Fluorescence probe studies of the effect of concentration on the state of aggregation of surfactants in aqueous solution

Abstract

The aggregation numbers n of the micelles formed by five cationic, one anionic, two zwitterionic, and one nonionic surfactants in aqueous solutions were determined at various concentrations by means of a method based on the analysis of the fluorescence decay curves of micelle-solubilized pyrene (S. Atik, M. Nam, and L. Singer, Chem. Phys. Lett. 67, 75 (1979)) . The micelle aggregation number of the anionic surfactant has also been measured in H 2O-0.1 M NaCl. For the six ionic surfactants in water, n was found to be independent of the surfactant concentration in the range extending from the CMC to 0.2-0.3 M. The n values are close to those which can be calculated on the basis of the oil drop model for micelles. For the zwitterionic surfactants the value of n was found to be very strongly dependent on the distance between the two charged groups of the surfactant head. When these two groups are far apart the aggregation number is close to that for an ionic surfactant of comparable chain length. On the contrary when the two charged groups are in contact the zwitterionic surfactant behaves as a nonionic surfactant and forms large micelles. The micelle aggregation number of the nonionic surfactant was found to increase with concentration. The values of the fluorescence lifetime and of the ratio of the intensities of the first to the third vibronic peaks of the micelle-solubilized pyrene were also measured. They confirm that pyrene is solubilized in the micelle palisade layer and yield additional information on the micelle structure.