Synthesis and Surface-Active Properties of New Photosensitive Surfactants Containing the Azobenzene Group

Recyclable Nonionic-Anionic Bola Surfactant as a Stabilizer of Size-Controllable and pH-Responsive Pickering Emulsions

A novel nonionic-anionic Bola surfactant (abbreviated as CH₃O(EO)₇-R₁₁-COOH) was designed and synthesized by condensation of methyl polyoxyethylene (7) ether with 12-bromododecanoic acid. In neutral aqueous solution, the surfactant behaves as a nonionic one and can stabilize oil-in-water (O/W) conventional emulsions alone and costabilize O/W Pickering emulsions with positively charged alumina nanoparticles with n-decane as the oil. In alkaline solution, the carboxylic acid group is deprotonated, becoming anionic and the surfactant is converted to Bola form, which is an inferior emulsifier and does not adsorb on particle surfaces, resulting in demulsification of both kinds of emulsions. With strong hydrophilicity, both the Bola surfactant and the bare particles return to the aqueous phase after demulsification, which is therefore recyclable and reusable in accordance with sustainable chemistry and engineering. In acidic media between pH 3 and 6, the ethyleneoxy groups tend to desorb from particle surfaces, slightly reducing the hydrophobicity of the particles. However, Pickering emulsions are still stable but their droplet size increases on lowering the pH. The Pickering emulsions are therefore pH-responsive and size-controllable. This newly designed Bola surfactant is effective in preparing smart emulsions, which are extensively applied in heterogeneous catalysis, oil product transportation, emulsion polymerization, and new material preparation.

Light Responsiveness and Assembly of Arylazopyrazole-Based Surfactants in Neat and Mixed CTAB Micelles

The self-assembly of an arylazopyrazole-based photosurfactant (PS), based on cetyltrimethylammonium bromide (CTAB), and its mixed micelle formation with CTAB in aqueous solution was investigated by small angle neutron and X-ray scattering (SANS/SAXS) and UV-vis absorption spectroscopy. Upon UV light exposure, PS photoisomerizes from E-PS (trans) to Z-PS (cis), which transforms oblate ellipsoidal micelles into smaller, spherical micelles with larger shell thickness. Doping PS with CTAB resulted in mixed micelle formation at all stoichiometries and conditions investigated; employing selectively deuterated PS, a monotonic variation in scattering length density and dimensions of the micellar core and shell is observed for all contrasts. The concentration- and irradiance-dependence of the E to Z configurational transition was established in both neat and mixed micelles. A liposome dye release assay establishes the enhanced efficacy of photosurfactants at membrane disruption, with E-PS exhibiting a 4-fold and Z-PS a 10-fold increase in fluorescence signal with respect to pure CTAB. Our findings pave the way for external triggering and modulation of the wide range of CTAB-based biomedical and material applications.

Structure-Performance Relationships for Tail Substituted Zwitterionic Betaine-Azobenzene Surfactants

Azobenzene-containing surfactants (azo-surfactants) have garnered significant attention for their use in generating photoresponsive foams, interfaces, and colloidal systems. The photoresponsive behavior of azo-surfactants is driven by the conformational and electronic changes that occur when the azobenzene chromophore undergoes light-induced trans ⇌ cis isomerization. Effective design of surfactants and targeting of their properties requires a robust understanding of how the azobenzene functionality interacts with surfactant structure and influences overall surfactant behavior. Herein, a library of tail substituted azo-surfactants were synthesized and studied to better understand how surfactant structure can be tailored to exploit the azobenzene photoswitch. This work shows that tail group structure (length and branching) has a profound influence on the critical micelle concentration of azo-surfactants and their properties once adsorbed to an air-water interface. Neutron scattering studies revealed the unique role that intermolecular π-π azobenzene interactions have on the self-assembly of azo-surfactants, and how the influence of these interactions can be tuned using tail group structure to target specific aqueous aggregate morphologies.

Photoreversible formation of nanotubes in water from an amphiphilic azobenzene derivative

An anionic azobenzene-appended derivative of L-ValylGlycine self-assembles into nanotubes in water. Irradiation with 365 nm light provokes trans-cis isomerization of the azobenzene unit and subsequent tube disassembly. Thermal or photoinduced (457 nm light) recovery of the trans isomer restores the nanotubes.

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.

