Premicellar and micelle formation behavior of dye surfactant ion pairs in aqueous solutions: Deprotonation of dye in ion pair micelles

The premicellar and micelle formation behavior of dye surfactant ion pairs in aqueous solutions monitored by surface tension and spectroscopic measurements has been described. The measurements have been made for three anionic sulfonephthalein dyes and cationic surfactants of different chain lengths, head groups, and counterions. The observations have been attributed to the formation of closely packed dye surfactant ion pairs which is similar to nonionic surfactants in very dilute concentrations of the surfactant. These ion pairs dominate in the monolayer at the air–water interface of the aqueous dye surfactant solutions below the CMC of the pure surfactant. It has been shown that the dye in the ion pair deprotonates on micelle formation by the ion pair surfactants at near CMC but submicellar surfactant concentrations. The results of an equilibrium study at varying pH agree with the model of deprotonated 1:1 dye–surfactant ion pair formation in the near CMC submicellar solutions. At concentrations above the CMC of the cationic surfactant the dye is solubilized in normal micelles and the monolayer at the air–water interface consists of the cationic surfactant alone even in the presence of the dyes.

Formation of dye surfactant ion pairs, their micelle formation, induced deprotonation of the dye in the ion pair micelles and solubilization of the dye in the normal micelles of the pure surfactant have been observed in very dilute, near CMC submicellar and micellar concentrations of the surfactant, respectively.Figure optionsDownload as PowerPoint slide