Gemini surfactant chain length and spacer tune micelle structure, while NaSal grows micelles from 2.6 nm to 15.8 nm and lengthens MC540 fluorescence lifetime

Alkyl chain length systematically sets both interfacial activity and micellar geometry: the CMC falls from about 55.4 mM for 8-4-8 to about 1.2 mM for 12-4-12, γ_CMC decreases from about 44.5 to about 33.8 mN/m, π_CMC rises from about 27.3 to about 38.0 mN/m, Γ_max increases from 0.93×10⁻⁶ to 1.54×10⁻⁶ mol/m², and A_min decreases from about 179.5 to about 107.5 Ų; DLS gives D_h rising from about 1.9 nm to about 3.8 nm, and SANS shows spherical micelles for 8-4-8 (R_c about 1.4 nm, N_agg = 13) versus ellipsoidal micelles for 10-4-10 and 12-4-12 (R_a about 2.1 and 3.2 nm, N_agg about 23 and 39). Earlier work on chain-length effects in Gemini surfactants often relied on a single technique; this study places interfacial parameters from tensiometry alongside micellar size, shape, and aggregation number from DLS and SANS at the same concentration (100 mM) and temperature (30 °C), so the interfacial and bulk self-assembly consequences of chain length can be read item by item. Tensiometry used the du Noüy ring method at 30 ± 0.1 °C with ±0.1 mN/m precision; DLS was replicated three times with ±0.5 nm tolerance; SANS was performed at the Dhruva reactor with λ = 5.2 Å and Q = 0.02–0.3 Å⁻¹, fitted with SASfit using spherical and ellipsoidal form factor models judged by χ² and residuals, with the Hayter–Penfold MSA structure factor.

Spacer length acts in the opposite direction to alkyl chain length: increasing the spacer from 4 to 8 carbons (10-4-10 to 10-8-10) lowers the CMC from about 10.2 mM to about 7.5 mM yet shrinks the micelles, with D_h falling from about 2.6 nm to about 1.1 nm and SANS giving R_a about 1.8 nm, R_b about 1.3 nm, and N_agg = 13; the authors attribute this to conformational flexibility of the long spacer enabling folding and looping, which lowers the effective aggregation number rather than simply changing intermolecular spacing. The result separates the effect of the spacer on the CMC from its effect on micellar size, arguing that size and shape transitions arise from changes in aggregation number and packing efficiency rather than from geometric enlargement or loosening of individual molecules. 10-4-10 and 10-8-10 share the same headgroup and hydrophobic tail length, forming a direct comparison; the surface tension curves of the two nearly coincide, indicating a minor interfacial role for the spacer, while DLS and SANS reveal clear differences in bulk micelles.

The identity of the salt, not merely ionic strength, determines how micelles restructure: adding 100 mM NaBr to 100 mM 10-4-10 raises D_h from about 2.6 nm to about 4.8 nm with SANS fitting a spherical micelle (R_c about 1.6 nm), whereas 100 mM NaSal raises D_h sharply to about 15.8 nm with SANS fitting an ellipsoidal micelle (R_a about 12.5 nm, R_b about 1.7 nm, N_agg about 146). The work compares an inorganic counterion and an aromatic hydrotropic counterion side by side in the same Gemini system, distinguishing the electrostatic screening effect of Br⁻ from the combined electrostatic and specific aromatic effect of Sal⁻, and reports the corresponding micellar shape and aggregation number. Two independent scattering methods, DLS and SANS, give consistent growth trends; SANS data were collected in D₂O with quartz cell and D₂O background subtraction and normalization to absolute scattering units, and model selection was based on fit quality across the full Q range.

Micellar architecture and salt environment jointly govern MC540 photophysics: upon encapsulation, absorption λ_max is about 561–566 nm and emission λ_em about 580–584 nm, clearly red-shifted relative to water, and excitation spectra show multiple bands at about 510, about 540, and about 563–567 nm, assigned to H-type aggregates or vibronic shoulders, interfacial/palisade monomers, and J-type aggregates, with energy funneling from multiple absorbing states into a common emissive channel at about 580–584 nm; fluorescence decays require tri-exponential fitting (τ₁ about 0.14–0.23 ns, τ₂ about 0.82–1.26 ns, τ₃ about 1.86–2.47 ns), with average lifetimes rising from about 0.77 ns for 8-4-8 to about 1.24 ns for 12-4-12, about 0.78 ns for 10-8-10, about 0.83 ns with NaBr, and about 0.98 ns with NaSal; rotational correlation times rise from about 1.36 ns for 8-4-8 to about 3.27 ns for 12-4-12, with about 1.44 ns for 10-8-10, about 1.86 ns with NaBr, and about 2.29 ns with NaSal. Prior MC540 photophysics has largely been studied in water, with conventional surfactants, or under salt-rich conditions, and systematic combination with Gemini surfactant assemblies has been scarce; this work presents spectral shifts, lifetimes, and rotational restriction alongside scattering-derived micellar size, shape, and aggregation number, giving a structure–photophysics correspondence. Steady-state absorption, excitation, and emission spectra were recorded at 30 ± 0.1 °C with excitation at about 510, about 540, and about 563 nm and emission monitored at about 600 nm; time-resolved fluorescence used a 510 nm pulsed diode laser and TCSPC collected at the magic angle (54.7°), analyzed after IRF deconvolution with a tri-exponential model chosen because it gave the lowest χ² and randomly distributed residuals among mono-, bi-, and tri-exponential comparisons; all systems used 2 μM MC540 with 100 mM surfactant, well above the respective CMCs.

The results apply to aqueous conditions at 30 °C with 100 mM Gemini surfactant (well above the respective CMCs), 2 μM MC540, and 100 mM NaBr or NaSal, covering the four structures 8-4-8, 10-4-10, 12-4-12, and 10-8-10. Within this scope, it provides a basis for selecting chain length, spacer, and counterion to tune micellar size, shape, and aggregation number as well as the dye's spectral shifts, lifetimes, and rotational restriction, and can be used directly by readers working on colloidal self-assembly, dye solubilization, and fluorescent-probe microenvironments.

Several quantitative details sit in the supporting information: absorption, excitation, and emission peak positions are collected in Table S1, tri-exponential fitting parameters and χ² in Tables S2–S4, and rotational correlation times in Table S5, while the main text reports only ranges and averages, so the per-system fitting parameters cannot be fully checked from the main text. The assignment of J-type and H-type aggregates rests mainly on spectral shifts and multiple excitation bands, and the authors phrase it as "possible" and "J-like" without independent structural characterization of the aggregates. Salt effects are compared at a single 100 mM concentration without a concentration or ionic-strength scan, so the transition region from electrostatic screening to specific aromatic binding remains an open question. In addition, photophysical measurements were made at a fixed concentration far above the CMC, so the influence of differing degrees of micellization across surfactants on dye partitioning is not separately resolved.