Adsorption Kinetics of a Photosensitive Surfactant Inside Microgels

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.