pH-Dependent Photoinduced Interconversion of Furocoumaric and Furocoumarinic Acids

Photo-controlled or photo-regulated molecules, especially biologically active and operating in physiological conditions, are in steady demand. Herein, furocoumaric and furocoumarinic acids being (Z/E)-isomers relative to each other were obtained in two stages starting from psoralen: the alkaline solvolysis of psoralen led to furocoumaric acid, which was further Z → E photoisomerized (365 nm) to furocoumarinic acid. The kinetics of Z → E photoisomerization was monitored by HPLC and UV-vis spectrophotometry. Photophysical characteristics in the aqueous phase for both acids, as well as the reversibility of (Z/E) photoisomerization process, were also assessed. Furocoumarinic acid was found to be visibly fluorescent at pH 2.0-12.0, with the maxima of fluorescence emission spectra being pH-dependent. The reverse E → Z photoisomerization predicted by quantum chemistry calculations as energetically favorable for the monoanionic form of furocoumarinic acid was proved in the experiment while being complicated by pyrone ring closure back to psoralen in acidic and neutral conditions. The preparative synthesis of furocoumarinic acid outlined in this work is particularly valuable in view of a wide range of pharmacological effects previously predicted for this compound.

Light and CO2/N2 dual stimuli-responsive wormlike micelles based on a zwitterionic surfactant and an azobenzene surfactant

Stimulus-responsive surfactant wormlike micelles have been widely investigated in the past decade. In this article, we report light and CO₂/N₂ dual stimuli-responsive wormlike micelles using a zwitterionic surfactant (SDAP) and an azobenzene surfactant (C₄AzoC₆N). In contrast to traditional amine-containing wormlike micelles, a fast and reversible CO₂-triggered thinning behavior was observed. The system can also be reversibly switched by UV irradiation. The dual stimuli-responsive wormlike micelles (C₄AzoC₆N-SDAP) may have applications in the development of functional materials for microfluidics and analytical chemistry.

Photoinduced viscosity control of lecithin-based reverse wormlike micellar systems using azobenzene derivatives

This report describes the controlled viscosity changes of photoresponsive reverse wormlike micellar systems formed by soybean lecithin (SoyPC), d-ribose, and azobenzene derivatives in decane.

Properties of Cationic Choline-Derived Surfactant with Photolabile Cinnamate Counterion

A cationic choline-derived surfactant with photolabile cinnamate counterion (tetradecyl-(2-hydroxyethyl)-dimethylammoniumcinnamate, C14HDACin) was synthesized. The properties of C14HDACin aqueous solutions for pre- or post-UV irradiation were investigated by employing tensiometry, conductance, transmission electron microscopy (TEM) and rheometry. The effect of UV irradiation time on photoisomerization of C14HDACin solutions was evaluated, which showed that the photoisomerization efficiency decreases with an increase of C14HDACin concentration. After UV irradiation, the fraction of counterion binding (β) of the C14HDACin micelle decreases. However, the value of CMC increases. Rheometry coupled with TEM studies confirmed that the aggregation of C14HDACin could transform from wormlike to spherical micelles upon UV irradiation.

Microstructural morphologies of CTAB micelles modulated by aromatic acids

pH and temperature-dependent aggregation behaviour of CTAB with solubilized aromatic acids.

Microfluidic flows of wormlike micellar solutions

The widespread use of wormlike micellar solutions is commonly found in household items such as cosmetic products, industrial fluids used in enhanced oil recovery and as drag reducing agents, and in biological applications such as drug delivery and biosensors. Despite their extensive use, there are still many details about the microscopic micellar structure and the mechanisms by which wormlike micelles form under flow that are not clearly understood. Microfluidic devices provide a versatile platform to study wormlike micellar solutions under various flow conditions and confined geometries. A review of recent investigations using microfluidics to study the flow of wormlike micelles is presented here with an emphasis on three different flow types: shear, elongation, and complex flow fields. In particular, we focus on the use of shear flows to study shear banding, elastic instabilities of wormlike micellar solutions in extensional flow (including stagnation and contraction flow field), and the use of contraction geometries to measure the elongational viscosity of wormlike micellar solutions. Finally, we showcase the use of complex flow fields in microfluidics to generate a stable and nanoporous flow-induced structured phase (FISP) from wormlike micellar solutions. This review shows that the influence of spatial confinement and moderate hydrodynamic forces present in the microfluidic device can give rise to a host of possibilities of microstructural rearrangements and interesting flow phenomena.

Design and synthesis of photoactive ionic liquids

Two ionic liquids with photoisomerizable p-hydroxycinnamic acid moieties were synthesized and characterized by X-ray crystallography and DSC, and their photochemistry was studied in solution and neat conditions. Irradiation at absorption maxima led to trans-cis photoisomerization and resulted in significant reduction of melting temperatures of the ionic liquids. X-ray structures of both compounds show an intricate network of supramolecular interactions before irradiation. Physical and chemical transformations are completely reversible upon irradiation at lower wavelengths of ionic liquid solutions in acetonitrile.

Unusual viscoelasticity behaviour in aqueous solutions containing a photoresponsive amphiphile

Here, we report unusual behaviour of the viscoelasticity of surfactant aqueous solutions consisting of cationic cetyltrimethylammonium bromide (CTAB) and an anionic photoresponsive amphiphile, sodium [4-(4-butylphenylazo)phenoxy]acetate (C4AzoNa). When C4AzoNa molecules are trans-isomers, spheroidal micelles are formed in the surfactant solution, the viscosity of which is low. Irradiation of this solution by ultraviolet (UV) light yields an aqueous solution of CTAB/cis-C4AzoNa (cis-isomers of C4AzoNa), which is a highly viscous gel consisting of wormlike micelles. The drastic change in the surfactant solution viscosity is attributed to the formation and disruption of wormlike micelles. The geometrical structural transformation of the azobenzene groups in the C4AzoNa molecules of the CTAB/C4AzoNa mixture would lead to a change in the critical packing parameter of the mixture, thereby inducing the morphological transformation of the aggregates (spheroidal micelles to wormlike micelles). To our knowledge, this is the first report of a drastic increase in surfactant solution viscosity by UV light irradiation.

Smart wormlike micelles

A major scientific challenge of the past decade pertaining to the field of soft matter has been to craft 'adaptable' materials, inspired by nature, which can dynamically alter their structure and functionality on demand, in response to triggers produced by environmental changes. Amongst these, 'smart' surfactant wormlike micelles, responsive to external stimuli, are a particularly recent area of development, yet highly promising, given the versatility of the materials but simplicity of the design-relying on small amphiphilic molecules and their spontaneous self-assembly. The switching 'on' and 'off' of the micellar assembly structures has been reported using electrical, optical, thermal or pH triggers and is now envisaged for multiple stimuli. The structural changes, in turn, can induce major variations in the macroscopic characteristics, affecting properties such as viscosity and elasticity and sometimes even leading to a spontaneous and effective 'sol-gel' transition. These original smart materials based on wormlike micelles have been successfully used in the oil industry, and offer a significant potential in a wide range of other technological applications, including biomedicine, cleaning processes, drag reduction, template synthesis, to name but a few. This review will report results in this field published over the last few years, describe the potential and practical applications of stimuli-responsive wormlike micelles and point out future challenges.