Dual fluorescence of sodium p-dimethylaminobenzoate in aqueous solution of positive ionic surfactant. Probing micelle formation and strength of electric field at micelle interface

Fluorescence spectrum of sodium p-dimethylaminobenzoate (SDMAB) in aqueous solution was recorded as a function of the concentration of positive ionic surfactant, Zeph, CTAC, and CTAB, respectively. Dual fluorescence which is characteristic of the excited state TICT of SDMAB was observed in aqueous surfactant solution. Micelle formation can be identified by the break that appears in curve of intensity ratio of TICT emission band to LE band, la/lb, versus surfactant concentration. The TICT band is shifted to blue in micelle as compared to that in pure water, which is an indication that SDMAB is buried in the apolar portion of the positively charged micelle, and the micellar phase polarity is probed to be the same for the three micelles. The la/lb ratio in three micelles takes an order as Zeph > CTAC > CTAB. This order is shown to be due to the difference in strength of electric field at micelle interface. A blue-shift of TICT band with surfactant concentration before micelle formation was noted, which can be taken as a new evidence for self-ceiling of surfcatnt molecule in aqueous solution.