Bioinspired Viscoelastic Capsules: Delivery Vehicles and Beyond

Microcapsules are often used as individually dispersed carriers of active ingredients to prolong their shelf life or to protect premature reactions with substances contained in the surrounding. This study goes beyond this application and employs microcapsules as principal building blocks of macroscopic 3D materials with well-defined granular structures. To achieve this goal and inspired by nature, capsules are fabricated from block-copolymer surfactants that are functionalized with catechols, a metal-coordinating motive. These surfactants self-assemble at the surface of emulsion drops where they are ionically cross-linked to form viscoelastic capsules that display a low permeability even toward small encapsulants. It is demonstrated that the combination of the mechanical strength, flexibility, and stickiness of the capsules enables their additive manufacturing into macroscopic granular structures. Thereby, they open up new opportunities for 3D printing of soft, self-healing materials composed of individual compartments that can be functionalized with different types of spatially separated reagents.

Photocontrolled self-assembly of azobenzene nanocontainers in water: light-triggered uptake and release of lipophilic molecules

A simple azobenzene based photo-surfactant thanks to unique photo-triggerable spontaneous emulsification characteristics, allows a clean, reversible and fatigue resistant uptake and release of small molecules in aqueous solution.

Synthesis and Characterization of Photosensitive Ionic Liquid Surfactant 4-Butylazobenzene-4′-(Oxyethyl)Methylimidazolium with Br− and BF4− Counterions

The photo-responsive ionic liquid surfactants 4-Butylazobenzene-4′-(oxyethyl)methylimidazolium with Br− (AZMIMBr) and BF4− (AZMIMBF4) counterions were synthesized, and their structures were characterized by means of 1H NMR. Their properties for pre- or post-UV irradiation were investigated by employing tensiometry, electrical conductance, thermal gravimetry-differential scanning calorimetry (TG-DSC) and small-angle X-ray scattering (SAXS). The effect of UV irradiation time on photoisomerization of two surfactants solutions was evaluated, which showed the photoisomerization efficiency decreases with an increase of their concentrations. After UV irradiation, the CMC value of the two surfactants increased, whereas, the surface tensions at CMC (γCMC) and the fraction of counterion binding (β) were approximately the same. SAXS coupled with polarized optical microscopic studies (POM studies) confirmed that the liquid crystal textures of AZMIMBr could be affected by UV-irradiation. The detailed analysis of thermodynamic parameters revealed that the micellizaton of AZMIMBr was entropy-driven, and the micellization of AZMIMBF4 was enthalpy-driven.

Photosensitive microgels containing azobenzene surfactants of different charges

We report on light sensitive microgel particles that can change their volume reversibly in response to illumination with light of different wavelengths.

A Family of Photolabile Nitroveratryl-Based Surfactants That Self-Assemble into Photodegradable Supramolecular Structures

Here we report the synthesis and characterization of a family of photolabile nitroveratryl-based surfactants that form different types of supramolecular structures depending on the alkyl chain lengths ranging from 8 to 12 carbon atoms. By incorporating a photocleavable α-methyl-o-nitroveratryl moiety, the surfactants can be degraded, along with their corresponding supramolecular structures, by light irradiation in a controlled manner. The self-assembly of the amphiphilic surfactants was characterized by conductometry to determine the critical concentration for the formation of the supramolecular structures, transmission electron microscopy to determine the size and shape of the supramolecular structures, and dynamic light scattering (DLS) to determine the hydrodynamic diameter of the structures in aqueous solutions. The photodegradation of the surfactants and the supramolecular structures was confirmed using UV-vis spectroscopy, mass spectrometry, and DLS. This surfactant family could be potentially useful in drug delivery, organic synthesis, and other applications.

A cationic azobenzene-surfactant-modified graphene hybrid: unique photoresponse and electrochemical behavior

Surfactant-modified graphene hybrids containing azobenzene groups were for the first time prepared, and the electrochemical performance was investigated. The hybrids were obtained by electrostatic interactions between cationic azobenzene-surfactants and negatively charged graphene oxide in water. The electrostatic interactions, chemical structure and photoresponse of the hybrids were measured by using zeta potential values, fluorescence spectra, FTIR, XPS, XRD, SEM, UV-Vis absorption, AFM and Raman spectra. The electrochemical performance was estimated using cyclic voltammetry. The results show that strong electrostatic interactions exist between the azobenzene surfactants and graphene oxide. Notably, this azobenzene-graphene hybrid can self-assemble into aggregation structures in aqueous solution. Besides, the self-assembly can be reversibly controlled by ultraviolet light (365 nm) and blue light (455 nm) irradiation. This process is driven by the photoinduced polarity change of the cationic azobenzene surfactant and is responsible for the graphene hybrids' electrochemical performance. It is the first example of the reversible self-assembly of graphene driven by light irradiation.

Light induced micelle to vesicle transition in an aqueous solution of a surface active ionic liquid

A new simple surface active ionic liquid displayed reversible micelle–vesicle transition under alternative UV/vis irradiation without additives.

