Microheterogeneity of Sodium Dodecylsulfate Micelles Probed by Frequency-Domain Fluorometry

The microenvironment of sodium dodecylsulfate (SDS) micelles has been examined with the use of two fluorescent probes, 2-anilinonaphthalene-6-sulfonic acid (2,6-ANS) and N-phenyl-naphthylamine (1-AN). The fluorescence lifetimes are recovered from multifrequency phase and modulation data with the use of a global analysis protocol. The fluorescence decay kinetics of 2,6-ANS, which probes the outer-core region (i.e., the palisade layer) of SDS micelles, is characterized by a Lorentzian distribution. In contrast, a single discrete excited-state lifetime is observed for 1-AN, which is expected to position itself in the inner-core region of the micelle. Fluorescence lifetimes of these probes are investigated also as functions of temperature, concentration of counter ions (Na⁺ and Mg²⁺) and linear alcohols (n-BuOH, n-PeOH, n-HeOH, and n-HepOH). The collective results confirm that the outer-core region of SDS micelles is microheterogeneous and the inner core is essentially homogeneous. In addition, the lifetimes and the partitioning of the outer-core probe, 2,6-ANS, appear to be more sensitive to variations in temperature and counter ions in comparison to those of the inner-core probe, 1-AN. The microenvironment of 2,6-ANS is found to be more heterogeneous at high temperature and low salt concentrations. This observation, we propose, is a result of different degrees of water penetration in the outer-core region. In the SDS system, the effects of micelle polydispersity and compositional diversity, on the environmental microheterogeneity of the fluorescent probe, seem to be minimal in comparison to water gradient effects.