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.