4-[2-(2,2-Dimethyl-4,6-dioxo-1,3-dioxan-5-ylidene)hydrazin-1-yl]benzonitrile

In the title compound, C₁₃H₁₁N₃O₄, the dioxane ring adopts an envelope conformation with the C atom bonded to the dimethyl group in the flap position [deviation = 0.613 (1) Å]. The nitrile group and the attached benzene ring are roughly coplanar [maximum deviation = 0.087 (1) Å]. An intra­molecular N—H⋯O hydrogen bond involving the hydrazinyl group generates an S(6) ring. The N—N and C—N bond lengths indicate that the compound may be a mixture of the azo and hydrazone tautomeric forms but the presence of the N-bound H atom supports the hydrazone form. The crystal structure is stabilized by weak inter­molecular C—H⋯O, C—H⋯N and C—H⋯π inter­actions.

Photoresponsive behavior of amphiphilic copolymers of azobenzene and N,N-dimethylacrylamide in aqueous solutions

Copolymers of 4-methacryloyloxyazobenzene and N,N-dimethylacrylamide (MOAB-DMA) can aggregate strongly in aqueous solution (they are soluble in water up to a MOAB molar fraction of 0.2) to give concentration-dependent aggregate size distributions and well-defined boundaries between the dilute and semidilute regimes, as determined by dynamic light scattering, surface tension, and probe solubilization experiments. The copolymers are strongly surface active, an uncommon observation for random copolymers, and exhibit pronounced photoviscosity effects at higher concentrations. The concentration dependence of the kinetic parameters for the reversible polymer rearrangement upon photoisomerization, as determined by electronic absorption spectroscopy, is attributed to steric hindrances. Trans-to-cis isomerization under UV light leads to partial dissociation of the azobenzene aggregates that cross-link the polymers, thereby significantly affecting the polymer solution rheology, with a consequent loss of viscoelasticity upon UV irradiation, especially in concentrated polymer solutions.

Temperature dependence of aggregation and dynamic surface tension in a photoresponsive surfactant system

The response of a nonionic photoresponsive surfactant system to changes in temperature is reported. This surfactant contains the light-sensitive azobenzene group, and when exposed to light, a solution of this surfactant contains a mixture of the cis and trans photoisomers of this group. The temperature of the surfactant solution has a strong impact on the time needed for the surfactant to diffuse and adsorb to a freshly formed interface. At surfactant concentrations that give rise to trans aggregates but not to cis aggregates, the transport of cis and of trans isomers to the surface of a pendant bubble have quite different temperature dependencies, owing largely to the difference in their aggregation states in bulk solution. Diffusion and adsorption of the cis isomer are described reasonably well by a simple diffusion model that accounts for the effect of temperature on the diffusion coefficient. The trans isomer, which was primarily bound in aggregates during these measurements, exhibits a stronger dependence of this adsorption time scale on the temperature of the solution. This temperature dependence of trans diffusion and adsorption is quantitatively consistent between samples containing only the trans isomer and samples containing a mixture of isomers. Fluorescence studies were done to determine the effect of temperature on the cmc of the surfactant. The critical concentration associated with the formation of cis-dominant aggregates increases modestly with increasing temperature. The cmc of the trans isomer also increases with increasing temperature, most significantly when the temperature exceeds about 35 degrees C. These trans cmc temperature-dependence data were incorporated into diffusion models that account for the potential roles of aggregates in the adsorption process. The observed temperature dependency of the trans adsorption time scale is consistent with a model that includes the effect of temperature on both the diffusivity and the supply of monomer via its effect on the cmc. Specifically, the results suggest that the dissolution of trans-dominant aggregates is important to the trans adsorption process. Further fluorescence studies were performed in which surfactant solutions containing aggregates were diluted rapidly, and the rate of dissolution of these aggregates was inferred from fluorescence decay. Aggregate breakup in colder trans samples is slower than in warmer samples, but these dissolution time scales are significantly shorter than those associated with the adsorption process. This is consistent with the assumption that aggregation kinetics do not contribute to the observed adsorption kinetics.

