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

In this study, the sphere-to-rod transition in sodium dodecyl sulfate (SDS) micelles was induced by increasing the ionic strength of the medium. This transition was monitored through the fluorescence of the lipophilic probe 2-amino-N-hexadecyl-benzamide (AHBA), which has a 16-carbon hydrocarbon chain and aggregates in aqueous solution. AHBA fluorescence parameters, such as emission intensity, spectral position, lifetime, and steady-state anisotropy, were monitored to understand the probe's behavior in aqueous solution as a function of ionic strength. The results showed that the addition of NaCl induces a spectral red shift in AHBA fluorescence, suggesting a repositioning of the probe within its aggregate. Fluorescence quenching experiments were also conducted using the Co²⁺ ion as a quencher of AHBA fluorescence, both when free in aqueous solution and at different salt concentrations. The quenching results revealed the presence of two distinct AHBA populations, each with different access to Co²⁺. The Stern-Volmer plots for low Co²⁺ concentrations, where quenching primarily affects the population most exposed to Co²⁺, indicated that NaCl slightly reduces the accessibility of this population to the quencher. Finally, the sphere-to-rod transition in SDS micelles was monitored through fluorescence quenching assays. The Stern-Volmer plots showed a dependence on the ionic strength of the medium, indicating that the change in the micelle shape occurs between 0.2 M and 0.3 M NaCl. Once the rod shape is reached, the quenching constant assumes lower values, suggesting that AHBA becomes more inserted into the micelles, with reduced access to Co²⁺.