Self-Assembly of Photoresponsive Molecular Amphiphiles in Aqueous Media

Amphiphilic molecules, comprising hydrophobic and hydrophilic moieties and the intrinsic propensity to self-assemble in aqueous environment, sustain a fascinating spectrum of structures and functions ranging from biological membranes to ordinary soap. Facing the challenge to design responsive, adaptive, and out-of-equilibrium systems in water, the incorporation of photoresponsive motifs in amphiphilic molecular structures offers ample opportunity to design supramolecular systems that enables functional responses in water in a non-invasive way using light. Here, we discuss the design of photoresponsive molecular amphiphiles, their self-assembled structures in aqueous media and at air-water interfaces, and various approaches to arrive at adaptive and dynamic functions in isotropic and anisotropic systems, including motion at the air-water interface, foam formation, reversible nanoscale assembly, and artificial muscle function. Controlling the delicate interplay of structural design, self-assembling conditions and external stimuli, these responsive amphiphiles open several avenues towards application such as soft adaptive materials, controlled delivery or soft actuators, bridging a gap between artificial and natural dynamic systems.

Dynamic Assemblies of Molecular Motor Amphiphiles Control Macroscopic Foam Properties

Stimuli-responsive supramolecular assemblies controlling macroscopic transformations with high structural fluidity, i.e., foam properties, have attractive prospects for applications in soft materials ranging from biomedical systems to industrial processes, e.g., textile coloring. However, identifying the key processes for the amplification of molecular motion to a macroscopic level response is of fundamental importance for exerting the full potential of macroscopic structural transformations by external stimuli. Herein, we demonstrate the control of dynamic supramolecular assemblies in aqueous media and as a consequence their macroscopic foam properties, e.g., foamability and foam stability, by large geometrical transformations of dual light/heat stimuli-responsive molecular motor amphiphiles. Detailed insight into the reversible photoisomerization and thermal helix inversion at the molecular level, supramolecular assembly transformations at the microscopic level, and the stimuli-responsive foam properties at the macroscopic level, as determined by UV-vis absorption and NMR spectroscopies, electron microscopy, and foamability and in situ surface tension measurements, is presented. By selective use of external stimuli, e.g., light or heat, multiple states and properties of macroscopic foams can be controlled with very dilute aqueous solutions of the motor amphiphiles (0.2 weight%), demonstrating the potential of multiple stimuli-responsive supramolecular systems based on an identical molecular amphiphile and providing opportunities for future soft materials.

Photoresponsive aqueous foams with controllable stability from nonionic azobenzene surfactants in multiple-component systems

The controllability of foam stability is a vital feature that allows for practical applications of foam systems. Light, as an external stimulus, offers unique opportunities to tune the foam stability in a non-invasive manner with high spatiotemporal precision. However, most of the reported photoresposive foams were generated from ionic type surfactants, limiting their applications in industrial complex systems with multiple components. Herein, we design and synthesize a series of nonionic azobenzene surfactants with different polyoxyethylene glycol (EO) chain lengths (BEO-n-Azo, n, referring to the EO chain length, is 14, 19 and 23, respectively) to prepare photoresponsive foams. Detailed insights into the effects of EO chain length on photoisomerization properties, surface tension, as well as foamability and controllable stability of photoresponsive foams are presented. The results demonstrate that photoresposive foams are generated not only from single-component solutions of BEO-n-Azo, but also from multiple-component complex systems doped with BEO-n-Azo, providing a promising strategy to broaden applications of photoresponsive foams in industrial processes.

Smart Air-Water Interfaces with Arylazopyrazole Surfactants and Their Role in Photoresponsive Aqueous Foam

A new light-switchable azo-surfactant arylazopyrazole tetraethylene glycol carboxylic acid (AAP-E₄) was used as a molecular building block to functionalize macroscopic foams. AAP-E₄ was studied in the bulk solution with UV/vis spectroscopy and at the interface with sum-frequency generation (SFG) as well as tensiometry. Additional foaming experiments were performed with a dynamic foam analyzer to study the role of AAP-E₄ surfactants at the ubiquitous air-water interface as well as within macroscopic foam. In the bulk, it is possible to switch the AAP-E₄ surfactant reversibly from trans to cis configurations and vice versa using 380 nm UV and 520 nm green light, respectively. At the interface, we demonstrate the excellent switching ability of AAP-E₄ surfactants and a substantial modification of the surface tension. In addition, we show that the response of the interface is strongly influenced by lateral electrostatic interactions, which can be tuned by the charging state of AAP-E₄. Consequently, the electrostatic disjoining pressure and thus the foam stability are highly dependent on the bulk pH and the charging state of the interface. For that reason, we have studied both the surface net charge (SFG) and the surface excess (tensiometry) as important parameters that determine foam stability in this system and show that neutral pH conditions lead to the optimal compromise between switching ability, surface excess, and surface charging. Measurements on the foam stability demonstrated that foams under irradiation with green light are more stable than foams irradiated with UV light.