Swelling, Structure, and Phase Stability of Compressible Microgels

TL;DR

This study uses Monte Carlo simulations to explore how microgel particles swell, deswell, and influence phase behavior under varying environmental conditions, revealing effects of compressibility and size fluctuations on crystallization and polydispersity.

Contribution

It introduces a simulation model that incorporates particle compressibility and size fluctuations based on the Flory-Rehner theory, advancing understanding of microgel suspension behavior.

Findings

Compressibility and size fluctuations suppress crystallization.

Microgels deswell at high densities, increasing polydispersity.

Freezing transition shifts to higher density with microgel softness.

Abstract

Microgels are soft colloidal particles that, when dispersed in a solvent, swell and deswell in response to changes in environmental conditions, such as temperature, concentration, and $p$H. Using Monte Carlo simulation, we model bulk suspensions of microgels that interact via Hertzian elastic interparticle forces and can expand or contract via trial moves that allow particles to change size in accordance with the Flory-Rehner free energy of cross-linked polymer gels. We monitor the influence of particle compressibility, size fluctuations, and concentration on bulk structural and thermal properties by computing particle swelling ratios, radial distribution functions, static structure factors, osmotic pressures, and freezing densities. For microgels in the nanoscale size range, particle compressibility and associated size fluctuations suppress crystallization, shifting the freezing transition to a higher density than for the hard-sphere fluid. As densities increase beyond close packing, microgels progressively deswell, while their intrinsic size distribution grows increasingly polydisperse.