Natalia Rincón-Londoño

A multiscale analysis of photoresponsive micellar systems based on CTAB/NaSal and 4-amine tetraortho-azobenzenes (AzoCH₃ and AzoCl) was carried out to elucidate how molecular photoisomerization modulates micellar morphology, mesoscopic organization, and macroscopic dynamics in the semidilute regime. Under basic conditions (pH 11), CTAB/NaSal solutions incorporating trans-AzoCH₃ and trans-AzoCl form well-defined wormlike micelles (WLMs), as confirmed by TEM and molecular dynamics simulations, which reveal the presence of tubular structures. Dynamic light scattering (DLS) measurements indicate that these systems lie within the semidilute–entangled regime, where collective hydrodynamic behavior dominates over individual micellar dimensions, in agreement with viscosity measurements. Upon UV irradiation at 360 nm, trans–cis photoisomerization of the azobenzenes induces a pronounced disruption of the WLM network, leading to partial collapse and fragmentation of tubular structures, as evidenced by HR-TEM imaging and simulations. Despite this significant structural rearrangement, DLS measurements reveal no substantial change in the hydrodynamic correlation length, indicating that the system remains in the semidilute regime, consistent with the slight decrease in viscosity while preserving shear-thinning behavior. Thermal cis-to-trans isomerization kinetics were monitored by UV–vis spectroscopy and were well described by monoexponential behavior, confirming reversible photosensitivity within the micellar environment. These results demonstrate that azobenzene photoisomerization can modulate micellar morphology without altering the overall concentration regime, highlighting a decoupling between local micellar structure and mesoscopic collective dynamics. This behavior underscores the robustness of semidilute wormlike micellar networks and their potential for applications in light-responsive soft materials.

This review paper presents a procedure for measuring the mesoscopic scales in micellar solutions embedded with giant cylindrical micelles using the mean square displacement determined with a quasi-elastic multiple light scattering method (diffusing wave spectroscopy) and theory. The mesoscopic scales of interest are the micelles’ total contour length, persistence and entanglement lengths, and the mesh size of the entangled micellar network. All of them depend on the physicochemical parameters of the solutions and determine the rheological behavior. We present an assessment of the whole procedure, the scattering experiments performance, the recovery of optical parameters, which includes dealing with the light absorption and its treatment, and how to develop the micro-rheology for obtaining the mesoscopic scales in these complex fluids.

Chromophores susceptible to light-induced trans-cis isomerization embedded in cylindrical micelles can modify micelles and their light-responsive performance. A small chromophore (4-(phenylazo) benzoate ion) is embedded in cylindrical micelles made of CetylTrimethylAmmonium Bromide (CTAB) and Sodium Salicylate (NaSal) in water. The microstructure is examined with Scanning Electron Microscopy (SEM) and Nuclear Magnetic Resonance (NMR). Rheological behavior and the length scales of the micellar network are determined with rheology and microrheology. The chromophore substantially modifies the micelles even without UV irradiation. The larger is the chromophore concentration, the smaller is the micellar length. Additional length scales of the micellar network do not substantially vary even when NaCl is added. Chromophore incorporation also modifies the rheology of the micellar solution, although gradient shear banding is preserved. Viscosity decreases as the chromophore concentration increases and viscoelastic spectra are modified, but when they are correctly rescaled, they can be superimposed. The addition of the chromophore makes the fluids more Maxwellian, particularly when NaCl is also added. When the chromophore is incorporated into the micelles, there is a response after UV-irradiation, although, it does not produce a significant rheological change.