Rod-like SDS Micelles in High Ionic Strength: A Fluorescence Quenching Investigation

Area of Science:

• Colloid and Surface Chemistry
• Physical Chemistry
• Materials Science

Background:

• Sodium dodecyl sulfate (SDS) forms spherical micelles in aqueous solutions.
• Micelle shape can transition to rod-like structures with changes in ionic strength.
• Lipophilic fluorescent probes can report on micelle microenvironments.

Purpose of the Study:

• To investigate the sphere-to-rod transition of SDS micelles induced by ionic strength.
• To monitor this transition using the fluorescence of 2-amino-N-hexadecyl-benzamide (AHBA).
• To understand the behavior of AHBA within SDS micelles as a function of salt concentration.

Main Methods:

• Inducing micelle transitions by varying NaCl concentration.
• Monitoring AHBA fluorescence (intensity, spectrum, lifetime, anisotropy).
• Conducting fluorescence quenching experiments with Co2+ ions.

Main Results:

• NaCl addition caused a red shift in AHBA fluorescence, indicating probe repositioning.
• Quenching experiments revealed two AHBA populations with differential Co2+ access.
• The sphere-to-rod transition occurred between 0.2 M and 0.3 M NaCl.
• Post-transition, AHBA showed reduced Co2+ accessibility, suggesting deeper micelle insertion.

Conclusions:

• The study successfully monitored the SDS micelle sphere-to-rod transition using AHBA fluorescence.
• Ionic strength significantly influences probe location and accessibility within SDS micelles.
• The transition involves changes in micelle structure affecting the probe's microenvironment.