Probing Salt-Induced Changes in the Self-Assembly Behavior of Gemini Surfactants Using Scattering and Spectroscopic...

This study investigates how Gemini surfactants (GSs) with varying alkyl chain lengths and spacer architectures regulate interfacial behavior, self-assembly, and the photophysical response of Merocyanine 540 (MC540) in aqueous media. Tensiometric measurements demonstrate that increasing hydrophobic chain length leads to a systematic decrease in critical micelle concentration (CMC), accompanied by reduced surface tension and enhanced surface pressure, indicating more efficient interfacial adsorption at 30 °C. An increase in spacer length lowers the CMC, reflecting its influence on aggregation behavior. Dynamic light scattering (DLS) and small-angle neutron scattering (SANS) reveal anisotropic micellar assemblies whose dimensions increase with hydrophobicity, whereas longer spacers introduce conformational flexibility, resulting in smaller aggregates, as observed for the 10-8-10. Salt addition alters micellar organization: NaBr induces micellar swelling through electrostatic screening, while NaSal promotes micellar elongation via specific counterion binding. Encapsulation of MC540 within GS micelles produces red shifts in absorption and emission spectra (λ_max ~ 561-566 nm; λ_em ~ 580-584 nm) relative to water, consistent with the J-type aggregation. Time-resolved fluorescence measurements show multiexponential decays (0.1-2.5 ns), reflecting heterogeneous dye microenvironments, with average lifetimes increasing for GSs with longer alkyl chains. Overall, the results demonstrate that micellar architecture and salt environment jointly govern the photophysical behavior of MC540.