Communication: Light driven remote control of microgels’ size in the presence of photosensitive surfactant: Complete phase diagram

Abstract

Here we report on a light triggered remote control of microgel size in the presence of photosensitive surfactant. The hydrophobic tail of the cationic surfactant contains azobenzene group that undergoes a reversible photo-isomerization reaction from a trans- to a cis-state accompanied by a change in the hydrophobicity of the surfactant. We have investigated light assisted behaviour and the complex formation of the microgels with azobenzene containing surfactant over the broad concentrational range starting far below and exceeding several times of the critical micelle concentration (CMC). At small surfactant concentration in solution (far below CMC), the surfactant in the trans-state accommodates within the microgel causing its compaction, while the cis-isomer desorbs out of microgel resulting in its swelling. The process of the microgel size change can be described as swelling on UV irradiation (trans-cis isomerization) and shrinking on irradiation with blue light (cis-trans isomerization). However, at the surfactant concentrations larger than CMC, the opposite behaviour is observed: the microgel swells on blue irradiation and shrinks during exposure to UV light. We explain this behaviour theoretically taking into account isomer dependent micellization of surfactant within the microgels.

Adsorption Kinetics of a Photosensitive Surfactant Inside Microgels

“…The reversible adsorption/desorption of the trans / cis -isomers, respectively, results in a reversible change of the osmotic pressure and thus the volume of the microgel. Depending on the surfactant concentration, there are two regions of opposite behavior of microgels . At low concentrations (Region I, below bulk CMC), effective sorption of the trans -surfactant by microgel results in the expelling of the native gel counterions (in this case, H + ) accompanied by an osmotic pressure drop and a volume collapse .…”

Tuning the Volume Phase Transition Temperature of Microgels by Light

“…Salt‐free case is considered, that is the counterions of the microgel particles, surfactant ions, and surfactant counterions are the only mobile ions in the system. We use the two‐zone model to account for the redistribution of ions between the microgel interior and the outer solution, [ 37,38 ] resulting in an uncompensated microgel charge of Q / e = νNf (β − 1 + sZ − tZ ), where e is the elementary charge while s , β, and t denote the total fractions of surfactant molecules, network counterions, and surfactant counterions, respectively, that are kept within the microgel. It is known that the critical aggregation concentration of surfactants in microgels is much less than solution CMC due to micelle charge neutralization by ions on gel subchains, so that micelle formation inside microgels starts at very low surfactant concentrations.…”

“…It is known that the critical aggregation concentration of surfactants in microgels is much less than solution CMC due to micelle charge neutralization by ions on gel subchains, so that micelle formation inside microgels starts at very low surfactant concentrations. [ 36–38 ] We consider the general case when the charge of micelles with the aggregation number m is partly neutralized by both oppositely charged monomer units and by surfactant counterions. Counterion immobilization in the vicinity of micelles is important when the total charge of the microgel is close to zero, this situation is realized at surfactant concentrations close to and above CMC.…”

“…[ 36 ] Depending on the surfactant concentration added to the microgel we can distinguish two regions where the swelling can be induced upon illumination with UV light (low concentration region, Region I) or green/blue light (above critical micelle concentration (CMC), Region II). [ 37 ] While the shrinkage is achieved on irradiation with green/blue light (Region I) and with UV (Region II). The up to eightfold change in volume is completely reversible.…”

Light-induced manipulation of passive and active microparticles

“…However, a light-induced DO flow can be generated when sedimented particles are porous [31]. At equilibrium such particles absorb the ionic species [36][37][38][39], but irradiation with blue light induces a rapid escape of cis-isomers out the pores. Consequently, each particle becomes a source of a laterally inhomogeneous excess of cis-isomers that leads to a concentration inhomogeneity along the wall.…”