Microion-Driven Free Swelling of Cylindrical Ionic Microgels*

Ionic microgels are soft colloidal particles, made of cross-linked polymer networks, that become charged and swollen in solution. Swelling can be controlled by adjusting temperature, pH, and ionic strength, facilitating applications to drug delivery and tissue engineering. Unlike spherical microgels, cylindrical microgels can independently swell in radial and axial directions, triggered by adjusting the balance between electrostatic and gel contributions to the osmotic pressure [1]. Within a cylindrical cell model, we compute the electrostatic osmotic pressure via exact statistical mechanical relations [1], implemented within Poisson-Boltzmann theory and molecular dynamics simulations. Combining electrostatic and gel osmotic pressures, the latter modeled using Flory-Rehner theory, we explore the dependence of swelling on charge density and salt concentration. With increasing density of fixed charge, spread uniformly over the particle surface or volume, we find that cylindrical microgels swell more in the axial than the radial direction. Our approach can help guide the design of responsive, rodlike microgels.
[1] A. R. Denton and M. O. Alziyadi, J. Chem. Phys. 151, 074903 (2019).