Unlocking Structure–Self-Assembly Relationships in Cationic Azobenzene Photosurfactants

Abstract

Azobenzene photosurfactants are light-responsive amphiphiles that have garnered significant attention for diverse applications including delivery and sorting systems, phase transfer catalysis, and foam drainage. The azobenzene chromophore changes both its polarity and conformation (trans-cis isomerization) in response to UV light, while the amphiphilic structure drives self-assembly. Detailed understanding of the inherent relationship between the molecular structure, physicochemical behavior, and micellar arrangement of azobenzene photosurfactants is critical to their usefulness. Here, we investigate the key structure-function-assembly relationships in the popular cationic alkylazobenzene trimethylammonium bromide (AzoTAB) family of photosurfactants. We show that subtle changes in the surfactant structure (alkyl tail, spacer length) can lead to large variations in the critical micelle concentration, particularly in response to light, as determined by surface tensiometry and dynamic light scattering. Small-angle neutron scattering studies also reveal the formation of more diverse micellar aggregate structures (ellipsoids, cylinders, spheres) than predicted based on simple packing parameters. The results suggest that whereas the azobenzene core resides in the effective hydrophobic segment in the trans-isomer, it forms part of the effective hydrophilic segment in the cis-isomer because of the dramatic conformational and polarity changes induced by photoisomerization. The extent of this shift in the hydrophobic-hydrophilic balance is determined by the separation between the azobenzene core and the polar head group in the molecular structure. Our findings show that judicious design of the AzoTAB structure enables selective tailoring of the surfactant properties in response to light, such that they can be exploited and controlled in a reliable fashion.

Multimodal control of liquid crystalline mesophases from surfactants with photoswitchable tails

“…This suggests that the simple packing parameter cannot take into account the dramatic volume change associated with the trans-cis photoisomerisation of the azobenzene (Figure 1). 30 Notably, we show that following photodeformation, the original LLC phase can easily be recovered through gentle heating or irradiation with blue light.…”

Run-and-Halt Behavior of Motile Droplets in Response to Light

“…In contrast, cis-Azo forms only spherical micelles, with a diameter of ~3.6 nm (Supplementary Figures S8D, S8E, S8F). 43 The diameter of the cis-micelles is smaller than that of the trans-micelles because the cis-switch is shorter than the trans-switch (0.9 nm vs 0.5 nm). 44 The fact that both forms of the switch form micelles with different geometries is a signature of their difference in shape and polarity, and constitutes additional evidence for their different effectiveness in solubilizing the oil from the droplets.…”

Self‐Assembly of Photoresponsive Molecular Amphiphiles in Aqueous Media

“…Since the supramolecular self‐assembled structures of amphiphiles in aqueous media allow compatibility to natural systems as well as the potential for biologically relevant applications, we focus on the self‐assembled microstructures and the structural transformations of molecular amphiphiles in aqueous media. The observed assemblies depend on the molecular structures and experimental conditions, such as concentration, temperature, pH, light, ionic strength, and the structure of the co‐assembly compound [81, 158–165] . According to Israelachvili et al., [6] the shape and size of self‐assembled structures of amphiphiles in aqueous media can be predicted by using the packing parameter, P .…”