No abstract

A cationic surfactant containing a spiropyran unit is prepared exhibiting a dual‐responsive adjustability of its surface‐active characteristics. The switching mechanism of the system relies on the reversible conversion of the non‐ionic spiropyran (SP) to a zwitterionic merocyanine (MC) and can be controlled by adjusting the pH value and via light, resulting in a pH‐dependent photoactivity: While the compound possesses a pronounced difference in surface activity between both forms under acidic conditions, this behavior is suppressed at a neutral pH level. The underlying switching processes are investigated in detail, and a thermodynamic explanation based on a combination of theoretical and experimental results is provided. This complex stimuli‐responsive behavior enables remote‐control of colloidal systems. To demonstrate its applicability, the surfactant is utilized for the pH‐dependent manipulation of oil‐in‐water emulsions.

Photosensitive azobenzene‐containing surfactants have attracted great attention in past years because they offer a means to control soft‐matter transformations with light. At concentrations higher than the critical micelle concentration (CMC), the surfactant molecules aggregate and form micelles, which leads to a slowdown of the photoinduced trans→cis azobenzene isomerization. Here, we combine nonadiabatic dynamics simulations for the surfactant molecules embedded in the micelles with absorption spectroscopy measurements of micellar solutions to uncover the reasons responsible for the reaction slowdown. Our simulations reveal a decrease of isomerization quantum yields for molecules inside the micelles. We also observe a reduction of extinction coefficients upon micellization. These findings explain the deceleration of the trans→cis switching in micelles of the azobenzene‐containing surfactants.

Graphical abstract

Here, we investigate the kinetics of adsorption and desorption of a cationic photosensitive azobenzene-containing surfactant within anionic microgels in the dark and under continuous illumination with light of different wavelengths and show that microgels can serve as a selective absorber of one of the possible isomers of the photosensitive surfactant. The adsorption of the isomer is governed by entropic reasons at which micellization of the surfactant takes place within the microgel matrix composed of cross-linked PNIPAM and anionic poly­(acryl acid) chains rendering it photoresponsive. Under irradiation with appropriate wavelength, the surfactant molecules photoisomerize from trans (hydrophobic)- to cis (hydrophilic)-state and the microgel collapses due to diffusion of the cis-isomers out of the particle interior. When the light is switched off, the microgels swell back to the equilibrium size by absorbing the rest of the trans-isomers out of solution with the characteristic time being between a few seconds and hours depending on the amount of the trans-isomers left in the solution. Measuring the kinetics of the microgel size response and knowing the exact isomer composition under light exposure, we calculate the adsorption rate of the trans-isomers. We show that depending on the intensity of the applied light, one can differentiate between two processes, i.e., at low intensities, the kinetics of the size change is mostly dominated by the photoisomerization rate of the surfactant within the interior of the particle, while at larger intensities, the process is limited by the surfactant adsorption/desorption rate. By performing temperature-dependent measurements, we also calculate the activation energy of the adsorption/desorption process.

Ionic complexation of azobenzene-containing surfactants with any type of oppositely charged soft objects allows for making them photo-responsive in terms of their size, shape and surface energy. Investigation of the photo-isomerization kinetic and isomer composition at a photo-stationary state of the photo-sensitive surfactant conjugated with charged objects is a necessary prerequisite for understanding the structural response of photo-sensitive complexes. Here, we report on photo-isomerization kinetics of a photo-sensitive surfactant in the presence of poly(acrylic acid, sodium salt). We show that the photo-isomerization of the azobenzene-containing cationic surfactant is slower in a polymer complex compared to being purely dissolved in aqueous solution. In a photo-stationary state, the ratio between the trans and cis isomers is shifted to a higher trans-isomer concentration for all irradiation wavelengths. This is explained by the formation of surfactant aggregates near the polyelectrolyte chains at concentrations much lower than the bulk critical micelle concentration and inhibition of the photo-isomerization kinetics due to steric hindrance within the densely packed aggregates.

In this paper, the phenomenon of light‐driven diffusioosmotic (DO) long‐range attractive and repulsive interactions between micro‐sized objects trapped near a solid wall is investigated. The range of the DO flow extends several times the size of microparticles and can be adjusted to point towards or away from the particle by varying irradiation parameters such as intensity or wavelength of light. The “fuel” of the light‐driven DO flow is a photosensitive surfactant which can be photo‐isomerized between trans and cis‐states. The trans‐isomer tends to accumulate at the interface, while the cis‐isomer prefers to stay in solution. In combination with a dissimilar photo‐isomerization rate at the interface and in bulk, this yields a concentration gradient of the isomers around single particles resulting in local light‐driven diffusioosmotic (l‐LDDO) flow. Here, the extended analysis of the l‐LDDO flow as a function of irradiation parameters by introducing time‐dependent development of the concentration excess of isomers near the particle surface is presented. It is also demonstrated that the l‐LDDO can be generated at any solid/liquid interface being more pronounced in the case of strongly absorbing material. This phenomenon has plenty of potential applications since it makes any type of surface act as a micropump.

Here we show that microgels trapped at a solid wall can issue liquid flow and transport over distances several times larger than the particle size. The microgel consists of cross-linked poly­(N-isopropylacrylamide-co-acrylic acid) (PNIPAM-AA) polymer chains loaded with cationic azobenzene-containing surfactant, which can assume either a trans- or a cis-state depending on the wavelength of the applied irradiation. The microgel, being a selective absorber of trans-isomers, responds by changing its volume under irradiation with light of appropriate wavelength at which the cis-isomers of the surfactant molecules diffuse out of the particle interior. Together with the change in particle size, the expelled cis-isomers form an excess of the concentration and subsequent gradient in osmotic pressure generating a halo of local light-driven diffusioosmotic (l-LDDO) flow. The direction and the strength of the l-LDDO depends on the intensity and irradiation wavelength, as well as on the amount of surfactant absorbed by the microgel. The flow pattern around a microgel is directed radially outward and can be maintained quasi-indefinitely under exposure to blue light when the trans-/cis-ratio is 2/1, establishing a photostationary state. Irradiation with UV light, on the other hand, generates a radially transient flow pattern, which inverts from inward to outward over time at low intensities. By measuring the displacement of tracer particles around neutral microgels during a temperature-induced collapse, we can exclude that a change in particle shape itself causes the flow, i.e., just by expulsion or uptake of water. Ultimately, it is its ability to selectively absorb two isomers of photosensitive surfactant under different irradiation conditions that leads to an effective pumping caused by a self-induced diffusioosmotic flow.