ICP-Kolloquium, Prof. Dr. Alan Denton

Response of Soft Colloids to Crowded Environments

Microgels are soft, permeable, colloidal particles, made of crosslinked polymer networks, that swell in a good solvent. Their sensitive response to changes in environmental conditions, e.g., temperature and pH, and their capacity to encapsulate drug or dye molecules, have spawned applications to drug delivery, biosensing, filtration, and photonic crystals. Within a coarse- grained model of elastic particles that interact via a hertzian pair potential and swell via a Flory-Rehner free energy of polymer networks, we explore the response of microgels to three fundamental types of crowding. First, we explore counterion-induced deswelling of ionic microgels [1]. Working within the cell model and Poisson-Boltzmann theory, we derive the electrostatic and gel contributions to the single-particle osmotic pressure tensor. We show that swelling of ionic microgels depends on the nonuniform electrostatic pressure profile inside the particles and on the distributions of mobile microions and fixed charge [2]. Second, we investigate the influence of nanoparticle crowding on microgel swelling by extending the Flory-Rehner theory to ternary mixtures and incorporating the entropic cost of nanoparticle penetration. Finally, we study the impact of microgel compressibility and faceting on the fluid-solid phase transition in bulk suspensions. To model equilibrium properties, we perform Monte Carlo simulations, with novel trial moves including random changes in microgel size and shape and in nanoparticle concentration. Our results demonstrate that softness and penetrability can profoundly affect properties of microgels in crowded environments. In particular, we find that counterions and nanoparticles induce deswelling and that faceting promotes crystallization. Our conclusions have broad relevance for interpreting experiments and guiding the design of smart, responsive materials.