Interactions and Transport of Charged Species in Bulk and at Interfaces
Counterion-Induced Swelling of Ionic Microgels and MicrocapsulesAlan Denton
North Dakota State University, USA
Coauthor(s) : Qiyun Tang 
 Georg-August Universität, Göttingen, Germany
Ionic microgels and microcapsules, when dispersed in a solvent, swell to equilibrium sizes that are governed by a delicate balance between electrostatic and elastic forces. Tuning of particle size by varying external stimuli, such as pH, salt concentration, and temperature, has relevance for biosensors, drug delivery, microfluidics, and filtration. To model swelling of ionic microgels and microcapsules, we apply Poisson-Boltzmann theory and molecular dynamics simulation to compute counterion density profiles. Applying a new statistical mechanical theorem for the electrostatic contribution to the pressure differential across the periphery of a permeable macroion, we relate the distribution of counterions within a macroion to the internal osmotic pressure. By combining the electrostatic pressure with the elastic pressure, modeled via the Flory-Rehner theory of swollen polymer networks, we explain how deswelling of ionic microgels with increasing concentration of particles can result from a redistribution of counterions that reduces electrostatic pressure. We also consider the influence of macroion swelling and size polydispersity on interparticle interactions and on thermodynamic and structural properties of bulk dispersions.
 Qiyun Tang and Alan R. Denton, "Ion Density Deviations in Polyelectrolyte Microcapsules: Influence on Biosensors," Phys. Chem. Chem. Phys. 16, 20924 (2014).
 Qiyun Tang and Alan R. Denton, "Ion Density Deviations in Semipermeable Ionic Microcapsules," Phys. Chem. Chem. Phys. 17, 11070 (2015).
 Mary M. Hedrick, Jun Kyung Chung, and Alan R. Denton, "Structure and Osmotic Pressure of Ionic Microgel Dispersions," J. Chem. Phys. 142, 034904 (2015).