SPHERE-TO-ROD TRANSITION IN IONIC MICELLES EXPLORED THROUGH FLUORESCENCE ANISOTROPY AND FRET

Micelles are supramolecular aggregates that form spontaneously when certain surfactants, such as sodium dodecyl sulfate (SDS), are dispersed in an aqueous medium. The shape of these structures can undergo a transition from spheres to rods with increasing surfactant concentration or upon the addition of electrolytes such as NaCl. In this work, we investigated this morphological transition in SDS micelles as a function of NaCl concentration, using fluorescence anisotropy of 1,6-diphenyl-1,3,5-hexatriene (DPH) and Förster resonance energy transfer (FRET) between DPH and the lipid labeled with 7-nitrobenz-2-oxa-1,3-diazole (NBD-PE). Understanding the mechanisms underlying this transition provides valuable insight into molecular self-assembly processes, which play an important role in diverse biological, industrial, and technological contexts. Samples were prepared with SDS at a concentration approximately twice the critical micelle concentration, which depends on the salt concentration. The molar ratio of DPH and NBD-PE to SDS was maintained at 1:5. The samples were incubated for at least 12 hours in the dark at room temperature, and measurements were performed at 25 ◦C. The fluorescence anisotropy of DPH initially decreased from 0.075 to 0.060 upon the addition of NaCl, followed by a linear rise, returning to 0.075 at 0.8 M NaCl. This behavior suggests that the transition to rod-like micelles imposes greater rotational constraints on DPH, likely resulting from the tighter packing of surfactant molecules induced by the electrostatic screening of the anionic headgroups by the electrolyte. The FRET efficiency showed a slight decline between 0.2 and 0.4 M NaCl, followed by a pronounced increase from 0.6 to 0.8 M, suggesting a structural reorganization that brings the fluorescent probes into closer proximity. These findings provide evidence for the sphere-to-rod morphological transition in SDS micelles, underscoring the critical role of salt in modulating micellar architecture