Adsorption Kinetics of a Photosensitive Surfactant Inside Microgels

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.

Generation of Local Diffusioosmotic Flow by Light Responsive Microgels

“…In general, for all wavelengths studied, incerasing the intensity of light leads first to an increase of the extent of the DO flow followed by a period of saturation; as illustrated for the case of irradiation with λ = 455 nm (Figure ), here starting from critical intensity ∼2 mW cm –2 , R C,max stays constant. The latter intensity is comparable to that at which the size change of the microgel is dominated by the surfactant adsorption/desorption rate, as discussed in our previous publication . We have shown that the kinetics of the response of the microgel size at low intensities is mostly governed by the photoisomerization rate of the surfactant within the interior of the particle, since the absorption of the trans -isomers by microgel is faster than the photoisomerization rate.…”

“…In this case, the value of the concentration gradient and thus the extent of the DO flow depends on the light intensity. At larger intensities, the process is limited by absorption/desorption kinetics, while the photoisomerization is now much faster (see the inset in Figure ), and the equilibrium size of the microgels as well as the value of the gradient of cis -isomer concentration is maintained by the absorption rate of trans -isomers out of solution, which is not intensity dependent . Here, the extent of the DO flow is saturated at a maximum value, as observed from a constant cleaning radius of 55 or 65 μm for microgels with an AA content of 5 and 20 mol %, respectively.…”

“…Similar to the rigid porous particles, we explain this observation by the dissimilar photoisomerization rate of the surfactant inside the microgel and in the bulk, ,, which creates an excess of cis -isomer concentration Δ c C near the microgels in comparison to the bulk soltion. We have demonstrated that the DO flow velocity scales linearly with Δ c C ( t ), where the dependence of Δ c C on time is estimated by solving the equation where k TC , MG and k TC are the rate constants of trans – cis isomerization inside the microgel and bulk solution, respectively, k CT is the rate constant of cis – trans isomerization in the bulk solution, c T,MG is the trans -isomer concentration in the microgel, c T,B and c C,B are the concentrations of trans- and cis- isomers in the bulk, and I is the intensity of the light (an illustration of the process is provided in Figure S3, Supporting Information).…”

Aqueous microgels with engineered hydrophobic nano-domains

“…A surfactant containing a light-responsive azobenzene unit was utilized for controlled adsorption and desorption in hydrophilic poly(NiPAM- co -AA) microgels obtained from free radical polymerization (1–25 mol% AA) in aqueous microfluidic droplets by Sharma et al 76 Upon light irradiation, the surfactant shows reversible switching between the hydrophobic trans -conformation and the hydrophilic cis -conformation. The hydrophobic surfactant conformation allowed for diffusion of the surfactant into the microgel polymer network with formation of micelles, leading to microgel swelling.…”

Spiropyran/Merocyanine Amphiphile in Various Solvents: A Joint Experimental–Theoretical Approach to Photophysical Properties and Self-Assembly

“…The moieties that are utilized to render amphiphiles the photoresponsive nature are azobenzene [3][4][5][6], stilbene [7][8][9][10][11], dithienylethene [12][13][14] and spiropyran [15][16][17][18], to name a few [2]. Such amphiphiles have exhibited, for example, (i) smart principles for the dynamic control of surface tension of aqueous solutions [19], (ii) phenomenon of the light-driven diffusioosmosis upon which one can remove, gather or pattern a particle assembly at a solid-liquid interface [20], (iii) the photo-controlled stability and breakage of foams [21,22], (iv) new macroscopic functions such as liquid droplet transport [23] and bubble manipulation [15], (v) photoswitchable catalytic properties [24], (vi) photochemical OFF/ON cytotoxicity switching [25], etc.…”