Microdynamic Behavior in Threadlike Micelles

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Microscopic dynamics of a salicylate anion, Sal-, in long rodlike or threadlike micelles formed by cetyltrimethylammonium bromide, CTAB, in aqueous solution was examined by use of fluorescence techniques. Long and stable threadlike micelles are formed at the equimolar ratio of CTAB and sodium salicylate, NaSal. Enhancement of fluorescence intensity of Sal- was observed with increasing the molar ratio of CTAB/NaSal up to unity, while a further increase in the ratio beyond unity did not affect the fluorescence intensity. We also found that Sal- anions had two (long and short) distinctive fluorescence lifetimes when the molar ratio was smaller than unity, and had long one only when the ratio was larger than unity. The long and short lifetimes are due to Sal- residing at a rather nonpolar micellar interior site and in the bulk aqueous phase, respectively. The enhancement of fluorescence intensity is related to the increase in the fraction of the long lifetime component. Furthermore, fluorescence anisotropy of Sal- was also measured and the rotational relaxation time, τφmic, at the micellar interior site was estimated as a function of the molar ratio. Since τφmic showed a long value (∼1.6 ns) at a molar ratio below unity, frequency of molecular motion of Sal- in the threadlike micelle is highly reduced. However, the τφmic is still in a nanosecond range, and the molecular motion in the micelle must be rather fast and dynamic even in the fully established threadlike micelle.