfactant systems is spontaneous and thermodynamically feasible, which may contribute to the sustained release of active-ingredients through self-assemblies.