Photoregulation of drug release in azo-dextran nanogels

A simple photoresponsive azo-dextran polymer has been investigated for its ability to act as a nanogel drug carrier. Self aggregation of the azo-dextran polymer leads to the formation of nanogels, AD (5 and 10) in aqueous media, which were characterized by TEM and DLS. When examined under UV light (365 nm), the unloaded nanogels, which were observed to be in the range of 120-290 nm, show dependence on the degree of crosslinking, pH and ionic concentration of the dispersed media. Nanogels, AD (5 and 10), have been loaded with a model fluorophore, rhodamine B and a drug, aspirin, by freeze drying an aqueous dispersion of the nanogels in the presence of the substrate dissolved in water or PBS buffer. The release pattern of the encapsulated bio-active molecules from these nanogels was regulated by (trans-cis) photoisomerization of the azobenzene moiety present in the crosslinker. A comparison of the release behavior of the loaded (rhodamine, aspirin) AD (5 and 10) nanogels reveal that the rate of release of the encapsulated active molecules from the nanogels was slower when the azo moiety was in E-configuration as compared to that the azo in the Z-configuration. The in vitro release behavior of drug from these polymeric micellar systems is revelative of the potential of the nanogels for targeted drug delivery in nanomedicine.

Dynamic surface tension behavior in a photoresponsive surfactant system

The surface properties of a nonionic photoresponsive surfactant that incorporates the light-sensitive azobenzene group into its tail have been investigated. Cis-trans photoisomerization of this azobenzene group alters the ability of the surfactant to pack into adsorbed monolayers at an air/water interface or into aggregates in solution, thereby causing a significant variation in surface and bulk properties following a change in the illumination conditions. NMR studies indicate that a solution left in the dark for an extended period of time contains the trans isomer almost exclusively, whereas samples exposed to light of fixed wavelength eventually reach a photostationary equilibrium in which significant amounts of both isomers are present. At concentrations well above the cmc but under different illumination conditions (dark, UV light, visible light), freshly formed surfaces exhibit profoundly different surface tension trajectories as they approach essentially identical equilibrium states. This common equilibrium state corresponds to a surface saturated with the trans (more surface active) isomer. The dark sample shows a simple, single-step relaxation in surface tension after the creation of a fresh interface, whereas the UV and visible samples exhibit a more rapid initial decrease in tension, followed by a plateau of nearly constant tension, and finally end with a second relaxation to equilibrium. It is hypothesized that this behavior of the UV and visible samples is caused by competitive adsorption between the cis and trans isomers present in these mixtures. The cis surfactant reaches the interface more quickly, leading to an initially cis-dominated interface having a tension value corresponding to the intermediate plateau, but is ultimately displaced by the trans isomer. Fluorescence studies are used for cmc determination in the samples, and the results suggest that the two isomers segregate into distinct aggregate phases. The critical concentration associated with the formation of cis-rich aggregates is much larger than that of the trans-rich aggregates, which accounts for the faster diffusion of the cis isomer to a fresh interface. Models of the diffusion and adsorption of surfactant are developed. These consider the role of aggregates in the adsorption process by examining the limiting behavior of three aggregate properties: dissolution rate, mobility, and ability to incorporate into the interface. These models are used to analyze the surface tension relaxation of dark and UV samples, and the predictions are found to be in agreement with the observed characteristic relaxation time scales for these samples, though the results are inconclusive regarding the specific role of aggregates. High-intensity illumination focused on a surface saturated with surfactant is used to drive photoisomerization of the adsorbed surfactant, and rapid, substantial changes in surface tension result. These changes are consistent with proposed conformations of the adsorbed surfactant and with monolayer studies performed with a Langmuir film balance.

Self-assembly of light-sensitive surfactants

This review covers recent advances with an intriguing class of functionalised light-sensitive surfactants. The main chemical classes are described, and the photo-responses in interfacial and aggregation systems are discussed.

H-aggregation of azobenzene-substituted amphiphiles in vesicular membranes

Photochemical switching has been studied of double-tailed phosphate amphiphiles containing azobenzene units in both tails in aqueous vesicular dispersions and in mixed vesicular systems with 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC). Since the ease of switching depends on the strength of the bilayer packing, particular emphasis has been placed on the occurrence of H-aggregation in the hydrophobic core of the vesicles. UV-vis spectrometry was employed to monitor H-aggregation and showed how this process depends on the ionic strength and on the mode of preparation of the vesicles. Two types of H-aggregates were observed in mixed DOPC vesicles with 5 mol % of azobenzene phosphate: one with lambda(max) at around 300 nm and one with lambda(max) at 305-320 nm. Those with lambda(max) at 300 nm could not be trans-cis photoisomerized, whereas those with lambda(max) at 305-320 nm are more loosely packed and can be photochemically switched. The permeability of the vesicular bilayers, as probed with leakage experiments using calcein as a fluorescent probe, was examined as another measure for the strength of bilayer packing. Leakage occurred only for DOPC vesicles containing more than 20 mol % of azobenzenephosphate, irradiated with UV light to induce trans-cis photoisomerization. We contend that detailed information on bilayer packing will be of crucial importance for fine-tuning the lateral pressure in vesicular membranes with the ultimate aim to steer the opening and closing of mechanosensitive protein channels of large conductance.